Water Balance Covers for Waste Containment: Principles and Practice

Transcription

1 Water Balance Covers for Waste Containment: Principles and Practice William H. Albright Desert Research Institute Reno, Nevada USA Craig H. Benson University of Wisconsin Madison, Wisconsin USA W. Jody Waugh S.M. Stoller, Co. Grand Junction, Colorado USA September 29, 2009

2 Preface This document was created to provide engineers, designers, and regulators with the basic principles behind selection and design of water balance (WB) covers for waste containment. Much of the document is derived from observations and lessons learned from the U.S. Environmental Protection Agency s Alternative Cover Assessment Program (ACAP). The document begins with two chapters discussing basic issues affecting the selection of WB covers, where they are appropriate and under what circumstances, and key factors to be considered by the engineer, regulator, and owner. Two subsequent chapters provide principles of soil physics, plant ecology, and water balance ecology that are relevant to design and evaluation of WB covers. This fundamental information is incorporated into two chapters on design. The first of these chapters covers preliminary design. A method to compute cover thickness is described that is based on balancing infiltration to be stored with storage capacity within the cover. The second chapter discusses computer modeling to validate or refine a design, to assess sensitivity, and to evaluate what if questions. The last chapter describes what can be expected in terms of field performance and methods for monitoring performance. Data from ACAP are described along with inferences that can be made about performance expectations in other locations in the US. ii

3 Acknowledgements This document reflects the authors experience gained from two decades of basic and applied research in final cover hydrology combined with numerous practical applications in realworld landfill settings. Much of the information in this document was derived from experiences associated with U.S. Environmental Protection Agency s (USEPA) Alternative Cover Assessment Program (ACAP) and a series of ACAP technology transfer workshops conducted by the authors under sponsorship of USEPA. ACAP was a nationwide network of 28 field-scale test sections simulating landfill final covers that were designed, constructed, instrumented, monitored, and, finally, decommissioned by the authors. Many of the principles presented in this document were refined based on discussions with workshop participants as well as students and colleagues who participated in ACAP. A list of these persons would be too long for this document. Nevertheless, their input is greatly appreciated. Steven Rock of USEPA was a driving force behind ACAP. His creativity, drive, insight, and persistence were keys to the success of the program. Glendon Gee brought his vast wealth of wisdom and experience to the initial stages of the program. Robert Valceschini s practical engineering experience was the source of the material in the section on construction issues. Financial support to prepare this document was provided by Region 8 of USEPA through an interagency agreement with the US Geological Survey (USGS), Albright s sabbatical program at the Desert Research Institute, and Benson s Wisconsin Distinguished Professorship at the University of Wisconsin-Madison. Randy Breeden was the project manager for USEPA. This material is based upon work supported by the National Science Foundation under Grant No Rick Thompson from the Montana Department of Environmental Quality, Bob Doctor and Patrick Troxel from the Wyoming Department of Environmental Quality, and Charles Johnson from the Colorado Department of Public Health and Environment also provided in-kind support and commentary. Steven Link of Washington State University reviewed parts of the manuscript regarding water balance ecology. iii

4 List of Commonly Used Symbols and Acronyms ψ: soil water suction ψ a : air entry suction ψ T : suction at the top of the cover ψ l : the suction, above which a plant no longer transpires at the maximum rate ψ B : breakthrough suction θ: volumetric soil water content θ s : saturated volumetric soil water content θ r : residual volumetric soil water content θ c: volumetric soil water content at field capacity θ m : volumetric soil water content at wilting point θ u : unit available volumetric soil water content (θ c θ m ) θ BF : volumetric soil water content corresponding to breakthrough suction θ T : water content at the top of the cover α, n, and m: fitting parameters in the van Genuchten equation l: pore interaction term γ w : the unit weight of water Λ: loss term defined to include runoff and percolation ET: evapotranspiration K hydraulic conductivity K s : saturated hydraulic conductivity K(ψ): hydraulic conductivity as a function of soil water suction, typically unsaturated hydraulic conductivity L: thickness LAI: leaf area index P: precipitation PET: potential evapotranspiration P r : percolation R: runoff SWCC: soil water characteristic curve S r : required storage ΔS: monthly change in soil water storage S c : storage status of a soil layer at incipient percolation S a : available storage T: transpiration iv

5 CONTENTS Preface... ii Acknowledgements...iii List of Commonly Used Symbols and Acronyms... iv List of Figures...viii List of Tables... xii Chapter Introduction to Water Balance Covers... 1 Chapter Issues in Cover Selection & Design... 5 Regulatory Underpinning... 5 Types of Covers... 9 Conventional Covers... 9 Water Balance Covers Design Philosophy and Issues Issues for the Site Owner Issues for the Design Team Issues for the Regulator Chapter The Soil Profile: Concepts of Flow and Storage Saturated Soil Properties Unsaturated Soil Hydraulic Behavior Soil Water Suction Soil Water Storage Concepts Soil Water Characteristic Curve Unsaturated Hydraulic Conductivity Flow and Hydraulic Gradients Chapter Introduction to Ecology and Revegetation of Water Balance Covers Basics of Plant Transpiration Revegetation Goals and Strategies Baseline Ecological Survey Revegetation Practices Revegetation Success Criteria General Revegetation Concepts and Practices Site Preparation Soil Edaphic Properties and Handling Physical Properties Chemical Properties Organic Matter and Microorganisms Soil Storage and Handling Soil Mulches and Amendments Selection of Plant Species and Materials Seedbed Preparation Planting Methods Seeding Rates and Pure Live Seed v

6 Seeding Planting Maintenance Irrigation Fertilization Weed Management Grazing Management Monitoring Natural Analogs and WB Cover Designs and Sustainability Example of Cover Design Concept Analogs Natural Analogs and Cover Sustainability Soil Development Ecological Change Climate Change Steps to Evaluate Long-Term Performance Biointrusion Control Chapter Preliminary Design Required Storage Available Storage and Thickness for Monolithic Covers Example Equilibrium Gradient Conditions Thickness Calculation Procedure Available Storage and Thickness for Capillary Barriers Thickness Calculation Procedure Example Use of Geotextiles for Capillary Breaks Field Application of SWCCs Measured in the Laboratory Chapter Introduction to Water Balance Modeling Model Attributes Model Input Boundary Conditions Meteorological Data Soil Properties Vegetation Properties Geometry Spatial and Temporal Discretization Reality Check Chapter Lessons Learned from the Field ACAP Field Performance Data Need for Site-Specific Design Importance of Vegetation Providing Sufficient Storage Capacity Performance in More Humid Climates vi

7 Expectations in Other Areas Design Percolation Rate and Equivalency Evaluating Cover Performance in the Field Construction Issues Some Issues for Construction References Appendix: ACAP Test Section Installation Instructions vii

8 List of Figures 1.1. A comparison of projected construction costs of two water balance (WB) covers and a conventional (RCRA Subtitle D) design. The two WB covers differ in the thickness of the soil profile (1.2 m and 1.5 m). The conventional consists of a 400-mm barrier layer of fine-grained soil (saturated hydraulic conductivity < 1 x 10-5 cm/s), a 1-mm geomembrane, a drainage layer, and a 300-mm surface layer. At this site a savings of 64% was realized with use of the thinner WB cover Basic configurations of conventional covers. These covers are characterized by a low-conductivity resistive barrier overlain by a vegetated soil layer. The resistive barrier can consist of soil (compacted clay or a geosynthetic clay layer) or a composite of low-conductivity soil and a geomembrane Photograph of tamping foot compactor compacting a clay barrier layer Photograph of welding geomembrane panels for a composite cover Basic configurations of monolithic and capillary barrier water balance covers. Monolithic covers consist of a layer of fine-textured soil placed over the waste; capillary barrier designs include a layer of coarse material under the fine-textured soil Conceptual function of water balance covers. As shown in (a) the fine-textured soil of the cover acts as a sponge to store precipitation for later release to the atmosphere by evaporation and transpiration. WB cover soils have a finite storage capacity (S C ) and drainage occurs when actual storage (S) exceeds S C. The field data shown in (b) are from a test section of a WB cover located in coastal California. The site, near Monterey, has a seasonal climate with cool, wet winters and warm, dry summers. Net accumulation of water in the cover soils (S) exceeded storage capacity (S c ) each year during the monitoring period. The result was percolation of approximately 50 mm yr These schematics demonstrate the concept of saturated hydraulic conductivity. A common representation of measured flow through porous media is shown in (a). For measurements that are area-independent (i.e., precipitation, percolation from a landfill cover) removing the area term gives the result expressed as flux Flexible-wall (a) and rigid-wall (b) permeameters. The flexible-wall system encases the soil sample in a flexible membrane within a chamber used to apply confining pressure. In the rigid-wall apparatus soil is compacted directly in the permeameter. The rigid-wall permeameter shown in (b) is for soils with large particles Photographs of (a) installed TSB permeameter with 300-mm casing and 100-mm standpipe and (b) water-filled SDRI using Marriotte bottle for measuring infiltration volume (note TSB permeameter to right of SDRI) The pressure in a water column depends on vertical location relative to a free water surface. Water pressure at the free water surface is zero (or atmospheric); below that surface (submerged) the pressure is positive. The water pressure in a hydrophilic capillary tube above the free water surface is negative. The negative pressure is directly proportional with height above the free water surface viii

9 3.5. Schematic illustrating the void space concept. The void space in soils typically used for landfill cover applications represents 30-45% of the total soil volume. For simplicity, two soils (a silt and a sand) are shown, each with a void space (porosity) of 40%. Submerged, the void space is completely occupied by water Schematic illustrating the field capacity concept. The two soils are raised out of the water and allowed to drain freely. Following free drainage, the water content of both soils is described as field capacity. The red arrows show the quantity of water lost to free drainage. At field capacity the water content of the sand is much less than the silt Schematic illustrating the wilting point concept. Plants transpire and, in the process, remove water from the soil where roots are present. Wilting occurs when the plant can no longer extract water from the soil. The water content at this point is referred to as the wilting point. Note that both the wilting point and the field capacity of the sand are at lower water contents than the wilting point of the silt This schematic shows the relationship between pore diameter, tension (or suction) at which water is held in pores of various size, and water content. At very low suctions even the largest capillaries are filled. As suction increases, increasingly smaller pores are emptied. Only those pores capable of maintaining sufficient suction to retain water remain water-filled Schematics showing the four primary features of the soil water characteristic curve (SWCC): (1) saturated water content, (2) the air entry suction (the suction at which the largest pores are emptied of water), (3) the slope of the curve (n) that describes the distribution of pore sizes, and (4) the residual water content (θ r ). The sand (b) has lower ψ a because it has larger pore sizes. The steeper middle section of the curve (smaller n) for the silt (a) corresponds to a larger distribution of pore sizes An example of data from laboratory analysis of a soil by hanging column (A), pressure plate (B), and chilled mirror hygrometer (C). Note the overlap in data from the hanging column and pressure plate and the alignment between the pressure plate and chilled mirror hygrometer. The van Genuchten equation (1980) was fit to the data Effect of the van Genuchten parameters (α and n) on the shape of the soil water characteristic curve. The α parameter (related to the inverse of the air entry suction, ψ a ) affects the breakpoint in the curve (a). The n parameter affects the slope of the soil curve for suctions greater than ψ a (b). These drawings are exaggerated to illustrate the effects of variations in the parameters A simplified representation of the relationship between soil water content and the water-filled pore space that controls hydraulic conductivity. As the soil water content is reduced, the conductive pathways are fewer, smaller, and more tortuous, which reduces the hydraulic conductivity ix

10 3.13. The relationship between hydraulic conductivity and soil suction. The hydraulic conductivity remains near the saturated hydraulic conductivity until the air entry suction is reached. Drainage of water from the relatively uniform pore sizes in the sand results in a rapid decrease in conductivity with increased suction above the air entry suction. The hydraulic conductivity of the silt exceeds that of the sand at higher suctions because the silt retains more water due to higher air entry suction and a broader pore size distribution The effect of the α and n parameters on hydraulic conductivity. The α parameter affects the breakpoint in the curve (a), commonly referred to as the air entry suction (ψ α ). The break point occurs at higher suctions as α increases. The n parameter affects the slope of the hydraulic conductivity function for suctions greater than ψ α (b). The slope becomes shallower as n decreases Schematic showing example of water potential gradient from soil, through roots, stem, leaf, and to air Schematic of the leaf surface and stomates. Open stomates allow diffusion of CO 2 into the substomatal cavity and water out of the stomates to the atmosphere Explanations of the components of bioclimatic diagrams Comparison of bioclimatic diagrams for Lake County, Colorado and Canyonlands National Park, Utah Natural analog of a conceptual design for a water balance cover with a capillary barrier at the Hanford Site near Richland, Washington General steps of a systematic approach for projecting long-term performance of WB covers that links modeling and natural analogs (adapted from Ho et al., 2004) Monthly precipitation (P) and monthly P/PET for the typical year and the wettest year on record for the example site Total soil water storage (S c ) of a soil profile (indicated on the left) of thickness L is determined by integrating the field capacity water content over the thickness of the layer. The water content at the bottom of the layer is the field capacity of the soil. The water content at points higher in the profile must be less than field capacity for equilibrium conditions (i.e., an absence of drainage). The average water content for the profile can be approximated as the average of the field capacity for the soil (θ c ) and the water content corresponding to the soil water suction at the top of the profile (θ T ). In practice, there may be little difference between θ c and θ T Available soil water storage (S a ) of a soil profile is the total storage capacity reduced by the amount of water not available for transpiration by plants. In practice, evaporation from the surface may dry the top of the soil below θ m. Using θ m as the lower bound of water content is a conservative approach and will have little effect on the storage capacity Soil water characteristic curves (SWCC) for two soils with lower (a) and higher (b) air entry suction. Suctions that define field capacity (ψ c ) and wilting point (ψ m ) are marked as are the corresponding water contents (θ c and θ m ) x

11 5.5. Schematic of a capillary break illustrating the fine-over-coarse soil layering. The capillary break is created by continuity in pore water across the interface between the finer and coarse layers, which creates equal suction at points adjacent to the interface SWCCs for a finer and coarser soil show the increase in storage created by the capillary break. At suctions greater than ψ B the pores in the coarse layer lack sufficient suction to maintain saturation. Without the underlying coarse layer the finer-textured soil would drain at field capacity water content (θ c ). With a capillary break, drainage does not occur until the suction at the interface of the finer and coarse layers reaches ψ B, which results in higher water content (θ BF ) and greater storage in the finer layer at break through SWCCs for three soils illustrating the capillary barrier effect. The coarse soil (Grand Junction sand) is the same in both graphs (θ s =0.29, θ r =0.08, α=0.72 kpa -1, n=2.78). Unsaturated parameters for the finer-textured soil are θ s =0.32, θ r =0.00, α=0.215 kpa -1, and n=1.2 (a) and θ s =0.38, θ r =0.00, α=0.030 kpa -1, and n=1.20 (b). Note the difference in effect of the capillary barrier due to differences in unsaturated parameters of the fine-textured soil Relationship between field capacity water content from SWCCs and the water content at incipient drainage from test section data. The SWCCs were determined by two methods. The points marked as Lab Drying SWCC were determined by standard laboratory method (ASTM D 6836); those marked Field Wetting SWCC were determined from co-located sensors for suction and water content in the test sections (adapted from Apiwantragoon 2007) Postconstruction changes in saturated volumetric water content (a), saturated hydraulic conductivity (b) and the van Genuchten parameters α (c) and n (d). Results are from the ACAP project (Benson et al. 2007) and are shown as the ratio of the postconstruction to the as-built values plotted vs. the as-built property Example of the correction procedure applied to a laboratory SWCC Example of spatial discretization used in a numerical model Example of water balance predictions made for a WB cover in northern California with the computer model LEACHM Effect of precipitation application rate on runoff predicted with WinUNSAT-H for a WB cover in northern California Saturated hydraulic conductivity of in-service cover soils compared to the as-built saturated hydraulic conductivity Corrected laboratory-measured SWCCs with water content and suction data from co-located sensors in an ACAP test section Unsaturated hydraulic conductivity for Boardman sandy silt predicted with the van Genuchten-Mualem equation using l = 0.5 and -2 along with unsaturated hydraulic conductivities measured using the instantaneous profile method described in Meerdink et al. (1995) xi

12 6.7. Soil water storage predicted with WinUNSAT-H using l = 0.5, -1, and -3 for a WB cover at a landfill in northern California Typical LAI function used in computer models Plant limiting function (PLF) used to define how transpiration is affected by water availability in the root zone Root density function (R) fit to three sets of root density data for a monolithic cover Water balance predictions made with HYDRUS in 1D and 2D for a WB cover on a 4:1 slope near Monterey, California WB cover profiles evaluated in ACAP (adapted from Apiwantragoon 2007) Locations of the ACAP field sites Water balance graph for capillary barrier tested by ACAP in Polson, Montana Water balance graph for capillary barrier tested by ACAP in Helena, Montana Water balance graph for capillary barrier tested by ACAP in Sacramento, California Water balance graph for capillary barrier tested by ACAP in Omaha, Nebraska Effect of spring rain and snow on percolation rate at ACAP sites Contour map showing typical percolation rates anticipated for WB covers in the continental US Flow diagram of design-to-construction process continuum List of Tables 3.1. Typical values of α and n Climate, soil, and biological parameters for baseline ecological surveys Criteria for selection of plant species for revegetation of WB covers Thresholds of P/PET corresponding to accumulation of water Parameters for Eq. 5.2 obtained by calibration with ACAP data Meteorological data for typical and wettest years for the example site Computer models used in North American practice for water balance predictions Summary of climatic conditions and percolation rates for ACAP WB covers (adapted from Apiwantragoon 2007). The ranges of annual precipitation and annual percolation rates shown in parentheses Summary of site conditions and percolation rates for conventional covers in ACAP (adapted from Apiwantragoon 2007) Summary of construction principles for water balance covers xii

13 Albright / Benson / Waugh - Chapter 1 Chapter 1 Introduction to Water Balance Covers Modern engineered landfills are expected to control fire and the spread of litter, limit contact with wildlife, minimize or eliminate the release of mobile contaminants to the surrounding environment, and provide acceptable end-of-service land use. From the perspective of environmental protection, release of contaminants to air and groundwater is often considered the most significant issue. Consequently, containment systems are used at modern landfills to control the movement of liquids and gases into and out of a landfill. A final cover is used to control the amount of precipitation that may enter the waste and create contaminated liquid (called leachate) that may contaminate groundwater. A liner is used beneath the waste to contain leachate and to preclude groundwater contamination. The combination of hydraulic barriers above and below the waste follows a design philosophy, often referred to as dry tomb, intended to contain contaminants by minimizing flow through the containment system. Covers and liners used in many modern landfills traditionally have employed lowconductivity materials and resistive barriers (e.g., clays and geomembranes) to impede the movement of water. However, over the last two decades, cover systems that rely on a combination of temporary storage of precipitation in soil near the surface followed by removal of the stored water by evaporation (E) and transpiration (T) have become popular, particularly in drier climates. Covers that function on this principle are described by a variety of names, including alternative covers, evapotranspirative covers, store-and-release covers, and water balance (WB) covers. In this document, the descriptive WB nomenclature is used throughout. However, the contents of this document apply to covers described by any of the previously described names. Use of store and release mechanisms to maintain a favorable water balance is a natural process that is not new to waste containment. Common experience informs us that plants make use of precipitation stored in near-surface soils and that those soils dry as a result. Indeed, even in conventional final covers that rely on hydraulic barrier layers, much of the water balance is managed by WB mechanisms in the vegetated surface layer. However, in an WB cover, these 1

14 Albright / Benson / Waugh - Chapter 1 mechanisms become solely responsible for managing the water balance and for maintaining percolation below a desired threshold. The vegetated soil operates as a storage tank that is filled by precipitation events, and emptied during subsequent periods of evapotranspiration (ET). This mechanism for managing the water balance does not rely on the physical characteristics of a single design element (i.e., the low saturated hydraulic conductivity of a resistive barrier), but on an integrated system consisting of the soil, plants, and atmosphere in which the components must be carefully considered separately and in combination. Selection of an appropriate landfill final cover requires careful consideration of the varied metrics for landfill performance, site-specific details, regulatory requirements, and cost. For example, there are some wastes that pose sufficient threat to human health and the environment that redundant containment systems are required. In such situations, covers that rely solely on the WB mechanism to control the movement of water may not be appropriate, but a combination of WB and resistive barriers may provide an adequate solution (e.g., see Waugh et al. 2006). There are also some climates characterized by either an excess of precipitation or a shortage of evaporative demand in which a combination of soil and plants may not provide sufficient control over the water balance to achieve adequately low percolation rates (Albright et al. 2004). Additional factors specific to each landfill may also affect selection of a final cover, such as type of waste, depth to groundwater, proximity to existing or planned uses of groundwater resources, longevity requirements, and the capacity of underlying soils to limit the movement of contaminants that might escape the engineered containment system. For example, a less restrictive cover may be acceptable for a landfill containing relatively inert wastes that is sited in a favorable geological environment. Applications may also exist where the final cover is intended to transmit a higher percolation rate. For example, WB covers may be particularly useful for bioreactor landfills in which the objective is controlled (rather than minimized) percolation into the waste to provide water for biological processes that degrade waste. Cases may also exist where regulations prohibit or strongly discourage the use of a WB cover. One of the attractive features of WB covers is the significant cost savings that can be accrued when a WB cover is used in lieu of a conventional cover. Much of the cost associated with conventional covers involves the purchase, hauling, and placement of the materials used for the hydraulic barrier. Both geomembrane and soil barriers require labor-intensive construction methods and transportation costs can be considerable if clay is not locally available. Water 2

15 Albright / Benson / Waugh - Chapter 1 balance covers tend to be thicker than conventional covers, but soil placement methods are usually less costly. For example, costs for a conventional composite cover and two alternative WB covers at a semi-arid site in eastern Oregon are shown in Figure 1.1. The conventional design consisted of 460 mm of compacted soil having a saturated hydraulic conductivity less than 10-5 cm/s, a geomembrane, a geosynthetic drainage layer, and 610 mm of vegetated cover soil. The WB covers were monolithic designs (1.2 m and 1.5 m thick) constructed with on-site sandy silt and vegetated with local grasses. A field demonstration showed that either WB cover was hydraulically equivalent to the conventional cover (Albright et al. 2004). Figure 1.1. A comparison of projected construction costs of two water balance (WB) covers and a conventional (RCRA Subtitle D) design. The two WB covers differ in the thickness of the soil profile (1.2 m and 1.5 m). The conventional consists of a 400-mm barrier layer of finegrained soil (saturated hydraulic conductivity < 1 x 10-5 cm/s), a 1-mm geomembrane, a drainage layer, and a 300-mm surface layer. At this site a savings of 64% was realized with use of the thinner WB cover. Using the thinner cover at this Oregon landfill resulted in a cost savings of 64%, or $41,000 per acre (in 2000 US $). Costs are very site-specific, however, and a careful cost analysis should be conducted before deciding to proceed with a WB cover. One particularly 3

16 Albright / Benson / Waugh - Chapter 1 important factor is the cost associated with design and permitting, which typically is higher for WB covers compared to conventional covers. Factors contributing to higher design and permitting costs include the site-specific nature of the design, fees associated with characterization of the soils and vegetation to support the design, labor associated with sophisticated predictive modeling that is not required for conventional covers, and costs associated with additional meetings between the designer, owner, and regulator to gain familiarity and confidence in the WB cover. This document describes the technical aspects of design and evaluation of WB covers. Mechanisms controlling storage of precipitation in near-surface soils and subsequent removal of that stored water by ET are described. This includes soil properties related to water storage and plant ecology, the removal of stored water through transpiration by plants, and climatic factors that influence the water balance. Procedures used to compute required and available water storage capacities are presented, and modeling methods are described that can be used to evaluate WB cover designs for various scenarios. Techniques used to validate designs and to monitor cover performance are presented. In large part, the origins of this document rest on two decades of research in landfill final covers and information gleaned from approved closures. Several field research programs have directly measured the performance of a variety of final covers in a broad range of climates within the continental US. Those research results provide a good indication of the potential performance of different cover designs in different climatic regimes and insight into the mechanisms and variables affecting the hydrologic performance of final covers. 4

17 Albright / Benson / Waugh - Chapter 2 Chapter 2 Issues in Cover Selection & Design Regulatory Underpinning Covers are commonly described as conventional or an alternative (to the conventional). This terminology in itself provides no description of design concepts or specific features, but rather is an artifact of regulatory history. Nevertheless, this nomenclature is commonplace and will be used in this document. Conventional covers are described by regulation (at least in general terms) as meeting minimum standards by including certain design features. Alternative covers are covers that employ different principles or materials than in the conventional design, and are generally required to have equivalent performance as the conventional cover. In the United States, conventional final cover design has its roots in the Resource Conservation and Recovery Act (RCRA). These provisions are described in Subpart F (Closure and Post-Closure Care) of Title 40 (Protection of Environment) of the US Code of Federal Regulations. The salient aspects are described in the following parts of Section 258: Closure criteria. (a) Owners or operators of all municipal solid waste landfill (MSWLF) units must install a final cover system that is designed to minimize infiltration and erosion. The final cover system must be designed and constructed to: (1) Have a permeability less than or equal to the permeability of any bottom liner system or natural subsoils present, or a permeability no greater than cm/sec, whichever is less, and (2) Minimize infiltration through the closed MSWLF by the use of an infiltration layer that contains a minimum 18-inches of earthen material, and (3) Minimize erosion of the final cover by the use of an erosion layer that contains a minimum 6-inches of earthen material that is capable of sustaining native plant growth. (b) The Director of an approved State may approve an alternative final cover design that includes: 5

18 Albright / Benson / Waugh - Chapter 2 (1) An infiltration layer that achieves an equivalent reduction in infiltration as the infiltration layer specified in paragraphs (a)(1) and (a)(2) of this section, and (2) An erosion layer that provides equivalent protection from wind and water erosion as the erosion layer specified in paragraph (a)(3) of this section. (3) The Director of an approved State may establish alternative requirements for the infiltration barrier in a paragraph (b)(1) of this section, after public review and comment, for any owners or operators of MSWLFs that dispose of 20 tons of municipal solid waste per day or less, based on an annual average. Any alternative requirements established under this paragraph must: (i) Consider the unique characteristics of small communities; (ii) Take into account climatic and hydrogeologic conditions; and (iii) Be protective of human health and the environment Post-closure care requirements. (a) Following closure of each MSWLF unit, the owner or operator must conduct postclosure care. Post-closure care must be conducted for 30 years, except as provided under paragraph (b) of this section, and consist of at least the following: (1) Maintaining the integrity and effectiveness of any final cover, including making repairs to the cover as necessary to correct the effects of settlement, subsidence, erosion, or other events, and preventing run-on and run-off from eroding or otherwise damaging the final cover.. In 1992, the US Environmental Protection Agency (USEPA) published a supplement to intended to clarify the intent of the regulation. This supplement provided recommended minimum designs for final covers and confirmed flexibility in the regulations related to the use of alternative designs. The following excerpt from the Federal Register (Vol. 57, No. 124, 1992, p ) describes the supplement: EPA established the requirement for a final cover infiltration layer, which includes a permeability standard, to prevent the bathtub effect from occurring. The bathtub effect occurs when a landfill fills up with liquids because the infiltration layer of the final cover is more permeable than the bottom liner system or natural subsoils present EPA intended, and has always interpreted, the language in this section to be a performance standard that requires the permeability of the final cover be less than or equal to that of the bottom liner system or natural subsoils present, whichever 6

19 Albright / Benson / Waugh - Chapter 2 is less While this standard does not explicitly require the use of a synthetic membrane in the final cover, the Agency anticipates that if a MSWLF has a synthetic membrane in the bottom of the unit, then the infiltration layer in the final cover will, in all likelihood given today s technologies, include a synthetic membrane as part of the final cover. This is so because it generally is not currently possible to have an earthen material infiltration layer as part of the final cover that has a permeability of less than or equal to the permeability of a synthetic membrane The following are illustrations of the correct interpretation of this rule language. These illustrations present typical designs of MSWLFs and the corresponding correct final cover as required under (a). MSWLF liner design Minimum final cover No liner (in-situ soils) Minimum infiltration layer of 18 inches of 1x 10-5 cm/sec earthen material overlain by a minimum 6-inch erosion layer Recompacted 1x 10-6 cm/sec soil liner Minimum infiltration layer of 18 inches of 1x 10-6 cm/sec earthen material overlain by a minimum 6-inch erosion layer Composite liner (80 mil synthetic over Minimum infiltration layer of 18 3 foot recompacted 1x 10-7 cm/sec soil inches of 1x 10-5 cm/sec earthen liner material overlain by a synthetic liner (Agency recommends minimum 20 mils; if HDPE 60 mils) overlain by minimum 6-inch erosion layer To correct any misunderstanding regarding the permeability standard of the final cover design, the Agency is today revising the language of (a) to provide further clarification. This revision is intended to eliminate any confusion regarding the correct interpretation of this rule language. This clarifying language does not remove any of the flexibility in (b) regarding alternative final cover designs approved by the Director of a State/Tribal program that has been deemed adequate by EPA. (emphasis added) There are two significant misnomers in that need clarification. First, the term permeability (ex. Have a permeability less than or equal to the permeability of any bottom liner ) was used in in the context of civil engineering practice ca. 1992, and was intended to mean the saturated hydraulic conductivity. This latter term is used in this document to prevent confusion with other, more specific definitions of permeability used in the hydrology and petroleum literature. Second, the term infiltration (ex. Minimize infiltration through the closed MSWLF ) is used incorrectly. Infiltration is the entry of water into the soil through the 7

20 Albright / Benson / Waugh - Chapter 2 atmosphere-soil interface. Water that has infiltrated the soil may (1) be removed back to the atmosphere by evaporation or transpiration, (2) be stored, thus increasing the water content of the soil, or (3) continue downward. In landfill cover applications, the primary concern is water that passes from the cover into the waste. That water is referred to herein as drainage or percolation. The regulations also are ambiguous regarding performance requirements. For conventional covers, performance expectations are avoided by specifying material parameters for the barrier layer (i.e., a minimum thickness and an upper bound on saturated hydraulic conductivity). This approach is acceptable for conventional covers, which can be constructed to meet regulations without explicitly defining their performance. However, this approach creates a dilemma for WB designs that must be equivalent to the conventional cover. WB covers rely on multiple mechanisms to control water that cannot be described by a single material parameter. Moreover, in the dry tomb landfill philosophy espoused by RCRA, the primary hydrologic function of the cover is to minimize percolation of water into the waste. Yet nowhere does RCRA define minimize or state that a final cover must restrict percolation to a stated quantity. Even the statement in the clarification that USEPA always interpreted the language in this section to be a performance standard refrains from stating a quantifiable performance standard and, instead, refers to a material property (saturated hydraulic conductivity) of the cover materials. Indeed, many lengthy discussions have taken place at permitting meetings over what minimize means in the context of the performance of final covers and what acceptable performance goals are in the context of RCRA. To date, consensus does not exist regarding quantitative performance expectations for final covers or the method by which equivalency is to be demonstrated (i.e., computer modeling, field demonstration, etc.). Another important complicating factor is the difficulty in developing and applying universal expectations that apply throughout the US. Field-scale evaluations of both conventional and alternative cover designs show that the performance of any cover depends on multiple factors, with climate being the most important factor. Performance easily obtained at an arid site may be difficult to achieve in a more humid climate even with a more sophisticated cover design. In the larger picture, the required performance of the containment system depends on numerous other factors including the waste characteristics, liner design, depth to groundwater, quality of groundwater, and distance to receptors. That is, one size does not fit all. Instead, conscientious 8

21 Albright / Benson / Waugh - Chapter 2 engineering design is needed that is site specific, ensures environmental protection, and addresses issues important to the stakeholders for a particular project. Types of Covers Conventional Covers Conventional covers (also known as prescriptive or resistive covers) for waste containment facilities employ a layer (or layers) of low-conductivity material to resist the movement of precipitation into underlying waste. Typical profiles for conventional covers are shown in Figure 2.1. The barrier layer generally consists of compacted fine-textured soil and, depending on the site, may be covered with a geomembrane. Covers that rely solely on compacted fine-textured soil for the barrier are often referred to as compacted clay covers even though soils that do not classify as clay can be used for a soil barrier layer. A geosynthetic clay layer (a thin layer of sodium bentonite sandwiched between geosynthetic layers) is sometimes substituted for the soil layer. Barriers that rely on a geomembrane overlying a low-conductivity soil layer are often referred to as composite covers. This nomenclature will be used throughout this document. Performance of conventional covers is discussed in Chapter 7. In addition to the hydraulic barrier, conventional covers can include a drainage layer overlying the barrier layer and a vegetated surface layer. The drainage layer serves to laterally divert water and prevent the development of pore-water pressures that can lead to slope instability. The vegetated surface layer protects the barrier layer from damage by erosion and freeze-thaw cycling. Barrier layers used in conventional covers are similar to those developed for bottom liners and the regulations that govern landfill design reflect the co-evolution in design (as shown in the table in the above excerpt from the 1992 supplement published in the Federal Register). The similarities in design features between cover and liner design stem from concern for the so-called bathtub effect that might result if drainage through a cover was greater than through the liner. 9

22 Albright / Benson / Waugh - Chapter 2 Simple Soil Cover Compacted Clay Cover Geosynthetic Clay Liner (GCL) Cover Soil Soil Soil Waste Compacted Clay Waste GCL Waste Composite with Clay Barrier Composite with GCL Soil Compacted Clay Geomembrane (GM) Soil Waste Waste GCL Figure 2.1. Basic configurations of conventional covers. These covers are characterized by a lowconductivity resistive barrier overlain by a vegetated soil layer. The resistive barrier can consist of soil (compacted clay or a geosynthetic clay layer) or a composite of lowconductivity soil and a geomembrane. Conventional cover designs rely on applying construction methods that will achieve the desired material property (low saturated hydraulic conductivity) in the barrier layer and persistence of these properties throughout the design life of the containment facility. The most important criterion in selection of material for a soil barrier layer is the ability to achieve the target saturated hydraulic conductivity with state-of-practice methods. Methods for placement of 10

23 Albright / Benson / Waugh - Chapter 2 low-conductivity soil barriers specify placement of thin lifts of relatively wet soils with fully penetrating compaction equipment (Figure 2.2). Large-scale field tests have demonstrated the effectiveness of these methods (Benson et al. 1999). Geomembranes are typically specified according to polymer type, thickness, and surface texture. Methods for placement of geomembranes emphasize welding of seams (Figure 2.3) and preparation of the subgrade to prevent punctures. Field quality assurance activities are critical to ensure that the construction practices achieve the desired material parameters (i.e., low hydraulic conductivity). A detailed description of construction methods and quality assurance requirements for barrier layers used in conventional covers can be found in Daniel and Koerner (2007). Environmental conditions common to surficial soils (wet-dry and freeze-thaw cycling and penetration by plant roots and burrowing fauna) can damage soil barrier layers. These processes form cracks, holes, and other macroscopic features that are collectively referred to as macropores. These features serve as preferential pathways that are reflected as increases in saturated hydraulic conductivity. These fractures persist even when the soil swells when water is added (Benson and Othman 1993, Othman and Benson 1994, Albrecht and Benson 2001, Albright et al. 2004). The geomembrane that overlies the soil barrier in composite designs Figure 2.2. Photograph of tamping foot compactor compacting a clay barrier layer. 11

24 Albright / Benson / Waugh - Chapter 2 Figure 2.3 Photograph of welding geomembrane panels for a composite cover. eliminates most biotic intrusion and protects the soil layer from fluctuations in water content caused by evapotranspiration (Melchior 2008). However, geomembranes do not protect the clay component from damage due to freeze-thaw cycling (Benson et al. 1995) or from water content changes caused by variations in thermodynamic conditions beneath the cover. Water Balance Covers The two common configurations of WB covers, monolithic and capillary barrier designs, are shown in Figure 2.4. The monolithic design consists of a thick layer of engineered finetextured soil. The capillary barrier design consists of a layer of engineered fine-textured soil over a thinner layer of clean coarse-grained soil. Roots from the vegetation extend throughout the fine-textured layer in both configurations. Noticeably absent is a layer of low-conductivity material (soil or geomembrane) to provide hydraulic resistance. In contrast to conventional covers, WB covers provide hydraulic control by means other than the hydraulic resistance afforded by a layer of low conductivity material. 12

25 Albright / Benson / Waugh - Chapter 2 Monolithic Cover Capillary Barrier Fine Textured Soil Coarse Soil Waste Figure 2.4. Basic configurations of monolithic and capillary barrier water balance covers. Monolithic covers consist of a layer of fine-textured soil placed over the waste; capillary barrier designs include a layer of coarse material under the fine-textured soil. Water balance covers manage water by providing two essential elements: (1) water storage capacity in the unsaturated near-surface soil to prevent drainage during periods when precipitation exceeds evapotranspiration and (2) sufficient removal of the stored water by evapotranspiration during periods of lesser precipitation and greater evaporative demand. The conceptual water balance for a WB cover is shown in Figure 2.5 using a water balance graph from a WB cover test section located near Monterey, California. The graph shows cumulative quantities for water stored within the cover as well as the four primary fluxes into and out of a WB cover: precipitation, evapotranspiration, runoff, and percolation. These cumulative fluxes increase monotonically because the fluxes always are positive (e.g., runoff always flows off the surface of a cover). In contrast, soil water storage increases as water accumulates within the cover due to infiltration during the wetter season, and decreases as water is removed from the cover by evaporation and transpiration during the drier season. In this example, percolation also contributes to the reduction in soil water storage. 13

26 Albright / Benson / Waugh - Chapter 2 (a) Evapotranspiration Precipitation L Infiltration Sponge Percolation if S > S c S = soil water storage S c = soil water storage capacity (b) Cumulative Precipitation and Evapotranspiration (mm) Storage Capacity = 300 mm Soil Water Storage Evapotranspiration Precipitation Missing Data Percolation No Surface Runoff 0 0 1/31/00 7/31/00 1/30/01 7/31/01 1/30/02 8/1/02 1/30/03 8/1/03 1/31/ Cumulative Percolation, Soil Water Storage, and Surface Runoff (mm) Figure 2.5. Conceptual function of water balance covers. As shown in (a) the fine-textured soil of the cover acts as a sponge to store precipitation for later release to the atmosphere by evaporation and transpiration. WB cover soils have a finite storage capacity (S C ) and drainage occurs when actual storage (S) exceeds S C. The field data shown in (b) are from a test section of a WB cover located in coastal California. The site, near Monterey, has a seasonal climate with cool, wet winters and warm, dry summers. Net accumulation of water in the cover soils (S) exceeded storage capacity (S c ) each year during the monitoring period. The result was percolation of approximately 50 mm yr

27 Albright / Benson / Waugh - Chapter 2 If the cover has adequate storage capacity, percolation can be limited to minute amounts. In this example, however, the soil water storage capacity (300 mm) is exceeded each year, and approximately 50 mm of percolation is transmitted annually. By adjusting the soil water storage capacity, a cover can be designed to transmit a range of desired percolation rates. The soil water storage capacity depends on the cover thickness, the unsaturated hydraulic properties of the cover soils, and the soil layering. Soil hydraulic properties are discussed in Chapter 3. Water removal generally is controlled by evaporation from the soil surface (E) and transpiration (T) by plants. Ideally, transpiration will remove water throughout the entire depth of the cover profile. The transpiration rate depends on a combination of factors including the composition of the plant community, the above-ground distribution of biomass (e.g., percent cover and leaf area), and the rooting depth and distribution. Properties of the plant community are discussed in Chapter 4. Design Philosophy and Issues Successful implementation of a WB cover begins with recognition of the differences in design philosophy compared with conventional designs. Conventional cover design generally follows a regulatory approach where the materials and layer thicknesses are specified to meet regulatory requirements. A consequence of this materials-and-methods approach is that the hydraulic performance of the cover is not known. Performance is assumed to be adequate provided the materials and methods required by regulation are implemented. This approach may be regarded as overly simplistic and inappropriate for application in complex environments where there is little control over the processes that influence performance. However, when the RCRA regulations that form the basis of most current conventional cover designs were promulgated, much less knowledge existed regarding factors that influence the water balance in final covers. Few field-scale data sets existed for conventional covers to evaluate the importance of factors such as imperfections in geomembranes and preferential flow paths in compacted clay layers. Moreover, field-scale research conducted since the RCRA regulations were promulgated has shown that some conventional covers can be very effective in limiting percolation to very small amounts. For example, percolation rates from carefully constructed composite covers typically are less than 4 mm/yr. 15

28 Albright / Benson / Waugh - Chapter 2 Design of WB covers follows a different approach (ITRC 2003). Since there are no layers of specified material property, there is no opportunity to assume that performance will be acceptable provided a particular design feature is present. Thus, design must begin with a description of the required performance, which typically is a maximum annual percolation rate. The next step is to select a conceptual design and to validate that the design is possible based on regional experience, availability of soils, and climatic information. A borrow source investigation is then conducted to determine the availability of suitable cover soils and to characterize the hydraulic and edaphic properties of the cover soils. Data from the borrow source investigation are used to select a cover profile (soil types and layer thickness) using design computations. A revegetation plan is developed to establish a cover plant community that will provide adequate transpiration, erosion control, and ecological resilience. Ecological reference areas are often selected to evaluate the potential vegetation of the borrow soil and as a basis for developing a revegetation plan. Because performance cannot be assumed based on the material properties, computer simulations are used to predict the cover performance. Computer models are also used to compare different designs and to understand mechanisms important to performance. Informed by prediction, the design is refined in an iterative process to meet the performance requirements as well as other constraints (economics, physical stability, postclosure land use, etc.). Even this very general description suggests that a variety of cover configurations might result from the flexibility afforded by application of predictive methods to meet the needs of sites with variable performance criteria, site characteristics, and climate. There is no single alternative design that can meet the performance requirements and economic constraints of all sites. The intent of this guide is to provide sufficient description of a design process such that site owners, engineers, and regulators can make informed decisions for site-specific applications. An important consideration during the design process is that a WB cover may not be possible or appropriate for a given site. The physical and economic viability will depend on the site-specific performance requirements, soil resource availability, plant ecology, climate, and land use plans. Performance goals cannot always be met using a WB cover. Costs are not always lower than for conventional covers, and the approval process is more complicated. In brief, WB covers may not be appropriate for all sites. 16

29 Albright / Benson / Waugh - Chapter 2 Issues for the Site Owner Cost: One of the most attractive features of WB covers is the possibility of reduced construction costs. WB designs do not include geomembranes or low-conductivity layers (either compacted clay or GCLs), and thus the cost associated with purchase, transport, and placement of those materials is avoided. Borrow sources for WB covers must be identified, and appropriate soils are often (but not always) locally available. The expensive methods required for placement of compacted clay layers (thin lifts, moisture-conditioned soil, equipment capable of remolding the soil) are also avoided. Soil placement for WB covers often specifies relatively thick (up to 600 mm) lifts, lower densities, and much less control on soil water content. Design costs for WB covers typically are higher than for a conventional design primarily due to the increased variety of design options, the increase in laboratory testing costs, and the costs associated with modeling for performance prediction. A WB design may require a multidisciplinary design team to address the complex soil-plant-climate issues affecting the water balance, whereas a conventional cover may often be designed by a single engineer. Long-term maintenance costs may also be lower for WB covers. Performance of conventional covers is contingent on an intact barrier layer, and repairing damaged barrier layers can be expensive and require specialty contractors. Damage to WB covers is often limited to erosion and differential settlement, both of which can be repaired by on-site personnel with addition of soil and replanting. Vegetated surface layers on conventional covers tend to be thin and may not provide sufficient rooting depth for a healthy plant community, which can result in difficulty establishing vegetation and maintaining erosion at an acceptable level. In contrast, WB covers are designed to support a vigorous plant community with roots throughout the depth of the cover. As a result, maintaining healthy vegetation and controlling erosion typically are less problematic for WB covers. Permitting: Permitting activities for WB designs often require more regulatory interaction to clearly define objectives and expectations. Acceptable limits for percolation typically are expressed in terms of annual flux (i.e., mm/yr) or an equivalent flux (requires a defined method to demonstrate). The required performance criteria typically depend on the type of waste (e.g., RCRA C or D). Regulatory acceptance and knowledge of WB covers is highly variable and site owners should understand that a shift from a well-defined prescriptive design 17

30 Albright / Benson / Waugh - Chapter 2 process to a site-specific flexible design process may require more time, effort, and interaction with regulators. Issues for the Design Team Feasibility study: Because performance of a WB cover must be demonstrated (as opposed to conventional covers with assumed performance), an initial site assessment is necessary to determine whether a WB cover likely will perform as required. This assessment includes evaluation of local climatic conditions, review of field demonstration data for other sites in the region, characterization of ecological reference areas and natural analogs, and review of leachate generation rates for other sites in the area. A preliminary financial analysis should also be conducted to compare costs associated with conventional and WB covers. WB covers are not always the most appropriate or economical solution. Site characterization: Performance of a WB cover depends on the ability of the engineered soil and plant system to respond to and manage climatic conditions at the site. Suitable soils must be available and the laboratory analyses for WB cover design differ (and are often more expensive) from those common to other earthwork or capping projects. Plants are critical to the water balance as well as erosion protection, and important information (for example, rooting depth, transpiration capability, ecological succession) can be difficult to acquire. The design team must understand how properties of the soils and plants affect the performance of the cover, as well as potential failure modes that may occur. Because of the broad range of issues that can affect the performance of WB covers, an interdisciplinary design team will need to be assembled. Modeling: Different computer models are needed to predict the performance of WB covers than for conventional covers. Unique expertise and input data are required for these models. Models of WB covers are used to predict performance, to understand mechanisms affecting performance of the cover, and to test design alternatives. The increased complexity of the modeling strategy is important to the permitting process and should be carefully considered. Particular importance must be paid to the appropriateness of the input parameters. 18

31 Albright / Benson / Waugh - Chapter 2 Issues for the Regulator Change in Design Philosophy: A shift from materials-and-methods to performancebased design poses substantial challenges for regulators charged with assuring protection of human health and the environment. For conventional covers, regulatory oversight is primarily concerned with the details of a prescribed design; the permitting process for a WB cover is much more involved with the design process. Site Characterization: Regulatory evaluation of WB covers requires an understanding of a much greater range of soil and plant properties as well as interaction of the engineered system with site-specific climatic conditions. Regulators must become familiar with the soil and plant parameters important to transpiration, the data needed to describe the soil and plant characteristics, and the laboratory and field test methods that provide reliable design data. Regulators must also know when to allow use of data from the literature or other projects and when to require site-specific testing. Performance expectation and evaluation: Since performance cannot be assumed from material properties (as with conventional designs), regulators must address the question of acceptable performance, whether or not this performance can be expected from a WB cover in a given state or region, and which methods are acceptable for performance predictions. Regulations often require demonstration of performance equivalent to that of the conventional designs. However, equivalent performance can be difficult to define and little guidance is available regarding appropriate methods to demonstrate equivalency. Performance predictions are often made with computer models (there are multiple choices, each with specific requirements for input data and parameters), and regulators should be aware of the limitations of modeling methods. Monitoring and long-term issues: Monitoring is often required to verify that acceptable performance of a WB cover is achieved. While common for WB covers, this requirement is unusual because performance monitoring of conventional final covers is rarely required. Different methods have been used to monitor covers and regulators should be aware of the limitations of these methods. The most important factor to consider is whether the performance metrics can be accurately and precisely measured using the monitoring method. Beyond the 19

32 Albright / Benson / Waugh - Chapter 2 issues surrounding hydrologic performance, regulators must also be aware of the end land use of a site and the sustainability of the cover. 20

33 Albright / Benson / Waugh - Chapter 3 Chapter 3 The Soil Profile: Concepts of Flow and Storage Some or all of the layers in a landfill cover are comprised of soil and each soil layer has hydrologic properties so that the layer meets specific functional requirements. Conventional designs, for example, resist the downward movement of water with a low-conductivity layer and typically include an overlying highly conductive drainage layer to allow lateral diversion of the impeded water. The saturated hydraulic conductivity, or the ability of the soil to transmit water when the entire pore space is filled with water, is used to characterize the rate at which water flows through these layers. In contrast to conventional covers, unsaturated hydraulic behavior is of paramount importance to WB covers. The ability to store water within the soil profile and the rate at which water is removed by evaporation and transpiration or drainage depends on the soil water characteristic curve (SWCC) and the unsaturated hydraulic conductivity. The SWCC defines soil water storage and forms the basis for design of WB covers. The concept of unsaturated hydraulic conductivity is also important to a conceptual understanding of WB cover function and is a key input to computer models used to predict performance. The SWCC and unsaturated hydraulic conductivity are non-linear and vary greatly between soil types. Saturated hydraulic conductivity is also an important factor in the hydraulic characterization of a soil for a WB cover, and is a required input parameter for computer models. A brief description of these important soil properties follows. Additional description and discussion of unsaturated soil properties can be found in Hillel (1998). Mitchell and Soga (2005) provide additional discussion of saturated soil properties. Saturated Soil Properties The ability of soil to transmit water increases with increasing soil water content. Thus, the saturated hydraulic properties describe the maximum rate at which water will flow through 21

34 Albright / Benson / Waugh - Chapter 3 soil for a given energy state. Flow of water in saturated soil is described by Darcy s Law, which describes the relationship between flow rate (Q), saturated hydraulic conductivity (K s ), the hydraulic gradient (i, the difference in hydraulic potential, ΔH, acting over the length of the flow path, L), and the cross sectional area (A) of the soil through which flow is occurring (Figure 3.1): ΔH Q = K s A = K sia (3.1) L Civil engineers historically have referred to saturated hydraulic conductivity (K s ) as the permeability. However, permeability is used in a different context in related disciplines. Thus, only saturated hydraulic conductivity will be used in this document. ΔH Soil Volumetric flow (Q) ΔLL Cross sectional area (A) The volumetric flow of water (Q) is determined by the saturated hydraulic conductivity (K s ), the hydraulic gradient (ΔH/ΔL) and the cross sectional area (A) of the porous media: Q = K s A (ΔH/L) Ponded water Soil layer Layer ΔL L ΔH Removing the area term (A) changes the units for outflow from volumetric (length 3 /time) to volumetric flux (length/time), or q = Ks (ΔH/L) Figure 3.1. These schematics demonstrate the concept of saturated hydraulic conductivity. A common representation of measured flow through porous media is shown in (a). For measurements that are area-independent (i.e., precipitation, percolation from a landfill cover) removing the area term gives the result expressed as flux. 22

35 Albright / Benson / Waugh - Chapter 3 Saturated hydraulic conductivity (K s ) is most often measured in the laboratory on compacted specimens prepared from bulk samples collected from the borrow source. Rigid-wall permeameters (ASTM D 5856 for fine-grained soil, ASTM 2434 for coarse-grained soil) or flexible-wall permeameters (ASTM D 5084) are commonly used to measure the saturated hydraulic conductivity. Photographs of rigid-wall and flexible-wall permeameters are shown in Figure 3.2. For WB covers, either type of permeameter is satisfactory. Requested specifications should include the confining stress ( kpa to represent field conditions), a reasonable hydraulic gradient (< 15) and the compaction condition at which the test specimen is to be prepared. This condition should mimic the anticipated field condition to the extent practical. In most cases, cover soils used for WB covers are compacted at a dry density corresponding to 80-90% relative compaction for standard Proctor effort (ASTM D 698) and at optimum -4% to 0% dry of optimum water content. A typical laboratory test condition is -2% dry of optimum water content and 85% compaction based on standard Proctor. The soil should not be over-processed prior to compaction in the laboratory. Crushing the soil to pass a 19 mm (3/4 inch) sieve is sufficient. Low dry density and drier water content are used to ensure favorable qualities for root growth. A structured soil also stores and release more water than a densely compacted soil, and is less prone to large changes in properties due to pedogenic effects (e.g., cracking due to wet-dry or freeze-thaw cycling). Field tests to determine saturated hydraulic conductivity are conducted less frequently than laboratory tests. Common test methods to measure field saturated hydraulic conductivity include the two-stage borehole (TSB) test (ASTM D 6391), the sealed double-ring infiltrometer (SDRI) test (ASTM D 5093), and the open double-ring infiltrometer (ODRI) test (ASTM D 3385). Detailed procedures for these methods are described in the cited ASTM standards. Cover soils used in WB covers typically have more structure and higher saturated hydraulic conductivity compared to conventional barrier layers. As a result, the TSB and SDRI test methods may need to be modified when evaluating WB cover soils to ensure a sufficient volume of soil is permeated and the head loss is acceptable. To address these issues, TSB tests are normally conducted with a 305-mm casing and a 50-mm or 100-mm standpipe, ODRI tests are conducted with an inner ring at least 305-mm in diameter, and SDRI tests are conducted with 23

36 Albright / Benson / Waugh - Chapter 3 25-mm tubing and a Marriotte bottle in place of the plastic bag. Photographs of modified TSB and SDRI testing equipment are shown in Figure 3.3. (a) Assembled flex-wall permeameter (b) Figure 3.2. Flexible-wall (a) and rigid-wall (b) permeameters. The flexible-wall system encases the soil sample in a flexible membrane within a chamber used to apply confining pressure. In the rigid-wall apparatus soil is compacted directly in the permeameter. The rigid-wall permeameter shown in (b) is for soils with large particles. 24

37 Albright / Benson / Waugh - Chapter mm Marriotte bottle (a) 300 mm TSB casing grouted in borehole (b) Marriotte bottle Inner ring outer ring SDRI TSB Figure 3.3. Photographs of (a) installed TSB permeameter with 300-mm casing and 100-mm standpipe and (b) water-filled SDRI using Marriotte bottle for measuring infiltration volume (note TSB permeameter to right of SDRI). 25

38 Albright / Benson / Waugh - Chapter 3 Unsaturated Soil Hydraulic Behavior Soil Water Suction Soils in WB covers are nearly always unsaturated. When soils are unsaturated, water in the soil pores is in tension and the pore water pressure is negative. For convenience, this negative pressure is referred to as the matric suction (ψ) in the soil and is reported as a positive quantity (e.g., a suction of 33 kpa corresponds to a water pressure of -33 kpa). Matric suction is often referred to simply as suction. The suction nomenclature will be used henceforth in this document. The concept of suction is illustrated in Figure 3.4, which shows the water pressure in a capillary tube below and above a free water surface. Below the free water surface the pressure is increasingly positive. Above the free water surface, capillary forces hold the column of water within the capillary tube under tension (negative pressure, or suction). The suction (ψ) is a function of the radius of the capillary tube (r), the surface tension of the liquid (σ), and the contact angle (β) between liquid and solid, as described by the Young-Laplace equation: 2σ cos β ψ = (3.2) r A similar effect occurs in soil under unsaturated conditions. Water is retained in the soil due to capillary forces that create suction. Adsorptive forces between the water molecules and the solid surface also contribute to retaining water in unsaturated soil. Because both of these forces contribute to water retention, the pore water pressure in unsaturated soils can be very negative and greatly exceed the tensile strength of water ( 100 kpa, also known as the cavitation pressure). The volumetric water content in unsaturated soil (θ), defined as the volume of water per total volume of soil, decreases when the force applied to the water is large enough to overcome the capillary and adsorptive forces that retain water within the soil pores. That is, the water content decreases as the suction applied on the pore water increases. This behavior can be visualized as applying vacuum to the pore water via a straw. As the vacuum is increased (i.e., greater suction is applied to the straw), more water is drawn out of the soil. As a result, water content of the soil decreases as the suction increases. When the water content diminishes, the 26

39 Albright / Benson / Waugh - Chapter 3 remaining water retreats into smaller pores. Conversely, when the suction is reduced, the water content increases and water fills larger pores. Capillary tube Pressure gauge kpa kpa kpa Figure 3.4. The pressure in a water column depends on vertical location relative to a free water surface. Water pressure at the free water surface is zero (or atmospheric); below that surface (submerged) the pressure is positive. The water pressure in a hydrophilic capillary tube above the free water surface is negative. The negative pressure is directly proportional with height above the free water surface. Soil Water Storage Concepts Soil layers in WB covers act as a sponge that absorbs and releases water. Thus, a quantitative understanding of the principles of water retention is central to understanding how WB covers function and to predict performance. The following is a brief introduction to the concepts of soil water retention and the relationship between soil water content and soil water suction. 27

40 Albright / Benson / Waugh - Chapter 3 The effect of soil texture on water retention is illustrated in Figures 3.5 through 3.7. Two soils are submerged to saturation (Figure 3.5), one a clean uniform sand (relatively large particles of similar particle size) and the other a silty sand (a finer textured and more broadly graded soil). The fraction of total soil volume occupied by pore space varies between soil textures, with finer textured soils generally having a higher fraction of the total soil volume as pores. However, for this example each soil is assigned a pore volume of 40% of the total soil volume with the remaining 60% represented by soil grains. Thus, both soils have a porosity of Sand Silt Saturated void space (~40% volumetric) Solid particles (~60% volumetric) Figure 3.5. Schematic illustrating the void space concept. The void space in soils typically used for landfill cover applications represents 30-45% of the total soil volume. For simplicity, two soils (a silt and a sand) are shown, each with a void space (porosity) of 40%. Submerged, the void space is completely occupied by water. 28

41 Albright / Benson / Waugh - Chapter 3 Sand Silt Water content at field capacity 0.4 Volumetric Water Content Figure 3.6. Schematic illustrating the field capacity concept. The two soils are raised out of the water and allowed to drain freely. Following free drainage, the water content of both soils is described as field capacity. The red arrows show the quantity of water lost to free drainage. At field capacity the water content of the sand is much less than the silt. 0.4 Unit available storage 0.4 Water content at wilting point Figure 3.7. Schematic illustrating the wilting point concept. Plants transpire and, in the process, remove water from the soil where roots are present. Wilting occurs when the plant can no longer extract water from the soil. The water content at this point is referred to as the wilting point. Note that both the wilting point and the field capacity of the sand are at lower water contents than the wilting point of the silt

42 Albright / Benson / Waugh - Chapter 3 The two soils are raised out of the water and allowed to drain freely (Figure 3.6). Water will drain until the suction that develops in the pore water is large enough to resist the gravity forces causing drainage. Much more water drains from the sand than from the silt, which is intuitive. The sand has larger pores, and thus smaller suctions can develop to retain water within the pore structure (Eq. 3.2). The amount of water remaining in the sand is about 10% of the total soil volume, whereas about 33% for the silty sand (these numbers are arbitrary, but do approximate actual soils and are meant to demonstrate concepts). These soils, which have drained freely and have reached equilibrium, are often described as being at field capacity water content (θ c ). A variety of definitions of field capacity exist, including (1) the amount of water soil can hold against the force of gravity, (2) the amount of water left in the soil after draining from saturation by gravity for 24 or 48 hr, (3) the state of saturated soil when all the soil moisture that is able to freely drain away has done so, or (4) the water content corresponding to a suction of 33 kpa. This last definition, which is quantitative, is common in practice and is used henceforth. When plants are added and roots exist throughout the soil (Figure 3.7) additional water can be removed by transpiration. Plants remove water until they wilt (i.e., the cessation of transpiration); this water content is referred to as the wilting point (indicated by the green arrows in Figure 3.7), and is less than field capacity. Wilting occurs when the plant can no longer maintain plant cell turgidity against the evaporative demand placed by the atmosphere on one end of the plant (the leaf surfaces) and the tension under which the soil water is held at the other end of the plant (the roots). Intuition may prove less useful in understanding the wilting point because, at this state, all soils may appear to be simply dry with little discernable difference in water content between soil textures. However, at the wilting point, the water content of the coarse-textured sand is lower than that of the finer-textured silt. Also, in this example, the water content of the silt at the wilting point is greater than that of the sand at field capacity. By convention, the wilting point is often assigned as the water content at a suction of 1500 kpa. However, the soil water content at the wilting point varies with plant species and with climate; desert plants often can transpire water to a much lower point than plants from more humid environments. The 1500-kPa definition is reasonably representative for plants in more humid environments, but for semi-arid and arid environments, the wilting point can be ,000 kpa. 30

43 Albright / Benson / Waugh - Chapter 3 As mentioned in Chapter 1, WB covers act as storage tanks that are filled when the rate of water addition by precipitation exceeds that of water removal by ET and emptied when ET exceeds precipitation. A full storage tank corresponds to field capacity (θ c ) and an empty tank corresponds to the wilting point (θ m ). The cover will not drain as long as the soil water content does not exceed field capacity and the water content in the cover will not drop below the wilting point. The difference between these two quantities (θ c θ m ) represents the volume of pore space that is available to store water per total volume of soil. This difference is referred to as the unit available storage (θ u = θ c θ m ), and is used to determine the required cover thickness to store a known amount of water as described in Chapter 5 (Benson and Chen 2003). Soil Water Characteristic Curve Field capacity and wilting point are benchmark water contents that are part of the continuous relationship between water content and suction called the soil water characteristic curve (SWCC). The SWCC is also referred to as the soil water retention curve, the moisture release curve, or the capillary pressure curve, depending on the discipline in which it is used. The SWCC describes water content for any soil as a function of suction, as shown in Figure 3.8. The suction at which the largest pores de-saturate is the air entry suction (ψ a ). At suctions lower than ψ a, all pores are filled with water and the soil is fully saturated. At zero suction the condition is referred to as the saturated water content (θ s ) (Figure 3.9). The driest condition, referred to as residual water content (θ r ), corresponds to the water content below which water can no longer be removed under practical conditions. Field capacity (33 kpa) and the wilting point (1500 kpa) fall between these quantities. SWCCs for the silt and sand are shown in Figure 3.9. Higher suction is required to desaturate the largest pores in the silt than for the sand (i.e., ψ a is larger for the silt than the sand) and silt retains more water than the sand at all suctions (except at the very highest suctions). The central portion of the SWCC is different for the two soils. The relatively flat slope for the sand indicates that most of the pores desaturate over a very small change in soil suction and are thus of similar size. The steeper slope for the silt means that a much larger change in soil suction is required to desaturate the majority of pores, and that the silt contains a much broader distribution of pore sizes. 31

44 Albright / Benson / Waugh - Chapter 3 High Soil suction Moderate Smallest pores and films retain water at high suctions Low Soil water suction Smaller pores empty as suction increases First (largest) pores empty Soil water content Saturated Figure 3.8. This schematic shows the relationship between pore diameter, tension (or suction) at which water is held in pores of various size, and water content. At very low suctions even the largest capillaries are filled. As suction increases, increasingly smaller pores are emptied. Only those pores capable of maintaining sufficient suction to retain water remain waterfilled. 32

45 Albright / Benson / Waugh - Chapter 3 Slope of retention curve (n) Air Entry Suction (ψ a 1/α) Suction Silt (a) Residual water content (θ r ) Saturated water content (θ s ) (b) Suction Clean Sand (n) ψ a θ r Figure 3.9. Schematics showing the four primary features of the soil water characteristic curve (SWCC): (1) saturated water content, (2) the air entry suction (the suction at which the largest pores are emptied of water), (3) the slope of the curve (n) that describes the distribution of pore sizes, and (4) the residual water content (θ r ). The sand (b) has lower ψ a because it has larger pore sizes. The steeper middle section of the curve (smaller n) for the silt (a) corresponds to a larger distribution of pore sizes. θ s Several methods can be used to measure the SWCC. ASTM Method D 6836 describes the hanging column method (for coarse-grained soils that drain readily for suctions between 1 and 80 kpa), the pressure plate method (for finer soils and suctions between 0 and 1500 kpa), and the chilled mirror hygrometer method (for suctions between 500 kpa to 100 MPa or when suctions near saturated are not required. An example of a SWCC defined using all three techniques is shown in Figure A detailed description of the SWCC over a wide range of 33

46 Albright / Benson / Waugh - Chapter 3 suctions usually requires two methods to characterize the wet and dry ends of the curve. In practice, the hanging column method and the chilled mirror hygrometer method are used for cleaner, coarse-grained soils (<15% fines), and the pressure plate and the chilled mirror hygrometer method are used for fine-textured soils Suction (kpa) Method A - Hanging Column Method B - Pressure Plate Method D - Chilled Mirror Hygrometer Volumetric Water Content Figure An example of data from laboratory analysis of a soil by hanging column (A), pressure plate (B), and chilled mirror hygrometer (C). Note the overlap in data from the hanging column and pressure plate and the alignment between the pressure plate and chilled mirror hygrometer. The van Genuchten equation (1980) was fit to the data. For the chilled mirror hygrometer method, measurements are made on multiple samples previously prepared at different water contents to give several points on the curve. The hanging column and pressure plate methods apply either a reduced pore water pressure (hanging column) or an increased pore gas pressure (pressure plate) to a saturated sample. Water flows from the soil until equilibrium is established, and the water content at equilibrium corresponds to the applied suction at each step. The time to reach equilibrium varies with soil type and water content. Measurement of the SWCC can require several weeks to complete. Allowing adequate time for the soil to reach equilibrium at each step is extremely important. If equilibrium is not established, the water contents will be over-estimated, and the storage capacity of the cover can be underestimated considerably. 34

47 Albright / Benson / Waugh - Chapter 3 The SWCC is usually described parametrically with a function fit to the test data, as shown in Figure In this example, the following equation developed by van Genuchten (1980) was fit to the laboratory data using a least-squares method: ( ) θ=θ r + θ s θ r 1 1+ αψ ( ) n where α, n, and m are curve-fitting parameters. The α parameter is inversely related to the air entry suction as shown in Figure 3.9, and has units of pressure -1 (e.g., kpa -1 ). The parameters, n and m, are dimensionless, and are usually related by m = 1 n -1. These parameters control the slope of the SWCC as shown in Figure Other equations have been used to describe the SWCC (e.g., the Brooks-Corey equation, the Fredlund-Xing equation, etc). However, the van Genuchten equation is widely used and the van Genuchten parameters are required as input for many predictive models. Typical values for α and n are given in Table 3.1. A spreadsheet used to fit Eq. 3.3 to laboratory data is attached electronically to this document. m (3.3) (a) (b) Decreasing α Matric Suction Matric Suction Decreasing n ψ θ r Vol. Water Content Vol. Water Content θ s Figure Effect of the van Genuchten parameters (α and n) on the shape of the soil water characteristic curve. The α parameter (related to the inverse of the air entry suction, ψ a ) affects the breakpoint in the curve (a). The n parameter affects the slope of the soil curve for suctions greater than ψ a (b). These drawings are exaggerated to illustrate the effects of variations in the parameters. 35

48 Albright / Benson / Waugh - Chapter 3 Table 3.1. Typical values of α and n. Soil α (kpa -1 ) n Clean sand Sand with fines Silty soils Loose clays Compacted clays Unsaturated Hydraulic Conductivity As a soil de-saturates, pore spaces that were filled with water under saturated conditions become partly or completely filled with air and at very low water content, soil water may exist almost exclusively as films on individual grains that are connected only at grain-to-grain contacts (Figure 3.12). Consequently, the hydraulic conductivity decreases because the conduits that transmit water through the soil become less numerous, smaller, and more tortuous. The hydraulic conductivity can vary many orders of magnitude between saturated and dry conditions. Saturated: largest & least tortuous pipes Unsaturated: smaller & more tortuous pipes Solid Water Solid Air Water Figure A simplified representation of the relationship between soil water content and the waterfilled pore space that controls hydraulic conductivity. As the soil water content is reduced, the conductive pathways are fewer, smaller, and more tortuous, which reduces the hydraulic conductivity. 36

49 Albright / Benson / Waugh - Chapter 3 The relationship between hydraulic conductivity and soil water suction is shown in Figure 3.13 for the clean sand and the silt. The features of the two soils are exaggerated to demonstrate important points. The rate of decline in hydraulic conductivity with increased suction depends on the distribution of pore sizes. The uniform pores of the sand drain with a small increase in suction, which results in a large decrease in hydraulic conductivity. The relatively larger distribution of pore sizes in the silt requires a greater change in suction to cause a similar change in conductivity. The effect of varying α and n on the unsaturated hydraulic conductivity is shown in Figure Decreasing α extends the break point at which the hydraulic conductivity starts to decline; decreasing n (increasing the slope of the SWCC) decreases the rate at which conductivity declines with increasing suction. Clean sand Hydraulic Conductivity Silt Suction Figure The relationship between hydraulic conductivity and soil suction. The hydraulic conductivity remains near the saturated hydraulic conductivity until the air entry suction is reached. Drainage of water from the relatively uniform pore sizes in the sand results in a rapid decrease in conductivity with increased suction above the air entry suction. The hydraulic conductivity of the silt exceeds that of the sand at higher suctions because the silt retains more water due to higher air entry suction and a broader pore size distribution. 37

50 Albright / Benson / Waugh - Chapter 3 Hydraulic Conductivity Decreasing α Suction Hydraulic Conductivity Decreasing n Suction Figure The effect of the α and n parameters on hydraulic conductivity. The α parameter affects the breakpoint in the curve (a), commonly referred to as the air entry suction (ψ α ). The break point occurs at higher suctions as α increases. The n parameter affects the slope of the hydraulic conductivity function for suctions greater than ψ α (b). The slope becomes shallower as n decreases. Unsaturated hydraulic conductivity is difficult and expensive to measure, and in many cases the unsaturated conductivity can be estimated as accurately as it can be measured (Benson and Gribb 1997). The parameters that describe the retention characteristics of a soil (θ s, θ r, α, n) can be used to estimate hydraulic conductivity as a function of soil water suction according to the following equation (van Genuchten 1980): 38

51 Albright / Benson / Waugh - Chapter 3 K ( ψ) = 2 n 1 n m { 1 ( αψ) [ 1+ ( αψ) ] } n l 1+ ( αψ) / [ ] 2 (3.4) where l is the pore interaction term. The parameter l is often assumed to be 0.5. This assumption is valid for coarse-grained soils; for fine-textured soils, Eq. 3.4 is more accurate when l is set between -2 and -3. Additional discussion of the effect of l on predicting the water balance of WB covers is in Chapter 5. Flow and Hydraulic Gradients Water moves in soil in response to gradients in total soil water potential. Total potential is the sum of two factors - relative elevation head and soil water pressure head. In simple terms, water will flow under gravity from points of higher elevation to points lower in a soil profile, and under the influence of pressure from higher pressure to lower pressure (or from lower suction to higher suction). The actual direction of movement depends on the sum of gravity and pressure components. For example, suction draws water upward to a soil surface dried by evaporation because the upward gradient in potential due to the difference in suction is greater than the downward gradient due to the difference in elevation. Two conditions important to a conceptual understanding of cover hydrology are unit gradient and equilibrium gradient. Unit gradient conditions exist when there is no variation in suction across a vertical segment of the soil profile. When there is no gradient due to differences in suction, the entire gradient is due to differences in elevation and the gradient in total soil water potential is unity (hence the unit gradient terminology). The unity concept is easier to understand when the units of potential (properly expressed as kpa in the SI system) are expressed in units of length (i.e., a difference of 1 m in elevation involves a difference of 1 m of elevation potential). When the gradient term in the flow equation is unity, the flow of water equals the hydraulic conductivity. An interesting application of the unit gradient condition is for conventional clay covers that specify the saturated hydraulic conductivity for the low-conductivity layer. If saturated conditions exist and no water is ponded on the surface of the barrier layer, then the gradient is unity and the flux out of the base of the barrier layer is equal to the saturated hydraulic conductivity of the barrier layer. This forms the basis for a common worst case performance 39

52 Albright / Benson / Waugh - Chapter 3 standard for clay covers (e.g., K s cm/sec or 31 mm/yr), even though continuously saturated conditions are unlikely in any type of cover system. Another interesting outcome of the unit gradient conditions is that the flux is independent of the thickness of the barrier layer; that is, the flux from a very thin barrier layer is the same as from a very thick barrier layer provided that the saturated hydraulic conductivity of both barriers is the same. Equilibrium gradient conditions exist when there is no movement of water across a vertical segment of the soil profile. When there is no movement of water, the gradient in total soil water potential is zero. The elevation component of total potential is always present (and equal to unity). Thus, equilibrium conditions require that, at each point in the profile, differences in potential due to suction exactly balance those due to elevation. A soil profile at field capacity meets this condition; at each point above the bottom of the cover the increase in potential energy of the water due to elevation is balanced by an equal decrease in potential energy due to increased suction. Earlier, this section described the concept of field capacity (and the associated soil water content) as a property of a soil at a point. This point-based definition is not quite correct. For a soil profile to be at field capacity (i.e., with no drainage), the suction must decrease with increasing depth (and the water content increase) to achieve equilibrium conditions. 40

53 Albright / Benson / Waugh - Chapter 4 Chapter 4 Introduction to Ecology and Revegetation of Water Balance Covers This chapter is an introduction to basic concepts, principles, and practices pertaining to the ecology, revegetation, and sustainability of water balance covers. Chapter 3 covered topics associated with storage of water in the soil profile, the focus of this chapter is on release of water back to the atmosphere. The goal is to convey that a successful WB cover program requires a sound ecological foundation. Understanding and implementing concepts introduced in this chapter should greatly reduce the time and resources required to construct and maintain a cover and to address the difficulties associated with inadequate performance due to insufficient water release (e.g., see Chapter 7: Lessons Learned from the Field). Sustainable water release (evapotranspiration) relies, in part, on the interaction of a community of organisms with their physical environment; by definition, an ecosystem. The ecological community of a WB cover consists of all organisms (plants, animals, and microorganisms) that inhabit the cover, and, if contiguous, organisms in surrounding areas that influence the cover community. The interaction of organisms as influenced by their physical environment will also determine how the ecological community, and hence the performance of the cover, will function and change over time. If the goal is to design and construct WB covers with sustainable performance and minimal maintenance, then an ecologically sound approach must be understood and implemented. In short, the goal is to imitate nature. The ecology of the landfill environment must be understood first before ecological processes can be favorably imitated or manipulated to achieve this goal. Several good textbooks are available for readers who wish to understand the ecological foundation for cover designs (e.g., Crawley 1997, Barbour et al. 1998, Gurevitch et al., 2006). Above all, to be successful, cover design teams should involve local expertise in the ecology and revegetation of disturbed land. This chapter introduces the following topics: (1) basics of plant transpiration, (2) revegetation goals and strategies, (3) baseline ecological surveys, (4) revegetation practices (5) natural analogs, and (6) biointrusion control. 41

54 Albright / Benson / Waugh - Chapter 4 Basics of Plant Transpiration Transpiration is the evaporation of water from plants and is an important mechanism contributing to the release of water from WB covers. Transpiration occurs as water vapor moves from stomatal cavities on the leaf surface to the atmosphere. This section presents some basic ecophysiology principles related to plant water status and transpiration. Additional information can be found in textbooks such as Pearcy et al. (1989), Bedunah and Sosebee (1995), Lambers et al. (1998), and as Reigosa-Rogers (2001). Although other mechanisms can affect water movement in plants, transpiration is the dominant driving force. Transpiration occurs as interplay of energy and water between soil, roots, leaf surfaces, and the atmosphere. Water moves from soil to roots to leaves to the atmosphere in response to gradients in water potential (Figure 4.1). The term water potential refers generally to the forces or potential energy acting on water in soil and plants (see Chapter 3). Water potential in soil and plants is primarily dependent on three physical and chemical energy forces: ψ = ψ + ψ + ψ (4.1) tot O Z where ψ tot is the total water potential, ψ is the matric potential, ψ o is the osmotic potential, and ψ z is the gravitational potential. These components are similar to those contributing to total potential (or head) in soil water. Matric potential (ψ) is a measure of the attraction of water molecules to hydrophilic surfaces. The matrix can be soil particle surfaces, cell walls, macromolecules, or other nonsoluble surfaces. Osmotic potential (ψ o ) refers to the movement of water attributable to solutes. In plants and other biological systems, an osmotic potential difference is created by permeable membranes, such as in cell structures that restrict movement of solutes. Salt tolerant plants, called halophytes, move water by producing gradients of osmotic potential from lower-solute soils or tissues to higher-solute tissues. The gravitational potential, ψ z, is the work needed to raise water against the Earth s gravity from a reference location to it present position. 42

55 Albright / Benson / Waugh - Chapter 4 Air (50000 kpa) Leaves (1500 kpa) ψ air >ψ leaves >ψ root >ψ soil Stem Roots (330 kpa) Soil ( kpa) Figure 4.1. Schematic showing example of water potential gradient from soil, through roots, stem, leaf, and to air. In general, water enters plants through the hairs of young root tips or through cracks in the root cortex of older roots. Water entering a plant is forced to move through fairly resistant cell membranes. Resistance to movement drops significantly when water enters the xylem. Water continues to move in the xylem, through vascular cell walls, through spaces between mesophyll cells, and finally into substomatal cavities in leaves where it transpires vaporizes and passes through the stomatal pore and into the atmosphere. Although resistance to flow occurs throughout the entire soil-plant-atmosphere continuum, stomates are the primary regulator of water movement (Figure 4.2). Factors that control the opening and closing of stomate pores largely control the rate of transpiration from a plant. Stomates on the leaf surface open to allow carbon dioxide to diffuse into the substomatal cavity in response to light and CO 2 concentrations. 43

56 Albright / Benson / Waugh - Chapter 4 Figure 4.2. Schematic of the leaf surface and stomates. Open stomates allow diffusion of CO 2 into the substomatal cavity and water out of the stomates to the atmosphere. The combination of plant transpiration and evaporation from the soil surface is bounded by the rate of potential evapotranspiration (PET). PET is the theoretical maximum rate at which evapotranspiration (ET) occurs for a given meteorological condition and represents the energy available for evaporation. Actual ET is less than or equal to PET. The Penman-Monteith equation commonly is used to compute PET: ( PET = δ / ξ )J n ( e s e) ( U / 100) δ / ξ+1 (4.2) where e is the atmospheric vapor pressure at 2 m above ground surface, e s is the saturated vapor pressure corresponding to the air temperature 2 m above ground surface (T a ), U is the wind velocity at 2 m above ground surface, ξ is the psychrometric constant, δ is the rate of change in e s with temperature, and J n is the net solar radiation. Eq. 4.2 indicates that PET increases as the solar radiation, air temperature, and wind speed increase and the relative humidity decreases. Thus, locations in more southern latitudes with less cloud cover, higher air temperature, and greater wind velocity have greater potential to evaporate water (e.g., large portions of the southwestern US meet these criteria). 44

57 Albright / Benson / Waugh - Chapter 4 Revegetation Goals and Strategies Water balance cover designers must first define the goals and objectives for revegetation, and then conduct baseline ecological surveys (next section) to identify and characterize important ecological components and processes. Baseline ecological surveys are needed to develop site-specific revegetation plans (following section) and long-term performance evaluations (last section). Revegetation projects, in general, vary greatly in purpose and complexity. The simplest goal is to get something to grow; to establish some type of vegetative cover. For many reclamation projects, the main goal is to control erosion by stabilizing the soil surface. Some revegetation projects have a more intricate goal: restoration of the undisturbed ecosystem an attempt to return the ecology of the site to its pre-disturbance condition. This may be difficult in the least and practically impossible, because restoration means that a late-successional ecosystem with all its complex interactions will be re-established. Restoration implies regeneration of, in a short period of time, the intricate interplay among plants, animals, microbes, and soils that may have taken 10s to 100s of years to come about naturally. Revegetation of WB covers should fall somewhere between simply trying to get something to grow to help control erosion, and regeneration of the pre-disturbance ecosystem. Changes following initial establishment of vegetation, known as ecological succession, will occur. Creating conditions for favorable changes in the plant community should be an objective; rapidly establishing the desired end-state plant community in unachievable. Rather, the goal should be to deliberately create an environment (soils, microbes, microtopography) on the cover that sets a trajectory for succession towards a more complex, more functional (high transpiration), more sustainable, mid-seral plant community. Plant succession can be thought of as a sequence of steps, or seres, from the initial pioneer community to a climax community, a plant community in dynamic equilibrium with the local environment. Distinct community types within a sere are called seral stages. The target of revegetation is often to create ecological conditions similar to surrounding areas that have been little disturbed, because more complex, mid-seral communities are typically more stable and resilient to change than monocultures or pioneer communities. 45

58 Albright / Benson / Waugh - Chapter 4 As ecological succession progresses, transpiration typically will increase, but only to a point. If plant populations become senescent with little recruitment, net primary production and transpiration may actually decline. The response of transpiration to plant succession on WB covers and long-term vegetation management practices that enhance transpiration, are topics for future research. In general, cover revegetation goals may include: Create a soil environment similar to nearby undisturbed reference areas and establish plant communities that are well-adapted to that environment. Sustain high evapotranspiration rates. Unlike other revegetation efforts that may focus on erosion control or improving wildlife habitat, the primary goal of WB covers is hydraulic control. Stabilize the surface. Manage erosion\deposition from wind and water. Be resilient. Ideally the plant community should continue to remove water and control erosion even after disturbances such as extreme meteorological events, fire, invasive plant species, grazing, pests, and pathogens. Rapid and sustainable establishment. Ideally the vegetation should rapidly achieve an acceptable water balance for the cover and continue to function as designed in perpetuity. Be consistent with current and future land use. The plant community on the cover should complement current land management practices and future goals at the site and in neighboring plant communities. Baseline Ecological Survey Once revegetation goals are identified, specific objectives must be defined. The objectives should address both the biological and physical components of the cover ecosystem. The biological objectives are centered on the vegetation: (1) the types of plants that are desirable and acceptable and those that are unacceptable, (2) the abundance (e.g., cover, density, leaf area) of the types of plants that are acceptable, (3) how long is needed to achieve acceptable types and abundance of plants, and (4) how long the acceptable types and abundance of plants should persist. Objectives for the physical components of the cover ecosystem focus primarily 46

59 Albright / Benson / Waugh - Chapter 4 on cover soil characteristics that will be necessary to establish and sustain the target plant community. The biological and physical objectives for revegetation of a particular WB cover can be defined using the results of a baseline ecological survey. Field data are acquired from relatively undisturbed sites nearby, sometimes called reference sites or analog sites (see last section of this chapter), where the soils and plant communities are considered to be the target condition for revegetation. Baseline ecology information is needed (1) to define the target soils and plants for revegetation, (2) to develop criteria and metrics for evaluating revegetation success, and (3) to define possible future environmental scenarios for long-term cover performance evaluations (see last section of this chapter). Plant communities on relatively undisturbed sites are sometimes described as approaching the climax plant community. Achieving the target ecosystem (plant community and soils) on a cover, as defined by characterization of reference areas, may take many years. Establishment of the target for types of plants (plant species composition) might be achieved in a few years if propagules from planted seeds, tublings, and transplants survive and reproduce. However, these young plants may take several years (for shrublands and grasslands) or many decades or even centuries (for forests) to mature before revegetation targets for plant community structure (e.g., height, cover, leaf area, rooting depth) can be achieved. Recreating the target edaphology (soil properties related to plant growth) and morphology of reference area soils, and recreating the complex exchange of energy and nutrients among plants, animals, microbes, soils, topography, etc., may take 100s of years to achieve or may never match the reference area. Hence, such targets may be unrealistic as revegetation objectives for covers. In the end, the revegetation objectives and metrics may need to be defined in proportion to the target or reference area, or as key functional parameters or proxies such as leaf area and rooting depth. Nonetheless, baseline ecological characterization is necessary to define the targets. In summary, baseline ecological surveys are needed to support revegetation planning, cover performance monitoring, cover performance modeling, and long-term performance projections. The ideal reference areas for baseline surveys should be dominated by mid to lateseral vegetation either growing on the borrow soil for the cover, or on a soil unit similar to the borrow soil as defined by a soil survey. Table 4.1 lists types of information and specific field parameters that might be included in the baseline survey depending on the specific revegetation 47

60 Albright / Benson / Waugh - Chapter 4 Table 4.1. Climate, soil, and biological parameters for baseline ecological surveys. Climate and Meteorology Precipitation: Annual average and extreme precipitation, seasonal time series of average precipitation, snowfall (average and extreme), snow depth (average and extreme). Temperature: Annual average and extreme temperature, seasonal time series of average temperature, heating degree days, cooling degree days, growing degree days. Severe weather: Extended periods of drought, extended wet periods. Shifts and trends: Long-term trends or shifts in seasonality of precipitation and temperature. Other parameters: Solar radiation, humidity, wind speed and direction, micrometeorology Past climate: Proxy records of past climate change extend meteorological records or detect long-term shifts and trends in drought, wet periods, seasonality of precipitation and temperature: Tree ring records, pollen records, and packrat middens are examples of proxy records. Global change projections: models, future scenarios Soil Physical and Chemical Edaphic Properties Standard soil physical and chemical properties: Texture or particle size (% sand, silt, and clay), gravel and cobble content, dry-weight bulk density (compaction), porosity, ph, electrical conductivity (salinity), cation exchange capacity, sodicity (sodium adsorption ratio or exchangeable sodium percentage) Soil fertility: Macronutrients (N, P, K, Ca, Mg, S) and micronutients (Mn, Fe, Zn, Cu, Mo, Cl, B). Phytotoxic trace elements (elements toxic to plants), agricultural pollutants, and industrial pollutants. Soil Biology and Microbiology Organic matter content: amount, type (fresh or humus), influences on nutrient cycling Carbon/nitrogen ratio and influence on higher plants and microbiology Seed bank, rhizomes, and other plant propagules Soil fauna (e.g., arthropods, earthworms) Microbiology: Mycorrhizae fungi and host plants, cryptobiotic crusts (algae and cyanobacteria), soil nitrogen fixers Soil Morphology Soil taxonomic classification and horizonation Soil structure, consistence, and lithologic discontinuities Micromorphology such as vesicles, cutans, and planes Krotovina and other evidence of recent and ancient animal burrows and root channels Physiography and Geomorphology Proximity to base level Drainage networks: Rilling, channel density, fluvial processes Geomorphic history Plant Community Characteristics Species richness: List of established plants accounting for seasonal variation, species in seed bank, ecotypes Measures of plant abundance and diversity: Canopy cover, density (#/unit area), diversity indices Transpiration indices: Leaf area index (total and green leaf), root depth and distribution Life histories relative to transpiration: annual, perennial, evergreen, deciduous, phenology Population structure: Growth forms (grasses, shrubs, forbs), age structure Spatial distribution patterns: Clumped, contagious, rhizomatous, uniform Species resilience and tolerance: Ecological amplitudes, tolerances to fire, grazing, pathogens, invasive species Plant materials: Ease of establishment, commercial availability (seed, transplants, local ecotypes) Wildlife and livestock: Habitat value of plant communities for burrowing animals, grazing animals, game animals, rare (e.g., threatened and endangered) species 48

61 Albright / Benson / Waugh - Chapter 4 goals and objectives for the cover. Of the parameters in Table 4.1, those that are most important may vary between sites. Choosing what should be included should be guided by the revegetation objectives and project budget. On-line resources for climate, soils, and vegetation data provide a good start for acquiring baseline ecological information prior to conducting field surveys. The regional climate data centers operated by the National Oceanographic and Atmospheric Administration (NOAA) ( are excellent sources for historical meteorological data. Bioclimatic diagrams illustrate differences in ecological setting and use climate data to give a preliminary indication of seasonal water balance for specific locations. Figure 4.3 explains the components of bioclimatic diagrams. Figure 4.4 is an example that shows the contrasts in temperature and precipitation data for two sites at very different elevations in the western USA. Bioclimatic diagrams are available at: 1 Country name, station location and elevation, station name 2 The length of the observation period for temperature and precipitation respectively 3 Annual average of temperature and annual precipitation sum 4 (red) Temperature curve 5 (blue) Precipitation time series 6 Indication of frost periods 7 Mean daily max. temperature of the warmest month 8 Mean daily min. temperature of the coldest month Figure 4.3. Explanations of the components of bioclimatic diagrams. Soil surveys produced by the Natural Resources Conservation Service (NRCS) are an excellent source of preliminary information about the soils in many parts of the US and can be accessed on-line at websoilsurvey.nrcs.usda.gov. The NRCS soil surveys often include descriptions and data on mid- to late-seral plant communities for each soil mapping unit. 49

62 Albright / Benson / Waugh - Chapter 4 However, field surveys may be necessary to acquire much of the soil and plant data identified for the baseline ecological survey. Ecologists on the cover design team should prepare field survey plans that include sampling designs, field sampling methods and instrumentation, laboratory analytical methods, and statistical methods. Figure 4.4. Comparison of bioclimatic diagrams for Lake County, Colorado and Canyonlands National Park, Utah. Revegetation Practices Successful revegetation of a WB cover should start with definition of success criteria and follow a sequence of steps to establish a plant community that satisfies performance requirements. This section introduces the general sequence of steps. Site-specific revegetation plans may include all or some of these steps depending on the outcome of the baseline ecological survey and specific revegetation objectives and strategies developed for a given site. Revegetation Success Criteria After goals and objectives are developed and baseline ecological surveys are completed, criteria for evaluating the success of the revegetation effort (target values for plant species composition and abundance) should be defined. The targets are based largely on the results of the baseline ecological survey characteristics of the undisturbed plant community growing in 50

63 Albright / Benson / Waugh - Chapter 4 the borrow soil type. However, given that succession to a mature and diverse plant community can take years, the revegetation success criteria must have a time component. A late-seral community is an unreasonable near-term target for the cover. Success criteria are minimum values of vegetation evaluation variables. Separate success criteria should be defined for an initial revegetation period and for later periods. Target values for near-term and long-term success of the revegetation effort will be different. Near-term criteria might include species richness, total plant canopy cover, cover of weedy species, species frequency, and shrub density and survival. As the plant community matures, the criteria might focus more on relative canopy cover of desirable herbaceous species, species diversity indices, the canopy cover and size of shrubs and other woody plants, and water balance indices such as leaf area index. Target values can be adjustable and proportional to the precipitation record prior to and during the growing season. Target values might also be developed that are proportional to measurements that year in reference areas. The plant community should be monitored at least annually, usually at the peak or end of the growing season, and compared with target values. The revegetation plan should include criteria and methods for evaluating revegetation success. The ecological basis for criteria, time steps for target values, and vegetation sampling designs, instrumentation, and statistical methods for field data collection and analysis, should all be included (see Monitoring). General Revegetation Concepts and Practices Several guidelines have been written for revegetation and reclamation of drastically disturbed land including mine lands, revegetation of conventional landfills, revegetation of remediated waste sites, and ecological restoration projects (e.g., Munshower 1994, Roundy et al. 1995, McLendon and Redente 1997, Monson et al., 2004, Link et al. 2006, EPA 2006). Readers are encouraged to consult these and other sources for more comprehensive background information, design guidance, and manuals dealing with restoration ecology and revegetation practices. Detailed guidelines specifically for revegetation of WB covers have yet to be developed. WB cover revegetation is unique in that the plant community plays a key functional role in performance. Failure of the plant community to perform for the long-term may carry unacceptable risks to human health and the environment. This is especially true for hazardous and radioactive waste sites. For now, this section provides abbreviated descriptions of several 51

64 Albright / Benson / Waugh - Chapter 4 important concepts and practices. As with baseline ecological surveys (see above), choices of specific methods and practices must be guided by project goals, objectives, legal requirements, and budget. Site Preparation Earth moving is the most expensive part of cover construction. Costs of plant materials, seedbed preparation, soil amendments, and maintenance are minor compared to shaping slopes and hauling soil. Integrating engineering and revegetation tasks with respect to earth moving can reduce costs. Reconstruction and recontouring of slopes should include practices to create surfaces that enhance plant establishment and survival, particularly in arid and semiarid environments where lack of water is the major factor limiting plant growth. Slope length and shape can be manipulated to limit runoff, conserve water for plant growth, and help control erosion. Concave slopes are generally more resistant to erosion. Contoured terraces on landfill slopes can be designed to harvest water for plant growth and shorten the effective slope length. Shaping a hillslope to form drainage networks similar to the surface morphology of reference areas can also help control erosion. The overall form, color, and texture (surface irregularities) of the cover should not contrast sharply with surrounding landscapes. Visual character of slopes may also be important to the public. Soil Edaphic Properties and Handling Establishing a favorable successional trajectory of plant communities on a cover requires restoration of the below-ground ecosystem. Understanding, designing for, and maintaining soil edaphic properties that enhance plant ecology, transpiration, and stability is essential. This section reviews important physical, chemical, and biological edaphic properties and introduces soil handling and placement practices that should be considered during design and construction. Target values for soil edaphic properties can be derived, in part, from the baseline survey of reference area soils. 52

65 Albright / Benson / Waugh - Chapter 4 Physical Properties The physical properties that most influence plant ecology are soil particle size distribution (texture), dry unit weight (bulk density) and compaction, and soil particle aggregation (structure). Particle size distribution influences water infiltration, hydraulic conductivity, water storage capacity, and cation exchange capacity (CEC, the quantity of cations that can be adsorbed on negatively charged soil solids). CEC influences the availability of plant nutrients. There is no optimum particle size distribution, as plant tolerances and adaptations vary. Soils with high clay content can have high water storage capacity and CEC (both are favorable attributes), but the water may be held so tightly by the clay that much of the water is unavailable to plants. Clay soils also can become hard and impenetrable to roots when dry, and expansive (e.g., montmorillonitic) clays can damage roots. Soils with a high sand fraction are well-drained, well-aerated, and friable when dry, but can have poor water storage capacity and CEC. Most plants grow best in soils that have balanced proportions of sand, silt, and clay such as loams, clay loams, and sandy loams. Dry unit weight (also known as bulk density) can affect root penetration and water movement. A common problem during construction of water storage layers is excessive compaction by heavy equipment. Highly compacted soils with high dry unit weight hinder root penetration and generally have low hydraulic conductivity. Thus, specifications should prevent over-compaction. The dry unit weight of the undisturbed reference area is a good target to use for controlling the dry unit weight of a cover. Soil structure refers to the natural aggregation of particles and the patterns of weakness or cracks between aggregates. Aggregate stability is influenced by clay content, organic matter, and time. Soils with granular and blocky structure exhibit higher infiltration and macropore flow, both favor root development and plant health, than soils with no structure or massive structure. Soil hauling and placement often results in breakdown of aggregates and reductions in infiltration and hydraulic conductivity. Practices that reduce compaction such as ripping, plowing, and chiseling can also impact aggregate stability. Construction practices that help retain soil structure may improve initial plant establishment and also accelerate ecological succession, particularly for diverse communities that include shrubs or other woody, deeperrooted plants. 53

66 Albright / Benson / Waugh - Chapter 4 Chemical Properties The chemical properties that will most influence plant communities are ph, soluble salts, and availability of plant nutrients. The availability of essential plant nutrients and phytotoxicity of metals and other elements are highly responsive to soil ph. Several essential nutrients become less available with increasing ph and some become more available. For example, nitrogen availability is highest at ph between 6.0 and 8.0, and then drops above 8.0, whereas phosphorus is least available at a ph of about 8.5 and is more available both above and below 8.5. Plant toxicity of iron, manganese, and other metals becomes a problem at ph values below 5.5. Soil ph also influences microbial populations that help drive nutrient cycling and availability to plants. Acidic soils that may develop in humid regions can be treated with finely ground calcitic or dolomitic limestone depending on whether calcium or magnesium is needed. Highly alkaline soils with ph > 9, which are fairly common in the arid and semiarid West, often can be treated with elemental sulfur. Cover designers must be aware of potential salinity and sodicity problems with borrow soils. Saline soils have high amounts of soluble salts such as sulfates, carbonates, and chlorides and are common in arid and semiarid environments where precipitation is inadequate to leach salts that accumulate as minerals weather. Salinity increases osmotic potential and can limit the ability of plants to extract water from soil, resulting in stunted plant growth. Salinity is typically measured as electrical conductivity (EC) using an aqueous soil extract. Soils with an EC greater than 0.4 S/m are considered saline. However, many Western rangelands classified as saline using this standard are actually highly productive grasslands and shrublands. Because plants vary in salinity tolerances native plants are typically more tolerant of saline soils in arid and semiarid regions than in humid regions salinity should be addressed relative to baseline ecological conditions on a site-specific basis. Planting salt-tolerant species found growing on undisturbed borrow soil is the most common approach for mitigating salinity problems. Soils containing sodium as a significant proportion of their total exchangeable cations can also impact plant growth. Sodic soils may impact plant growth directly because of caustic alkalinity or the adverse effects of sodium on plant metabolism, but often effects are indirect. Soils high in sodium usually lack good soil structure, because the sodium causes clay and organic matter to disperse leading to soil compaction and root growth problems. Sodicity is most 54

67 Albright / Benson / Waugh - Chapter 4 often defined in terms of the sodium adsorption ratio (SAR), which is calculated based on concentration of sodium, calcium, and magnesium in soil water extracts: [ Na ]/{( [ Ca ] + [ Mg ]) / 2} 1/ 2 SAR = (4.3) where [ ] is a concentration in milliequivalents/liter (meq/l). The standard system for classifying saline and sodic soils can be divided into three broad categories: saline soils, saline-sodic soils, and sodic soils. Saline soil has EC > 0.4 S/m and SAR < 13. A saline-sodic soil has EC > 0.4 S/m and SAR > 13. A sodic soil has EC < 0.4 S/m and SAR > 13. Elements required in relatively large quantities for plant growth (macronutrients) include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Nitrogen and phosphorus are generally the most limiting for landfill revegetation. Nitrogen is an integral component of plant proteins and enzymes. Deficiencies in nitrogen, usually due to a lack of the nitrate ion, produce small and pale plants. Phosphorus, available to plants as phosphate ion, plays a comprehensive role in plant metabolism. Phosphorous deficiencies cause poor root growth in seedlings. Phosphorus, usually abundant in cover soils, is often unavailable because of insolubility. Deficiencies of both macronutrients can be alleviated with fertilizers (see Maintenance). Soils with at least 7 mg/l available phosphorus are generally sufficient for revegetation. Nitrogen deficiency, usually the greater long-term problem, is related to the lack of microorganisms necessary for the biological cycling of nitrogen. Nitrogen fertilizer applications are often excessive and cause proliferation of weedy annual species and poor establishment of desirable perennials. A key to sustaining diverse and productive cover vegetation is to create soil conditions that promote biologically-active cycling of nitrogen. Organic Matter and Microorganisms Soil organic matter and interactions with soil microorganisms play essential roles in establishment and succession of sustainable and productive plant communities. Levels of organic matter are often quite low in arid and semiarid soils, however, even in these soils, organic matter can play a fundamental role in maintaining favorable soil physical and chemical properties and soil fertility. In the following ways, continual decomposition of fresh or recognizable organic 55

68 Albright / Benson / Waugh - Chapter 4 material (e.g., roots, insects, organic mulches) into humus is essential to maintaining a healthy plant community: Provides humic substances (decomposition products) from organic matter that aggregate soil particles, enhances soil structure and water-storage capacity, and promotes more extensive root growth, Maintains relatively low soil dry unit weight, favorable porosity, and soil aeration, Releases plant-available nutrients during mineralization including nitrogen and phosphorus, Increases cation exchange capacity, buffers soil alkalinity and acidity, and diminishes the phytotoxic effects of metals through chelation and immobilization. The soil microbial system and plant communities are closely coupled. High transpiration rates require diverse, sustainable, and productive plant communities, which rely on the intricate interactions of soil microbial communities. Bacteria and fungi sustain the nutrient cycles that fuel productivity. Mycorrhizal fungi infect plant roots while surviving on root exudates, enhance uptake of water and nutrients, and transfer water and nutrients among roots of different plants. Macrofauna, microfauna, saprophytoca bacteria, and fungi successively reduce plant and animal litter to humus, biomass, nutrients and CO 2. Microbial processes in the rhizosphere such as nitrogen fixation and metal chelation help drive succession towards more mature, more productive ecosystems. At higher trophic levels, macrofauna and microfuana also feed on mycorrhizae and saprophytes. In mature cover ecosystems, all are cycled. Soil Storage and Handling The upper part of the water storage layer in a WB cover should be topsoil that can serve as a medium for development of a productive and sustainable plant community. A soil material that qualifies as topsoil should have the following attributes: Favorable physical properties including particle size distribution, cation exchange capacity, structure, dry unit weight, and hydraulic conductivity, Favorable chemical properties including ph, salinity, and sodicity, Both recognizable and humic organic matter, Biologically active plant nutrient pools, 56

69 Albright / Benson / Waugh - Chapter 4 Viable populations of macrofauna (e.g., anthopods, worms) and microorganisms including bacteria (e.g., saprophytes, cyanobacteria), mycorrhizae and other fungi, Living plant propagules including seeds, viable plant parts (e.g., stolons, rhizomes), and whole plants. The best choice for topsoil handling and placement is to strip and reapply fresh soil in a single operation. This method, sometimes called direct haul or live haul topsoiling, retains favorable soil physical and chemical properties and will provide, from the onset, a biologically active nutrient pool (favorable organic matter and C:N ratios), microbial community, and plant propagules. When direct haul is not an option, the topsoil should be salvaged, stored in relatively low stockpiles, and planted to maintain favorable attributes until used. Extended storage of topsoil will likely result in die-off of viable microorganisms and plant propagules, and impaired nutrient cycling. Stored topsoil may require addition of organic matter, nitrogen, and inoculum to re-establish microbial populations. Generally, the least desirable option is to substitute subsoil for topsoil. Subsoil generally lacks nutrients, has poor soil chemistry, and may require a lengthy supply of soil amendments and fertilizers to establish and sustain an acceptable plant community. However, at arid sites where soils are naturally nutrient poor, a subsurface soil may be preferable to a surface soil containing weed seed because annual weeds often compete with and exclude desirable perennial species. WB cover designers should conduct a soil survey as part of the baseline ecological characterization to identify the types, extent, and quality of soil for the water storage layer. Soil structure and a favorable dry unit weight are maintained by minimizing the number of times soil is moved and by avoiding handling soils that are too wet or too dry. Wet soils often become too compacted and dry soils lose structure. At some Western sites, establishment of cryptobiotic crusts (living, complex association of cyanobacteria, lichens, mosses, and green algae) may help accelerate plant succession. Cryptobiotic crusts often fix nitrogen, provide soil stability, increase infiltration of water, and preserve soil aggregation in arid and semi-arid plant communities. Soils with intact cryptobiotic crusts also require special handling. Cryptobiotic crusts may be preserved by salvaging the uppermost 50 mm of dry crust and spreading a thin layer on the surface of the cover. 57

70 Albright / Benson / Waugh - Chapter 4 Soil Mulches and Amendments Organic and inorganic mulches may be required to provide temporary protection of the topsoil surface from erosion. Straw from cereal grains is the most common mulch. Straw can be spread using dry blowers and then secured or anchored to the soil either by crimping (crimpers are implements that use a disk or wheel to push straw much into the soil) or using tackifiers. Long stems are needed for crimping. Straw often contains grain or noxious weed seed, a common drawback. Native hay, an alternative to straw, usually has longer stems for crimping, and if cut from desirable native plant stands, can also be a good source of seed to enhance species diversity. Wood chips and fragments, an alternative to straw and hay that usually lasts longer, may also be cheaper if a local source is available. Wood residues are difficult to anchor and should be limited to shallow slopes. Wood fiber mulches are often applied to steep and otherwise inaccessible slopes using a wet slurry called hydromulch. Relatively expensive erosion control fabrics and mats such as jute netting and wood fiber blankets can be used on steep slopes with high erosion potential. If any of these coarse organic mulches (straw, hay, wood residue) are incorporated into the soil, then fertilizer may be necessary to supply sufficient N for both microbial decomposition of the organic mulch and for plant growth. Rock or gravel spread over or mixed into the soil surface may be an attractive alternative to organic mulches in arid and semiarid regions. Rock or gravel admixtures together with vegetation can provide long-term erosion protection. Vegetation and organic litter disperse raindrop energy, shield underlying fine soils, increase infiltration, reduce surface water flow and surface wind velocity, bind soil particles, and filter sediment from runoff. Gravel mixed into the soil surface helps control erosion when vegetation is sparse (following construction, fires, drought, etc.), mimicking conditions that lead to the formation of natural gravel pavements. A gravel admixture also helps disperse raindrop energy, shield underlying fine soils, and reduce flow velocity. Gravel mulches increase near-surface water storage, enhancing seedling emergence. Hence, rock or gravel admixtures can help control both wind and water erosion and, functioning as mulch, enhance seedling emergence and plant growth. However, gravel mulches can also form a reverse capillary barrier effect that limits surface evaporation, which may adversely affect the cover water balance, especially when plants are immature or dormant. 58

71 Albright / Benson / Waugh - Chapter 4 Soil amendments may be required especially when direct haul topsoiling is not an option. Different types of amendments can be applied to raise nutrient levels, improve soil chemistry (buffer ph, improve CEC and C:N ratio), improve water retention, enhance microorganisms populations, ameliorate toxicity problems, and control soil loss. Chemical fertilizers should be applied with caution, as over-application can enhance weed growth and decrease mychorrizal associations. Agricultural rates of fertilizer application are much too high for covers. Slowrelease fertilizers should be applied at rates calculated for site-specific conditions and only if fertility tests indicate a major soil deficiency. Incorporate fertilizers into the soil, whether using a broadcasted solid or sprayed liquid, to limit loss of nitrogen to the atmosphere and to move phosphorus to the seedling root zones below the soil surface. Organic amendments (e.g., animal manure, biosolids) may be necessary for topsoil stockpiled too long or when subsoil is substituted for topsoil. Organic amendments have the advantages of slow release of nutrients, enhanced microbial populations, improved C:N ratios, and improved soil water retention. Biosolids (composted sewage sludge) can build microbial populations that trigger self-sustaining biological activity, slowly release nitrogen over several years reducing competition from fast-growing annual weeds, improve soil friability and permeability, lower soil ph, and, hence, are often used to amend poor-quality subsoils when importing topsoil proves impracticable. Although the production process for biosolids decomposes most of the complex organic molecules in sewage and kills most of the pathogens, some types of biosolids are regulated to prevent contamination of surface water and shallow groundwater. Thus, health and safety issues and the potential for environmental impact should be checked before biosolids are used as a soil amendment. Selection of Plant Species and Materials The choice of plants for revegetation of WB covers involves selection of the most appropriate species and varieties in concert with selection of types and sources of plant materials. Considering the twin goals of (1) creating a soil environment similar to surrounding undisturbed areas (reference areas or analog sites) and establishing plant communities that are well-adapted to that environment, and (2) sustaining high evapotranspiration rates to limit percolation, species in the reference plant community should be the first choice. Direct haul topsoil from a reference plant community in an area planned for landfill expansion would be the ideal approach. If direct 59

72 Albright / Benson / Waugh - Chapter 4 haul is not an option, selection of species should be based primarily on the results of the baseline ecological survey. Given that this is also not always possible, Table 4.2 lists criteria for selecting species that address less than ideal conditions. Table 4.2. Criteria for selection of plant species for revegetation of WB covers. Propagation and Establishment Availability of seed or other plant material Ease of propagation Ease of seeding or planting Immediacy and certainty of establishment Value for High Transpiration Rates High net primary productivity High species diversity in seed mixture Canopy cover and green leaf area Root distribution and growth rates Species mixture with complimentary life forms, phenology (season of growth), physiognomy, and spatial distribution patterns Competitive with invasive annual weeds Species mixtures with mutualistic interactions Adaptability to Climate Shifts and Extreme Events Tolerance of periods of drought Tolerance of temperature extremes Tolerance of strong winds and overland flow of runoff Resilience to a changing climate Adaptability to Existing Soil Conditions Soil water relations including changes in water storage profile, permeability, and preferential flow as influenced by soil structure Value for soil stability Adaptability to changes in soil morphology Tolerance of compaction, salinity, sodicity, unfavorable ph, nutrient deficiencies, and metal toxicities, etc. Sustainability Persistence Self-renewal Compatibility with other species Disease and pest resistance Tolerance of herbivory Fire resistance Miscellaneous Criteria Habitat value for burrowing and tunneling animals Potential for biouptake of contaminants Aesthetic value Commensurate with surrounding land use goals Maintenance costs 60

73 Albright / Benson / Waugh - Chapter 4 Diverse mixtures of native and naturalized plant species are thought to maximize water removal and remain resilient given variable and unpredictable changes in the environment resulting from pathogen and pest outbreaks, disturbances (overgrazing, fire, etc.), and climatic fluctuations. In contrast, the exotic grass plantings common on landfill covers are genetically and structurally rigid, are vulnerable to disturbance or eradication by single factors, and often require considerable maintenance. Species mixtures might include legumes for nitrogen fixation, plants with both C3 and C4 photosynthetic pathways, rhizomatous and bunch grasses, plants with complementary aboveground and belowground growth patterns, and early, mid, and late seral plants. Shrubs may be of particular importance on WB covers because they extract water from deeper in the cover soil and create islands of fertility and biological activity, accelerating succession and maturation of the plant community. Once the design team has assembled a promising list of species, consideration must be given to sources of plant materials. Choices must be made for each species including: selecting appropriate ecotypes or cultivars; using seeds versus whole plants or plant parts; harvesting seed from reference plant communities or purchasing seeds or plant parts; growing transplants from seed or purchasing transplants; considering commercial availability of seeds or plants of appropriate cultivars; and costs of the above options. The best option is direct haul of biologically active topsoil from reference plant communities that are already laden with seed, whole plants, and plant parts. Since this is often impracticable, the second choice is to collect local seed and plant parts from reference areas or similar plant communities to ensure use of local ecotypes, which are best adapted to the cover habitat. The last option is to purchase commercial seed or vegetative material and to select cultivars developed from sources similar to reference areas as determined during the baseline ecological survey. Seedbed Preparation After the water storage layer has been constructed but before the cover is seeded or transplanted, several activities may be of value, depending on site conditions, to facilitate seeding and to improve the probability of successful revegetation. Topsoil may be required on the surface, either by live haul or from a maintained stockpile (see Soil Storage and Handling above), if the water storage layer consists of subsoil. Some form of tillage may be necessary to level the seedbed, reduce competition from weeds that may germinate from the seedbank, loosen 61

74 Albright / Benson / Waugh - Chapter 4 a soil compacted by heavy equipment, or break up large clods and smooth the surface if drill seeding is planned. Tillage involves the use of implements such as plows, harrows, and disks. In contrast, at arid and semiarid sites, roughening the soil surface may enhance germination and plant establishment by creating moist microenvironments. A roughened soil surface also improves seed placement if broadcast seeding is planned. Soil surface roughening is a manipulation of the small-scale environment that helps control soil loss, retain runoff, catch and hold broadcasted seed, and enhance plant growth. Methods for roughening the surface and harvesting water include contour furrowing, land imprinting, pitting, and cultipacking. Planting Methods Plants may be introduced via seeding or live planting. Seeding and planting methods should be planned with consideration of site preparation, soil edaphic properties and handling, soil mulches and amendments, selection of plant materials, and seedbed preparation. Seeding Rates and Pure Live Seed Selecting seeding rates should not be a trivial exercise. Too much seed may produce thick stands, competition between individuals and species, monocultures of dominant plants, and poor overall health of the plant community. Too few seeds may reduce competition with invasive species and increase the likelihood for erosion. Seeding rates should be determined based on the number of seeds planted per unit area and then converted to units of weight per unit area (kg/ha). Seeding rates based solely on weight per unit area will result in too many lighter seeds and too few heavier seeds. Seeding rates and mixtures should be developed to satisfy the specific revegetation objectives. Seeding rates recommended for other applications in the region, such as planting of highway right-of-ways, may not be appropriate. Cover designers should develop seeding rates for each individual species in a mixture taking into consideration factors such as the desired composition and diversity of the plant community, climate, soil fertility, season of planting, inter- and intra-specific competition, likelihood of weed encroachment, pressure from granivores (seed eaters) and herbivores (grazers), likely germination and survival of seedlings, and the percent pure live seed (PLS). The best seeding rates have been developed through trial and error for a specific mixture of species in a specific environment. 62

75 Albright / Benson / Waugh - Chapter 4 Seeding rates for purchased or harvested seed should be based on the percent pure live seed (PLS). %PLS = (% Germination % Purity)/100 (4.4) Percent germination is the percent of seeds by weight that are viable. Percent purity is the percent by weight of seed left after weed seed and trash are removed. For example, if germination is 50% and purity is 90%, then PLS is 45%. Most state agencies require commercial growers to certify and label %PLS on bags of seed sold for profit. Labels should also include the germination percentage, date of the germination test, origin of the cultivar or ecotype, and percent and type of crop and weed seed. Seed from commercial growers should be certified and labeled. Seeding The goal of seeding a WB cover is to place the seed in contact with firm soil at a depth and with moisture conditions most favorable for germination and establishment. The most favorable depth varies from species to species. Overall, small seed should be placed shallower and large seed should be placed deeper. The optimum depth for a given species depends on the balance of light requirements for germination, adequate moisture, soil type and compaction, and whether young shoots are long enough to push through to the soil surface. Achieving a diverse plant community with high transpiration rates requires a seeding strategy that places seed of different species at their different optimum depths for germination. The strategy may require an innovative combination of seeding and planting methods. Seeding methods include drill seeding, broadcast seeding, and hydroseeding. Drill seeders are implements pulled behind tractors that open small furrows in the soil at predetermined depths, drop seed through tubes into the furrows at predetermined rates, and push soil over the seed. The primary advantage of drill seeding is that seeds can be placed regularly and at uniform depths. Drill seeders equipped with separate seed boxes can place different size seeds at different depths and in different rows, which can reduce competition for limited moisture. However, drill seeders are inappropriate for rocky soils, steep slopes, or saturated soils. Many native seeds are too trashy (hairy or with long filaments called awns) to pass 63

76 Albright / Benson / Waugh - Chapter 4 through drill seeders. Also, when drill seeders are used, seedlings emerge in rows looking more like a crop than a native plant community. Broadcast seeding can involve any number of methods that drop seed on the soil surface rather that burying it in a furrow. Broadcast seeding on a large scale involves the use of mechanical devices that throw seed in irregular patterns. Broadcast seeding is often followed by dragging chains, harrows, or something similar to knock seeds into cracks and crevices and cover them with soil, placing seed at variable depths. Both broadcast and drill seeding are often followed with an implement called a cultipacker that has spiked wheels that compress the soil overlying the seed. Hydroseeding is a type of broadcast seeding that disperses seed in a liquid under pressure, and is particularly useful for spreading seeds on steep slopes or rough surfaces that are otherwise inaccessible to drill seeders. Broadcast seeding is usually more economical, more suitable for small or trashy seed, and the emerging vegetation has the appearance of native plant communities instead of row crops. However, broadcast seeding rates often must be doubled because many seeds end up either desiccated on the surface or consumed by rodents and birds. Broadcast seeding is most effective when the seedbed surface is roughened, pitted, or imprinted prior to seeding. Legumes, the most common nitrogen-fixing plants species, should be included in seed mixtures for most WB covers to accelerate establishment of the nitrogen cycle. Other plant families such as the rose family also fix nitrogen. Legumes require a symbiotic association with host-specific bacteria, called Rhizobium, that convert atmospheric nitrogen into plant-available nitrogen. Seeding or planting legumes will require inoculum containing the appropriate Rhizobium species. The appropriate inoculum is plant-species specific and can be collected from the surface soil beneath the nitrogen-fixing species in its natural habitat. The inoculum should be mixed with lightly dampened seed for drilling and broadcasting, or mixed with the slurry for hydroseeding. As a general rule, the best season for seeding either precedes or coincides with the period of maximum precipitation, or a period of reliable precipitation that is of sufficient duration for seedlings to germinate and become established. Therefore, determining the best season for planting requires information on seasonal climate patterns, seed physiology (need for scarification or cold stratification), seasonal growth patterns, and moisture requirements of species in the seed mixture. Advantages of late fall seedings in cool climates include (1) seeds 64

77 Albright / Benson / Waugh - Chapter 4 remain dormant until favorable moisture and temperature conditions return in the spring, (2) seeds undergo cold stratification, and (3) equipment can be mobilized when convenient rather than during a short window in the spring when the ground is often muddy and inaccessible. Disadvantages of dormant fall seeding include (1) greater likelihood for desiccation and predation by birds and rodents, and (2) greater germination and competition from weeds. Spring seeding may require stratification of seed in cold storage and cultivation of early-season weeds. In the arid and semiarid West spring precipitation is generally less predictable. However, if spring rains come, plants may establish faster if weeds are cultivated first. Overall, cool season grasses and forbs grow best in early spring and should be seeded in the fall; warm season grasses and forbs require warm summer months and can be seeded in the spring. Shrubs and trees require an extended wet spring-summer growing season and are often difficult to establish from seed in the semiarid west, and for that reason, are often transplanted. Planting Transplanting whole plant or plant parts is an alternative to seeding for long-lived trees and shrubs that germinate only when conditions are ideal. Options for whole plants include container-grown plants, bare-root plants, and plants excavated from their natural setting called wildings. Bare-root and container-grown plants grown in greenhouses are usually hardened (a process that causes dormancy) to improve survival of transplant shock. Pruning before removing and transplanting wildings increases their viability. Although more expensive to purchase, container-grown plants typically develop into larger and healthier plants than bare-root plants or wildings of comparable size. Wildings from nearby locations may have a long-term advantage unless containerized or bare-root stock can be grown using seed harvested from local populations. Plant parts include cuttings, root pads, and sprigs. Cuttings are pieces of stems or roots with viable growth nodes that can develop into whole plants. Root pads and sprigs (intact chunks of soil and root parts) can be used to transplant rhizomatous and sprouting grasses, forbs, and shrubs. Root pads and sprigs may be lifted from the soil using heavy equipment such as a front end loader and moved into a depression formed in the topsoil of the cover. Sprigging involves removal and transplanting of only the root mass of sprouting shrubs. 65

78 Albright / Benson / Waugh - Chapter 4 Maintenance Successful and rapid establishment of plant communities requires maintenance after the cover is seeded and planted. Necessary maintenance may include irrigation, fertilization, weed management, and herbivore management. Irrigation Natural establishment of many long-lived shrubs requires an infrequent combination of relatively cool temperatures and an exceptionally wet spring-summer season. Irrigation may be necessary to create ideal conditions for survival and growth of transplanted shrubs; to effectively extend the growing season. Irrigation may also be beneficial to establish warm season species, to increase availability of plant nutrients in the soil, and, if applied strategically, to manipulate species composition and increase diversity. However, irrigation can cause negative repercussions if not carefully managed. Clearly, excessive irrigation may exceed the water storage layer capacity in a WB cover and result in unacceptable percolation rates. A deficit irrigation rate must be determined to apply less water than can be removed by evapotranspiration, thereby maintaining soil water levels in the cover below the storage capacity. A deficit irrigation rate can be determined empirically by monitoring water storage or estimated using water balance models (see Chapter 6). Other potentially detrimental effects of irrigating include stimulating plant growth and abundance to a level that is unsustainable with ambient precipitation, and inadvertently enhancing competition from fastgrowing weeds. Fertilization Topsoil should be tested for nutrient deficiencies (see section on Soil Edaphic Properties) before construction of the cover, and periodically thereafter if revegetation success criteria have not been satisfied within a reasonable timeframe. Nitrogen, phosphorus, and potassium are the three nutrients most likely to be deficient. Nitrogen is typically the most limiting to plant growth and a sustainable source of biotic nitrogen can be achieved only if the nitrogen cycle is established. Atmospheric deposition of nitrogen is increasing and may also have an effect on nitrogen budgets. Establishing the nitrogen cycle requires either direct haul topsoil, or a combination of fresh and decomposed organic matter plus inorganic nitrogen, and a healthy soil 66

79 Albright / Benson / Waugh - Chapter 4 microbial community (see Soil Edaphic Properties). In the short term, N deficiencies can be overcome using inorganic fertilizer. However, inorganic fertilizer is only a short-term fix because of leaching, volatilization, and denitrification. Nitrogen fertilization can also favor fastgrowing annual species (including weeds), displacing mid and later seral species, and reducing species diversity. Weed Management Establishment of a diverse community of desirable, perennial plants may require some form of weed management. Fast-growing weeds compete with seeded and planted perennials. Furthermore, regulations may require control of listed noxious weeds. The first and ideal weed control is to use topsoil free of weed seed. Cultivation after weeds germinate but before seeding and planting can reduce the weed seedbank. Rapid weed recruitment and establishment from persistent seedbanks may require chemical, mechanical, biological, or prescribed burning methods after seeding and planting. Selective herbicides can be used to target individual species. However, possible negative environmental consequences must be understood. Seasonally strategic mowing can top weeds before they drop viable seed, and if repeated, may eventually reduce the weed seedbank. Most weeds are non-native and thrive in the absence of natural predators. Biological control, the release of host-specific insects or pathogens is legal for some weed species. Strategic grazing with goats and sheep to limit seed production is another form of biological control. Prescribed burning can reduce weed populations and enhance perennial growth if the fire ecology of the plant community is understood. Finally, using weed free soil and immediately establishing a diverse community of native perennial and annuals will minimize weed problems (Link et al. 2006). Grazing Management Fencing may be necessary for the first few years after cover construction to protect the establishing plant community from livestock grazing and wildlife herbivory. After the vegetation matures, depending on the ecology of the plant community, moderate grazing when plants are dormant removes standing dead biomass and may actually stimulate growth. Additional controls such as specialized fencing or introduction of predators may be necessary to control smaller herbivores. 67

80 Albright / Benson / Waugh - Chapter 4 Monitoring A monitoring program should be designed to evaluate the success of revegetation efforts. Success should be measured against criteria and target values that have been defined based on results of the baseline ecological survey (see Revegetation Success Criteria). Short-term monitoring should consist of vegetation sampling during or at the end of the growing season. Sample results are then compared to target values. If sampling results meet or exceed target values for key performance criteria, then the revegetation is deemed successful for that year. If sampling results are below target values, then follow-up maintenance (previous section) or follow-up investigations, such as a re-evaluation of soil fertility, may be warranted. Three general attributes of a WB cover plant community composition, structure, and function can be evaluated. Species composition is usually a simple list of the species occurring on the cover, but may also include species richness and species diversity. Species richness is the number of species per unit area. Species diversity is the species richness weighted by species evenness (distribution of individuals among species) and is often considered a more informative measure of revegetation success. For example, vegetation may have high species richness but poor revegetation success if most of the plants are weeds. A plant community with slightly fewer species, but a more even distribution of individuals among species high species diversity is more desirable. Plant community structure, for purposes of sampling, refers to the appearance, arrangement, and abundance of species in both time and space. Density and canopy cover are the vegetation parameters most commonly measured for the grass and shrub-dominated communities commonly found on WB covers. Density is the number of plants rooted in a unit area and provides a good measure of shrub seedling abundance; for example, 600 rabbitbrush seedlings per hectare. Canopy cover is usually reported as the percentage of the ground surface area as projected vertically beneath plant canopies; for example, 15% sagebrush canopy cover, or 8% thickspike wheatgrass canopy cover. The most common methods for measuring canopy cover are based on either subjective estimation, line transects, or point interception. Plant community function refers to the flow or cycling of energy, nutrients, and water. The most important plant community functions that could be monitored on WB covers are plant transpiration and plant/soil evapotranspiration (ET). Direct measurement of transpiration or ET 68

81 Albright / Benson / Waugh - Chapter 4 is usually limited to research applications. However, methods are now being developed for routine monitoring of seasonal evapotranspiration using time series of satellite image series and by scaling up from stem and whole-plant measurements to landscapes. Most often, ET is estimated by using measurements of plant community structure, such as leaf area index and phenology, in numerical models. Because monitoring invariably involves measurements of only a portion of the plant community, by definition a sample of the whole, statistical methods must be employed to make inferences about the entire community from the sampling data. Statistics play a role in methods both for choosing locations to sample and for data analysis and interpretation. Sampling designs can be randomized, stratified, or systematic depending on the monitoring objectives. Data analysis methods typically involve comparisons of mean values of density and canopy cover with the predetermined target values. To gage whether the means are similar or dissimilar to the target values requires computation of confidence intervals. Readers should consider this as a very brief introduction to monitoring revegetation success. Development of an effective vegetation monitoring program will require input from plant ecologists with experience in plant identification, field methods for measuring vegetation, and statistical sampling design and data analysis methods. For more information, readers should consult a textbook on field and statistical methods for vegetation sampling (e.g., Bonham, 1989). Natural Analogs and WB Cover Designs and Sustainability An underlying theme of this chapter is the need to first understand the ecology of a landfill environment before designing the WB cover and developing a revegetation plan. Baseline ecological surveys (see section above) provide ecological information needed for design and to improve understanding of natural processes that will influence cover performance. In some cases, natural analogs exist that can improve initial design concepts and can be used to project long-term performance to improve sustainability of WB covers (Waugh et al. 1994). According to Webster s Dictionary, an analog is defined as that which corresponds to something else in construction, function, qualities, etc. and thus has similar properties. For the purposes of this chapter, natural analogs are natural and archaeological settings, materials and 69

82 Albright / Benson / Waugh - Chapter 4 processes that provide clues for more effective cover designs or are indicative of long-term changes in cover environments. Example of Cover Design Concept Analogs A soil profile near a Richland, Washington landfill illustrates the use of natural analogs to develop or improve cover design concepts. Figure 4.5 is a natural soil profile consisting of a thick fine-textured soil layer overlying a coarse gravel layer that functions as a capillary barrier (see Chapter 5). The soil profile formed in sediments that were rapidly laid down 13,000 years ago near the end of a period of Pleistocene cataclysmic floods (Baker et al. 1991). The profile is unique in that the entire sediment sequence was laid down and pedogenic (soil development) processes started forming at a known time, and then continued, relatively uninterrupted, until the present. Figure 4.5. Natural analog of a conceptual design for a water balance cover with a capillary barrier at the Hanford Site near Richland, Washington. 70

83 Albright / Benson / Waugh - Chapter 4 The soil profile provides clues about the integrity of layer interfaces and the performance of a capillary barrier cover at the site (Bjornstad and Teel 1993). The open-work gravel layer suggests that a capillary barrier could be designed that prevents illuviation of fine soil into underlying clean gravel, preserving the integrity of this layer interface for 1000s of years. The thick, white calcium carbonate horizon at the fine-coarse layer interface is a pedogenic clue that the capillary barrier has limited deep percolation for 1000s of years. However, calcium carbonate deposits on gravels below the interface suggest that water sometimes percolates past the capillary barrier and, hence, the fine soil layer may not be thick enough to provide adequate water storage for all precipitation events. Natural Analogs and Cover Sustainability WB covers designed for hazardous and radioactive waste landfills that are expected to perform for 100s to 1000s of years must accommodate long-term ecological change. Projections of how a changing environment may influence cover performance are crucial to improving longterm maintenance strategies and reducing costs. Current guidelines for designing landfill covers do not address long-term changes in the environmental setting that may contribute to cover degradation. Long-term processes and episodic events associated with soil development (pedogenesis), ecological succession, climate change, and geomorphological change are usually not considered. Furthermore, most current approaches for long-term performance evaluation rely on physically based models that neglect inherent and measurement uncertainty and are not sensitive to ecological change. Projections of the long-term performance of WB covers require data for reasonable future ecological scenarios. Natural analog studies can help identify and evaluate likely changes in environmental processes that may influence cover performance, processes that cannot be addressed with short-term field tests or existing numerical models. Natural analog information can be very helpful to (1) engineer cover systems that mimic sustainable natural systems, (2) define possible future environmental scenarios for input to models and field tests, and (3) provide insight about the possible evolution of covers as a basis for monitoring leading indicators of change. Natural analogs also help demonstrate to the public that model predictions have real-world complements. 71

84 Albright / Benson / Waugh - Chapter 4 Evidence from natural analogs can improve our understanding of (1) effects of soil development processes on water storage, water movement, and site ecology; (2) effects of plant community dynamics on ET, soil hydraulic conductivity, soil erosion, and animal burrowing; and (3) meteorological variability associated with possible long-term changes in climate; (4) vegetation responses to climate change and disturbances. Examples of natural and archaeological analogs used for evaluating long-term cover performance follow. Examples are given for climate change, pedogenesis (soil development), and ecological succession. Soil Development Pedogenic (soil development) processes will change soil physical and hydraulic properties that are fundamental to the performance of WB covers. Although rates and magnitudes of change vary, pedogenesis takes place to some degree in all soils. Pedogenesis includes processes such as (1) formation of macropores associated with root growth, animal holes, and soil structural development that may allow preferential flow; (2) secondary mineralization, deposition, and illuviation of fines, colloids, soluble salts, and oxides that can alter water storage and movement; (3) soil mixing caused by freeze-thaw activity and animal burrows; and (4) formation of lag layers by winnowing, frost heaving, movement of soil gases during and after rain, and the shrink-swell action of expansive clays. Ecological Change Without human intervention, ecological development will take place on all covers. Ecological change is inevitable and may alter the functional performance of all cover designs in ways not initially anticipated. Plant communities develop and change in response to several interacting factors: propagule accessibility, climatic variability, change in soil characteristics, disturbances such as fire, and species interactions such as herbivory, competition, or fluctuations in soil microbe populations. Plant community dynamics are manifested by shifts in species composition, vegetation abundance, and species diversity and may be accompanied by changes in rates of nutrient cycling, energy exchange, and transpiration. Plant community dynamics are complicated and effects are difficult to model and predict. Even in the absence of large-scale disturbances, seasonal and annual variability in precipitation and temperature will cause changes in species abundance, diversity, biomass production, and soil 72

85 Albright / Benson / Waugh - Chapter 4 water extraction rates. Knowing how changes in the plant community inhabiting a cover may influence soil water movement, ET, and the water balance is important. Natural analogs provide clues about changes in ecology that occur in ways that cannot be accurately predicted by models or short-term field tests. For example, successional changes in the vegetation can create smallscale topographic patterns that foster greater heterogeneity in the soil water balance. At arid sites, desert shrub communities that are likely to develop on covers tend to trap windborne sediments, causing a hummock-swale relief with variable soil physical and hydraulic properties. Similarly, at humid sites, blowdown of mature trees will create depressions for water accumulation. Successional chronosequences provide clues of possible future ecological changes. For example, at the Lakeview, Oregon landfill, possible future responses of plant community composition and leaf area index (LAI) to fire were evaluated using a nearby fire chronosequence. In addition, possible LAI responses to climate change scenarios were evaluated at regional climate-change analog sites. Climate Change Design and performance evaluation approaches for WB covers rely on instrumental climate records (meteorological data). Many approaches implicitly assume that instrumental climate records and statistics adequately bound reasonable ranges of future climate. Instrumental climate records are rarely representative over the longer term because they are not extensive enough to capture the true variability of climate. Projections of long-term extreme events and shifts in climate states over long time periods, as well as annual and decadal variability in meteorological parameters, are needed to design sustainable covers with a very long design life. One way to extend the instrumental climate record is to assess past climate using proxies such as geomorphological, geological, and geochemical evidence; tree-rings, pollen, vegetation; and archaeological sites. Proxy records can document past shifts in the magnitude, frequency, and duration of flood and drought periods; shifts from predominately winter to summer precipitation; and shifts in the annual distribution of temperature. As examples, climate change models and natural analog data were linked to establish first approximations of possible future climatic states at sagebrush steppe sites near Richland, Washington (Peterson 1996) and Monticello, Utah (Waugh and Petersen 1996). A preliminary analysis of paleoclimate data for 73

86 Albright / Benson / Waugh - Chapter 4 Monticello yielded average annual temperature and precipitation ranges of 2 to 10 C and 600 to 800 mm, respectively, corresponding to late glacial and mid-holocene periods. Steps to Evaluate Long-Term Performance One approach for evaluating the long-term performance of covers links probabilistic modeling with evidence of long-term change derived, in part, from natural analogs. Natural analogs help define possible future environmental scenarios (Figure 4.5). This approach can be applied initially during the design phase, with the objective of building more sustainable covers, and then during the maintenance and monitoring phase, to reiterate long-term performance projections. A probabilistic modeling platform developed by Pacific Northwest National Laboratories (PNNL) called Framework for Risk Analysis in Multimedia Environmental Systems (FRAMES) is an example that has been used for hazardous waste landfills ( Ho et al. (2004) demonstrated applications of FRAMES for evaluations of landfills near Monticello, Utah, and Lakeview, Oregon. General steps of a systematic approach for projecting long-term performance that links modeling and natural analogs follows (see Figure 4.6): 1. Develop and screen future environmental scenarios. A scenario is a well-defined sequence of processes or events that describe possible future conditions of the cover. For example, a scenario might include a future climate state based on global change models, future ecological conditions and stages of soil development for the climate state, and a different land use. Future environmental conditions could be inferred from characteristics of natural analogs (see Natural Analogs). 2. Develop models of relevant future scenarios. Broad conceptual models of future scenarios are developed first to guide the selection of mechanistic or probabilistic models. Specific models can then be selected and integrated into a total system model framework that links performance with risk, such as FRAMES. 74

87 Albright / Benson / Waugh - Chapter 4 Develop and Screen Scenarios Scenario 1 Scenario 2 Scenario 3 Select Select Reject Develop Models Climate Evapotranspiration Source Term Vadose Zone Saturated Zone Human Exposure Estimate Parameter Ranges and Uncertainty Climate Change Ksat Leaf Defects Area Perform Calculations Interpret Results Risk/Perfor mance Cost/Schedule Regulatory C ompliance 00E000E000E000E000E000E000E000E 000E000E000E000E0000D >I< FFF8FFF8FFF PA_process.ai Figure 4.6. General steps of a systematic approach for projecting long-term performance of WB covers that links modeling and natural analogs (adapted from Ho et al., 2004) 3. Develop values and uncertainty distributions for input parameters. Single deterministic values might be assigned to some well-characterized parameters, but uncertainty distributions are preferable. The uncertainty and/or variability in other parameters may require the use of uncertainty distributions to define values. Uncertainty distributions for many environmental values will be based on the characterization of natural analogs. Some uncertainty distributions may be derived from literature, prototype tests in lysimeters (see Chapter 7), or monitoring results from landfills in similar environments. 75

88 Albright / Benson / Waugh - Chapter 4 4. Perform calculations and sensitivity/uncertainty analyses. If performance calculations (runs) include uncertain parameters, a Monte Carlo approach can be used to rapidly create large suites of simulations that input different combinations of parameter values sampled from the uncertainty distributions. The results are a collection of uncertainty distributions that can be compared to the performance objectives. Sensitivity analyses indicate which input parameters the performance metrics are most sensitive to. 5. Document results and iterate previous steps as needed. The results are presented as the probability of exceeding a performance objective. Results can be used to iteratively evaluate alternative designs and components and to select the most suitable WB cover design for conditions at a site. 6. Monitor key performance indicators. Use results of sensitivity analyses to help select parameters for post-closure performance monitoring of the WB cover. The objectives of performance monitoring include (1) provide early warnings of possible deterioration of the cover, (2) compare actual performance results with model predictions, and (3) reiterate and refine long-term performance projections, particularly in response to changes in the environmental setting. Biointrusion Control The baseline ecological survey (see above) should address the potential for intrusion of a WB cover by burrowing animals. If investigations of reference areas indicate that the plant community may create habitat for burrowers that could damage or compromise performance, then the cover design should include deterrents or barriers to burrowing. Burrowing mammals and invertebrates can alter physical and hydraulic soil properties that influence erosion and the soil water balance of covers. For example, pocket gophers burrowing in a hazardous waste landfill at Los Alamos, New Mexico cast more than 12,000 kilograms of soil per hectare to the surface of a landfill over a 14-month period (Hakonson et al. 1982). Displacement of that volume of soil left more than 8 cubic meters of void space per hectare in the cover, or about 2750 meters of tunnel system per hectare. Loose soil cast to the surface by burrowing animals is vulnerable to wind and water erosion. Burrowing animals can alter the soil water balance by decreasing runoff, increasing rates of water infiltration and, 76

89 Albright / Benson / Waugh - Chapter 4 conversely, increasing drying of cover soils after a storm event due to natural drafts within burrows (e.g., Landeen 1994). Burrowing animals can unearth buried waste and displace contaminants to the surface of hazardous waste landfills. Contaminants can also be mobilized by ingestion or in particulates attached to the animal s skin and fur (e.g., McKenzie et al. 1982). Once on the surface, contaminants can then be transferred through higher trophic levels and carried off site (e.g., Arthur and Markham 1983). Physical barriers for animal burrowing and tunneling generally fall into three categories: thick covers, clean rock layers, and compacted soil layers. Ideally, covers could be designed thick enough to contain burrows and prevent intrusion. Because designing covers thick enough to prevent animal intrusion is often impractical given the depths many animals burrow, particularly tunneling invertebrates, physical barriers or deterrents may be necessary. Compacted clay layers may provide some deterrent to burrowing by invertebrates and small mammals until the clay becomes desiccated and cracked (Bowerman and Redente 1998). Compacted clay is an ineffective deterrent to burrowing by larger mammals such as prairie dogs (Shuman and Wicker 1987). Clean, open rock layers have been shown to prevent intrusion by burrowing mammals and insects including harvester ants, pocket mice, and pocket gophers (Cline et al. 1980, Nyhan 1989). However, prairie dogs and ground squirrels have burrowed more than 200 mm into crushed rock layers with a D 50 of 60 mm or less (Cline et al. 1982). Larger rock would be needed to stop larger animals such as prairie dogs and badgers. In summary, an ideal cover would be thicker than the burrowing and tunneling depths of mammals and invertebrates that might inhabit a landfill. Otherwise, clean rock layers placed below soil water storage layers have been shown to be good deterrents for many burrowing animals. Observations at many existing landfills indicate that surface layers of rock also deter burrowing mammals. 77

90 Albright / Benson / Waugh - Chapter 5 Chapter 5 Preliminary Design Water balance covers are designed in two major phases. The first phase is a preliminary design where computations are made using a calculator or a spreadsheet. The objective of the preliminary design computations is to obtain an estimate of the required thickness of the cover and to assess whether a WB cover is viable for a site with the soil resources that are available. The second phase consists of predicting the water balance using a computer model (see Chapter 6). In this second phase, the design is refined based on the outcomes of model predictions and various what if questions are answered. The preliminary design process is based on answering two basic questions: how much water must be stored for the meteorological conditions at the site (i.e., what is the required storage)? how much water can be stored in the proposed cover profile (i.e., what is the available storage)? An implicit assumption made during preliminary design is that the vegetation planted on the cover will remove all of the stored water each year, leaving an empty sponge that is ready to store infiltrating water during the following wet season. Required Storage The required storage (S r ) is the total amount of water to be stored in the cover profile annually for a given meteorological setting. This quantity equals the net infiltration during the wetter period of the year when precipitation exceeds evapotranspiration. A new semi-empirical method to estimate the required storage for WB covers was developed during ACAP using site data collected from the test facilities (Apiwantragoon 2007). The method consists of two parts: (i) identifying periods during the year when water accumulates in the cover and (ii) identifying the amount of water that is stored during these accumulating periods. Although semi-empirical, 78

91 Albright / Benson / Waugh - Chapter 5 the method has general applicability because of the large database that was used to create the method. ACAP consisted of 28 final cover test sections in 11 states that were monitored on an hourly basis for 4-8 yrs. As a result, an enormous database was collected. Periods during which water accumulated were identified by graphing the monthly change in soil water storage against variables affecting storage: P, P/PET, and P PET (P = precipitation, PET = potential evapotranspiration). The data were evaluated on a monthly time interval to ensure sufficient averaging (to reduce noise) while providing adequate resolution of changes in hydrologic conditions. A monthly approach was also deemed practical for design. The analysis showed that P/PET was the best metric to define periods when water accumulation occurs (Apiwantragoon 2007). Regression was used to define thresholds in P/PET beyond which water accumulates in a WB cover (Apiwantragoon 2007). These P/PET thresholds are summarized in Table 5.1. They are segregated by climate (sites with snow and frozen ground vs. sites without freezing conditions) and by the warm and cool seasons in North America (fall-winter and springsummer). The thresholds in Table 5.1 segregate months when water accumulates and months when water is removed from the cover. Water accumulates when the monthly P/PET exceeds the thresholds in Table 5.1. For example, in Montana, which has snow and frozen ground in the winter months, water will accumulate in the fall and winter months when monthly P/PET exceeds 0.51, and in spring and summer when monthly P/PET exceeds If P/PET falls below this threshold, water does not accumulate. Table 5.1. Thresholds of P/PET corresponding to accumulation of water. Climate Type Seasonal Period P/PET Threshold No Snow & Frozen Ground Snow & Frozen Ground Fall-Winter 0.34 Spring-Summer 0.97 Fall-Winter 0.51 Spring-Summer

92 Albright / Benson / Waugh - Chapter 5 A method was also developed to compute the amount of water that accumulates when the P/PET threshold is exceeded. This method is based on a water balance analysis calibrated using ACAP data (Apiwantragoon 2007). The monthly accumulation of soil water storage (ΔS) can be computed using the water balance equation: ΔS = P R ET L P r (5.1) where P is monthly precipitation, R is monthly runoff, ET is monthly evapotranspiration, L is monthly internal lateral drainage, and P r is monthly percolation. Of the quantities on the righthand side of Eq. 5.1, only P is available for design. However, ET can be assumed to be a fraction β of PET, L is very small and can be ignored (Albright et al. 2004), and R and P r can be combined into a loss term Λ. Thus, Eq. 5.1 can be re-written as: ΔS = P β PET Λ (5.2) Eq. 5.2 can be used to compute the monthly accumulation in soil water storage using meteorological data available from NOAA if β and Λ are known. These two independent parameters were obtained by fitting Eq. 5.2 to the entire ACAP data set (Apiwantragoon 2007). A summary of the fitted β and Λ is shown in Table 5.2. Table 5.2. Parameters for Eq. 5.2 obtained by calibration with ACAP data. Climate Type No Snow & Frozen Ground Snow & Frozen Ground Seasonal Period β (-) Λ (mm) Fall-Winter Spring-Summer Fall-Winter Spring-Summer The thresholds in Table 5.1, the parameters in Table 5.2, and Eq. 5.2 can be used to compute the required storage (S r ) in a design year as: 80

93 Albright / Benson / Waugh - Chapter 5 6 S r = ΔS i,fw + ΔS i,ss for ΔS i,fw 0andΔS i,ss 0 (5.3) i=1 12 i=7 where ΔS i,fw is the change in storage during the i th month of fall and winter and ΔS i,ss is the change in storage during the i th month of spring and summer. Both ΔS i,fw and ΔS i,ss are computed with Eq. 5.2 using monthly data and the parameters in Table 5.2. The terms ΔS i,fw and ΔS i,ss are included in Eq. 5.3 only for those months when the monthly P/PET exceeds the thresholds in Table 5.1, and in only those cases where either term is greater than or equal to zero (i.e., terms less than zero are not to be included). The following example for a western landfill illustrates how the method is applied. Analysis of the meteorological data record for the site (collected from a NOAA weather station) showed that the average annual precipitation is 294 mm. The annual precipitation in 1965 (295 mm) is close to the average annual precipitation, and thus 1965 was used as a typical year. The wettest year is 1987, when the annual precipitation was 533 mm. Monthly precipitation and P/PET for the typical year and the wettest year on record are shown in Figure 5.1 and are summarized in Table 5.3. The site in this example receives snow and periodically has frozen ground. Thus, the P/PET threshold is 0.51 for the fall and winter months and 0.32 for the spring and summer months (Table 5.3). These thresholds are exceeded in November and December during the typical year, and from November through February in the wettest year (Figure 5.1 and Table 5.3). Thus, water will accumulate in a WB cover at this site in November and December during the typical year, and from November through February in the wettest year. For the other months, water accumulation is zero. The monthly soil water storage (ΔS) is computed with Eq. 5.2 using the parameters in Table 5.2 (β = 0.37 and Λ = -8.9 mm in fall-winter; β = 1.0 and Λ= in spring-summer). Summing the monthly soil water storage quantities gives the required storage (37 mm for the typical year, 158 mm for the wettest year) (Table 5.3). Thus, the cover needs to be able to store 158 mm for worst-case conditions. 81

94 Albright / Benson / Waugh - Chapter 5 Monthly Precipitation (mm) Typical Wettest 20 0 July Aug Sep Oct Nov Dec Jan Feb March April May June Monthly Precipitation/PET Typical Wettest Spring-Summer Threshold (0.32) Fall-Winter Threshold (0.51) 0.0 July Aug Sep Oct Nov Dec Jan Feb March April May June Figure 5.1. Monthly precipitation (P) and monthly P/PET for the typical year and the wettest year on record for the example site. 82

95 Albright / Benson / Waugh - Chapter 5 Table 5.3. Meteorological data for typical and wettest years for the example site. Month Year Precipitation PET Threshold P/PET (mm) (mm) Exceeded? ΔS (mm) January No 0 February No 0 March No 0 April No 0 May No 0 June No July No 0 August No 0 September No 0 October No 0 November Yes 29 December Yes 8 Typical Year Totals January Yes 48 February Yes 18 March No 0 April No 0 May No 0 June No July No 0 August No 0 September No 0 October No 0 November Yes 83 December Yes 9 Wettest Year Totals Available Storage and Thickness for Monolithic Covers The total storage capacity (S c ) of a cover profile represents all soil water present at when percolation is incipient (just about to occur), and is determined by integrating the field capacity water content over the cover thickness (Figure 5.2). In principle, S c represents the storage status of the soil such that addition of a drop of water at the top of the cover will result in a drop of percolation from the bottom. The water content corresponding to this condition is referred to as the field capacity water content (θ c ). Thus, the storage capacity is computed as: 83

96 Albright / Benson / Waugh - Chapter 5 S c = θ dz θ L (5.4) c c where L is the thickness of the cover and z is the vertical coordinate. Not all of the storage capacity is available for storing water because plants cannot remove some of the water stored in the soil. This water that cannot be removed is described by the minimum water content that can be achieved (θ m ), which is often defined by the wilting point. The available storage (S a ) of a soil layer is the total storage capacity less the water content remaining at the minimum water content (Figure 5.3): S a ( θ ) dz ( θ θ ) = θ L (5.5) c m The available storage capacity represents the volume of water per unit surface area that is available for storing water within the cover, and has units of length. c m θ Τ θ c θ Storage Layer S c L z θc + θt = θ dz 2 0 L Area Figure 5.2. Total soil water storage (S c ) of a soil profile (indicated on the left) of thickness L is determined by integrating the field capacity water content over the thickness of the layer. The water content at the bottom of the layer is the field capacity of the soil. The water content at points higher in the profile must be less than field capacity for equilibrium conditions (i.e., an absence of drainage). The average water content for the profile can be approximated as the average of the field capacity for the soil (θ c ) and the water content corresponding to the soil water suction at the top of the profile (θ T ). In practice, there may be little difference between θ c and θ T. 84

97 Albright / Benson / Waugh - Chapter 5 θ m θ T θ c θ Storage Layer z Area S a L θ θ = m θm 2 0 c T ( θ θ ) dz L Figure 5.3. Available soil water storage (S a ) of a soil profile is the total storage capacity reduced by the amount of water not available for transpiration by plants. In practice, evaporation from the surface may dry the top of the soil below θ m. Using θ m as the lower bound of water content is a conservative approach and will have little effect on the storage capacity. By convention, field capacity is normally assumed to be the water content corresponding to a suction of 33 kpa. The minimum water content, or wilting point, is normally assumed to correspond to a suction of 1500 kpa. Thus, if the SWCC for the cover soil is available, the available storage can be computed directly using Eq This storage capacity tends to be conservative (i.e., the capacity is underestimated) because field capacity can correspond to suctions as low as 10 kpa (i.e., higher water content at incipient drainage) and, in many drier climates, the wilting point suction may be much higher (i.e., lower water content) than 1500 kpa (wilting points of kpa are common in arid lands). The minimum required cover thickness (L) is computed by equating S a (Eq. 5.5) and S r (Eq. 5.3) and solving for L: ( θ θ ) c S r L (5.6) m The denominator of the right hand side of Eq. 5.6 is known as unit available storage capacity (θ u ), and is the difference between the field capacity and wilting point water contents. Unit 85

98 Albright / Benson / Waugh - Chapter 5 available storage capacity represents the volume of water that can be stored per volume of soil, and is dimensionless. This procedure to compute the cover thickness is referred to as the unit available storage capacity method. The results of this method provide a good estimate of cover thickness but do not account for equilibrium gradient conditions. The implications of equilibrium gradient conditions are discussed below and are used to refine the estimate from the unit available storage capacity method. Example The following example illustrates how the water contents at field capacity and wilting point are determined from the SWCC and how the cover thickness is computed using the unit available storage capacity method. SWCCs for two soils are shown in Figure 5.4. SWCCs are normally measured in the laboratory and fit with a constitutive equation, the most common of which is van Genuchten s equation (see discussion in Chapter 3, Eq. 3.3). van Genuchten parameters for the SWCCs (θ s, θ r, α, and n) in Figure 5.4 are shown on the graphs. The SWCC in Figure 5.4(a) has lower air entry suction (larger α) and a less negative slope (larger n) than the SWCC in Figure 5.4(b). For the soil in Figure 5.4(a), the water contents at field capacity (θ c ) and wilting point (θ m ) are calculated with van Genuchten s equation at suctions of 33 kpa and 1500 kpa: θ = c θ = m ( ) ( ) [( 0.50 kpa )( 33 kpa) ] [( 0.50 kpa )( 1500 kpa) ] = = Similarly, for the SWCC in Figure 5.4(b), θ c = and θ m = Once θ c and θ m have been determined, the unit available storage for the soil shown in Figure 5.4(a) is computed as θ u = θ c - θ m : θ = unit available storage = θ u 1 ( θ ) = ( ) = m water m soil c m For the soil shown in Figure 5.4(b), θ u = m water m -1 soil, or approximately twice that of the soil in Figure 5.4(a). Thus, a 1-m-thick cover profile will store 77 mm of water using the soil in Figure 5.4(a) and 157 mm for the soil in Figure 5.4(b). 86

99 Albright / Benson / Waugh - Chapter ψ m = 1500 kpa ψ c = 33 kpa θ s 0.38 θ r 0.01 α (kpa -1 ) 0.50 n 1.50 m 0.33 θ c 0.10 θ m 0.02 θ c - θ m 0.08 Suction (kpa) 10 1 θ m = 0.02 θ c = 0.10 (a) Suction (kpa) 10 1 ψ m =1500 kpa ψ c = 33 kpa θ m = 0.16 θ c = 0.31 θ s 0.41 θ r 0.01 α (kpa -1 ) 0.10 n 1.20 m 0.17 θ c 0.31 θ m 0.16 θ c - θ m 0.15 (b) Water Content Figure 5.4. Soil water characteristic curves (SWCC) for two soils with lower (a) and higher (b) air entry suction. Suctions that define field capacity (ψ c ) and wilting point (ψ m ) are marked as are the corresponding water contents (θ c and θ m ). 87

100 Albright / Benson / Waugh - Chapter 5 The thickness of cover (L) required for the western landfill example described earlier can be computed using Eq For the soil in Figure 5.4(a) and the wettest year (S r = 158 mm), L S ( θ θ ) c m m = r = 2.0 m Similarly, for the soil in Figure 5.4(b), the thickness (L) needs to be at least 1.0 m. An important point to remember when making these calculations is that the units for α and ψ must be the same when using the van Genuchten equation. Both can be defined using pressure units (kpa is most common for SWCCs) or length units corresponding to head of water (m or cm is the most common for SWCCs). The conversion between suction in units of pressure and length is a factor of 10, i.e., 1 m = 10 kpa. Equilibrium Gradient Conditions A unit gradient condition throughout the profile is implicitly assumed when the available storage is computed with Eq However, when percolation is incipient, the hydraulic gradient across the profile must be zero (i.e., the equilibrium gradient concept described in Chapter 3). This implies that the suction at the top of the cover must be greater than the suction at the bottom of the cover by an amount equal to the cover thickness, L (in terms of pressure units, the suction at the top would be higher by L γ w, where γ w is the unit weight of water). As a result, the water content is non-uniform with depth and the storage capacity is actually lower than that computed with Eq Consequently, the thickness computed with Eq. 5.6 may be too small. In many cases, this error is small. However, the error can be significant if the cover soil has low air entry suction and the water content decreases rapidly as the suction increases past the air entry suction. The following example illustrates how to determine water content at the top of the cover and the thickness required for equilibrium gradient conditions. An iterative method is required, and the thickness computed using Eq. 5.6 using the unit available storage approach is a good starting point. At incipient breakthrough, the suction at the bottom of the cover is 33 kpa and the corresponding water content at that point is determined by solving the van Genuchten equation (as shown above). Under equilibrium gradient conditions, the suction at the top (2.05 m, from previous calculation) of the cover must balance the increase in potential energy corresponding to the higher elevation. That is, the suction at the top of the cover must be greater than the suction at the bottom of the cover by L γ w. In practical terms, under equilibrium gradient conditions an 88

101 Albright / Benson / Waugh - Chapter 5 increase in elevation of 1 m corresponds to an increase in suction of approximately 10 kpa. For the soil in Figure 5.4(a): ψ = 33 kpa ( L in m)( 10 kpa m ) = 33kPa + ( 2.05 m) ( 10 kpa m ) = 33 kpa kpa = 53. kpa T 5 The water content at the top of the cover (θ T ) corresponding to the suction at the top of the cover (ψ T ) is computed using van Genuchten s equation: 1 θ T = ( ) = [( 0.50 kpa )( 56.6 kpa) ] The average water content of the cover at the equilibrium gradient condition can be estimated as the arithmetic average of the water contents at the top and bottom of the cover: ( )/2 = Using as θ c in Eq. 5.6 yields a cover thickness of 2.36 m, or approximately 0.3 m larger than the estimate using the unit available storage method. Re-computing ψ T and θ T again for a cover 2.36-m thick yields: 1 1 ( L in m)( 10 kpa m ) = 33kPa + ( 2.36 m) ( 10 kpa m ) = 33 kpa kpa = 56. kpa ψ T = 33 kpa As before, the θ T corresponding to ψ T is computed using van Genuchten s equation, which yields θ T = Thus, the average water content is ( )/2 = and the cover thickness, L, computed with Eq. 5.6 is 2.39 m. This thickness is only 30 mm larger than the 2.36 m thickness computed for the first iteration, and is considered sufficiently accurate. A similar set of computations can be made for the soil in Figure 5.4(b). The first iteration for the computations for the soil in Figure 5.4(b) yields a cover thickness of 0.87 m, which is only 30 mm thicker than the cover thickness computed using the unit available storage approach. Thus, for the soil in Figure 5.4(b), which has higher air entry suction and more gradually varying water content for suctions in excess of the air entry suction, the unit available storage approach is adequate. 89

102 Albright / Benson / Waugh - Chapter 5 Thickness Calculation Procedure 1. Determine the water content at field capacity (θ c ) at 33 kpa and the wilting point (θ m ) at 1500 kpa using van Genuchten s equation. These quantities can be estimated visually from the SWCC but are more accurately determined by calculation. 2. Estimate the required thickness using the unit available storage method via Eq. 5.6 using θ c and θ m from Step 1 as input. 3. Determine the suction (ψ T ) and water content (θ T ) at top of the cover assuming an equilibrium gradient, where ψ T = 33 kpa + (L γ w ) and θ T is computed with van Genuchten s equation. 4. Determine the average water content of the cover at field capacity: (θ c + θ T )/2. 5. Use the average water content in Step 4 as θ c in Eq. 5.6 and re-compute the cover thickness, L. 6. Repeat Steps 3-5 until L no longer changes appreciably. The final L is the required minimum thickness of the cover. Available Storage and Thickness for Capillary Barriers Capillary barriers consist of a fine-textured layer over a coarse-textured layer. The interface between the layers acts as a barrier to the downward flow of water under unsaturated conditions, and effectively increases the available storage capacity of the fine-textured soil. Capillary barriers achieve this increase in storage by increasing the water content at the bottom of the layer at which drainage first occurs. The fine-over-coarse arrangement of soil layers characteristic of a capillary barrier is shown in Figure 5.5. Under the conditions of a continuum, the water pressure in the soil pores at the interface between the layers is the same regardless of whether the perspective is from the upper fine-textured layer or lower coarse layer (i.e., the fine and coarse layers have identical soil water suction at the interface). This commonality in suction is the key factor responsible for the capillary barrier function (Khire et al. 2000). Use of a coarse soil layer as a barrier to flow generally is counter intuitive, as coarse soils generally are accepted as being much more permeable than fine-textured soils (e.g., that is why coarse soils are used for drainage). However, under unsaturated conditions, such as those in WB 90

103 Albright / Benson / Waugh - Chapter 5 covers, coarse soils can be much less permeable (i.e., have much lower unsaturated hydraulic conductivity) than fine-textured soils. Figure 5.5. Schematic of a capillary break illustrating the fine-over-coarse soil layering. The capillary break is created by continuity in pore water across the interface between the finer and coarse layers, which creates equal suction at points adjacent to the interface. SWCCs for fine-textured and coarse-grained soils are shown in Figure 5.6. In a monolithic design, a small increase in water content above field capacity of the storage layer results in two conditions: (1) lower suction (less than 33 kpa) at the bottom of the cover and (2) drainage until the soil dries to the water content corresponding to the field capacity suction. In a capillary barrier, the same small decrease in suction (increase in water content) at the bottom of the finer soil layer does not result in drainage due to the very low conductivity of the underlying coarse layer at that suction. The net result is to increase the water content at the bottom of the finer soil layer (and throughout the finer layer according to the equilibrium gradient principle) at which drainage occurs. In fact, appreciable flow into the coarse layer will not occur until the suction at the interface between the layers drops to the breakthrough suction (ψ B, Figure 5.6) for the coarse soil. The breakthrough suction for the coarse soil corresponds to the point near the dry end of the SWCC where the water content begins to increase as the suction diminishes. As a result, by adding the coarse layer, the water content at the bottom of the fine-textured layer can increase to a water content (θ BF ) corresponding to breakthrough suction for the coarse soil before appreciable flow into the coarse layer will occur (Khire et al. 2000). Moreover, coarse soils with lower ψ B result in greater θ BF (and more storage capacity) in the fine-textured soil. Low breakthrough suction in the coarse layer requires relatively large pores (low air entry suction) 91

104 Albright / Benson / Waugh - Chapter 5 and a narrow distribution of pore sizes (to create a flat SWCC). Accordingly, clean and uniformly graded coarse-grained soils (sands and gravels) are used to create capillary breaks. Suction Increase in water content at incipient drainage Finer ψ c (33 kpa) ψ B Coarser Water Content θ c θ BF Figure 5.6. SWCCs for a finer and coarser soil show the increase in storage created by the capillary break. At suctions greater than ψ B the pores in the coarse layer lack sufficient suction to maintain saturation. Without the underlying coarse layer the finer-textured soil would drain at field capacity water content (θ c ). With a capillary break, drainage does not occur until the suction at the interface of the finer and coarse layers reaches ψ B, which results in higher water content (θ BF ) and greater storage in the finer layer at break through. Thickness Calculation Procedure The procedure described earlier for monolithic covers can be adapted to determine the thickness of the fine-textured layer in a capillary barrier. The procedure is as follows: 1. Determine an initial estimate of the thickness of the fine-textured layer using the unit available storage capacity method for monolithic barriers. 2. Determine the breakthrough suction (ψ B ) for the coarse layer by visual examination of the SWCC for the coarse soil. 3. Determine the water content of the fine-textured soil corresponding to the breakthrough suction of the coarse layer (θ BF at ψ B ) using van Genuchten s equation and parameters for the SWCC of the fine-textured soil. 92

105 Albright / Benson / Waugh - Chapter 5 4. Determine the water content at the wilting point (θ m ) at 1500 kpa for the fine-textured layer using van Genuchten s equation. 5. Determine the suction (ψ T ) and water content (θ T ) at top of the cover assuming an equilibrium gradient, where ψ T = ψ B + (L γ w ) and θ T is computed with van Genuchten s equation from the SWCC for the fine-textured soil. 6. Determine the average water content of the fine-textured layer at breakthrough: (θ BF + θ T )/2. 7. Use the average water content in Step 6 as θ c in Eq. 5.6 and re-compute the required thickness (L) of fine-textured layer. 8. Repeat Steps 5-7 until L no longer changes appreciably. The final L is the required thickness of the cover. Example Determine the required thickness of the fine-textured layer in a capillary barrier to provide adequate storage for the example site (see above) assuming that the fine layer has the SWCC shown in Figure 5.7(a) and the coarse layer is Grand Junction sand, which has the SWCC shown in Figure 5.7 (a, b). Inspection of the SWCC for Grand Junction sand indicates that the breakthrough suction (ψ B ) is approximately 10 kpa. Thus, from the van Genuchten equation at ψ B = 10 kpa, θ BF for the fine-textured layer (Figure 5.7(a)) is That is, adding a capillary break increases the water content at the base of the fine textured at breakthrough from to 0.366, an increase of (i.e., 50 mm of additional water storage per meter of cover soil). The suction at the top of the fine-textured layer shown in Figure 5.7(a) is computed as: 1 ( 10 kpa m ) = 10 kpa kpa = 18. kpa ψ T = 10 kpa + L 10 = m 7 The water content corresponding to ψ T is computed from van Genuchten s equation: θ T = Thus, the average water content in the fine-textured layer is ( )/2 = The required thickness of the fine-textured layer is then computed from Eq. 5.6 using θ c = 0.353, which gives L = m. An additional iteration yields: 1 ( 10 kpa m ) = 10 kpa kpa = 17. kpa ψ T = 10 kpa + L 10 = m 0 93

106 Albright / Benson / Waugh - Chapter θ c = θ B = ψ c = 33 kpa 10 ψ B = 10 kpa Suction (kpa) 1 (a) θ c = θ B = ψ c = 33 kpa 10 ψ B = 10 kpa Suction (kpa) 1 (b) Water Content Figure 5.7. SWCCs for three soils illustrating the capillary barrier effect. The coarse soil (Grand Junction sand) is the same in both graphs (θ s =0.29, θ r =0.08, α=0.72 kpa -1, n=2.78). Unsaturated parameters for the finer-textured soil are θ s =0.32, θ r =0.00, α=0.215 kpa -1, and n=1.2 (a) and θ s =0.38, θ r =0.00, α=0.030 kpa -1, and n=1.20 (b). Note the difference in effect of the capillary barrier due to differences in unsaturated parameters of the fine-textured soil. 94

107 Albright / Benson / Waugh - Chapter 5 The corresponding θ T is 0.345, and the average water content is estimated as ( )/2 = From Eq. 5.6, the new estimate of L is m. One more iteration yields L = m as well. Thus, the final minimum thickness of the fine-textured layer in a capillary barrier is 0.69 m, which is 0.31 m thinner than the monolithic cover computed using the unit storage method. The SWCCs shown in Figure 5.7(b) demonstrate the capillary barrier effect for Grand Junction sand and another fine-textured soil with different unsaturated hydraulic parameters. Use of Geotextiles for Capillary Breaks Laboratory tests indicate the hydraulic properties of nonwoven geotextiles and geosynthetic drainage layers may be suitable for use as the coarse layer in a capillary barrier design. The very low air entry suction and rapid change in saturation with suction (i.e., a flat SWCC) associated with geotextiles are ideal attributes for a capillary break. However, to date, there have been no field tests of geosynthetic materials specifically to evaluate their use to create a capillary break in a cover. Unsaturated hydraulic properties of geotextiles can be found in Stormont et al. (1997). Field Application of SWCCs Measured in the Laboratory Hydraulic properties of a soil sample compacted and analyzed in the laboratory may differ from those in an actual field application due to scaling effects, hysteresis, and alterations in soil structure caused by pedogenic processes such as freeze-thaw and wet-dry cycling and biointrusion (root growth and death, burrowing fauna). Post-construction changes in soil structure typically result in lower density and formation of larger pores, and these changes tend to be larger for more clayey soils. These changes generally result in lower available storage capacity. The difference in conditions in the field and laboratory is illustrated in Figure 5.8, which shows water content at incipient percolation (a practical measure of field capacity) in the ACAP test sections vs. the field capacity water content obtained from SWCCs determined using two different methods. In one method, field capacity was obtained from a SWCC measured in the laboratory using ASTM D 6836 at 33 kpa (solid circles). This common laboratory method yields a drying SWCC (i.e., the specimen begins saturated, and then is dried by incrementally 95

108 Albright / Benson / Waugh - Chapter 5 increasing the suction). Nearly all of the solid circles, which correspond to the laboratorymeasured curve, fall below the 1:1 equality line, indicating that the storage capacity in the field is lower than the field capacity obtained from conventional laboratory-measured SWCCs. The other method (open circles in Figure 5.8) consisted of creating SWCCs using water contents and suctions measured in the field using co-located sensors in the ACAP test sections. In effect, this latter method provides a field-measured SWCC for wetting conditions (i.e., the condition leading to transmission of percolation). Good agreement exists between the water content at incipient percolation and the field capacity obtained from field-measured SWCCs at 33 kpa. Field Water Content at Incipient Percolation from ACAP Test Sections Lab Drying SWCC Field Wetting SWCC Mean Field Capacity from SWCC Figure 5.8. Relationship between field capacity water content from SWCCs and the water content at incipient drainage from test section data. The SWCCs were determined by two methods. The points marked as Lab Drying SWCC were determined by standard laboratory method (ASTM D 6836); those marked Field Wetting SWCC were determined from co-located sensors for suction and water content in the test sections (adapted from Apiwantragoon 2007). 96

109 Albright / Benson / Waugh - Chapter 5 Analysis of large, undisturbed samples from the ACAP test sections in the as-built condition and following multiple years of exposure to field conditions showed significant changes in the unsaturated properties (Figure 5.9 (a-d)) and in the shape of the SWCC (Figure 5.10). Changes to saturated volumetric water content (θ s ) (Figure 5.9a): θ s tends to increase (density tends to decrease) with time. Soils placed with higher θ s at construction tend to exhibit a smaller change in θ s. The finer-textured soils commonly used for WB covers undergo larger changes in θ s compared to coarse-textured soils with little fines. Changes to saturated hydraulic conductivity (K s ) (Figure 5.9b): Soils placed with initially low K s (< cm/sec) often show a large increase in K s. Soils placed with initially higher K s (> cm/sec) in the as-built condition demonstrated a much smaller increase. Regardless of as-built K s, after a few years of field exposure the K s of most soils range between and cm/sec. Changes to the unsaturated parameters (α and n) (Figs. 5.9c, d): The α parameter generally increased (some by nearly two orders of magnitude) after a few years of exposure. The largest increases in α were for soils with lower initial α. This indicates that the formation of larger pores has greater effect on soils with an initial network of small pores compared to soils that initially contain both large and small pores. The n parameter generally decreased; changes were small relative to the range over which n can vary in natural soils. Changes in n were greater for soils placed with greater initial value of n reflecting a greater change in pore size distribution for soils that initially have a narrower pore size distribution. 97

110 Albright / Benson / Waugh - Chapter 5 (c) Coarse-textured, broadly-graded (a) (b) (d) Figure 5.9. Postconstruction changes in saturated volumetric water content (a), saturated hydraulic conductivity (b) and the van Genuchten parameters α (c) and n (d). Results are from the ACAP project (Benson et al. 2007) and are shown as the ratio of the postconstruction to the as-built values plotted vs. the as-built property. 98

111 Albright / Benson / Waugh - Chapter 5 Matric Suction (kpa) Corrected Lab SWCC α = kpa -1 lab α = kpa -1 field n = 1.30 lab n = 1.69 field 33 kpa Volumetric Water Content Figure Example of the correction procedure applied to a laboratory SWCC. A correction method to account for differences between field and laboratory conditions was developed using the SWCCs measured in the field and laboratory during ACAP (Apiwantragoon 2007). This method consists of applying a scaling factor to the van Genuchten parameters (α and n) obtained from the laboratory-measured SWCC so that the laboratorymeasured SWCC is representative of field conditions. This method can be summarized as follows: α: multiply by a factor of 1.3 for less plastic soils, 12.9 for more plastic soils. n: multiply by a factor of 1.1 for less plastic soils, 1.2 for more plastic soils. More plastic soils generally classify as clays and plastic silts (CL, CH, MH) in the Unified Soil Classification System (USCS). The less plastic soils generally classify as sands and elastic silts (SM, SC, ML, CL-ML) in the USCS. An example of the correction procedure for a more plastic soil is shown in Figure In this example, θ c decreases from 0.39 (laboratory measured) to 0.20 (corrected). 99

112 Albright / Benson / Waugh - Chapter 6 Chapter 6 Introduction to Water Balance Modeling Once a preliminary design for a WB cover has been selected, water balance simulations typically are conducted with a computer model. This modeling exercise is conducted for several reasons, all or some of which may be relevant to a given project: to refine and validate the design with realistic meteorological data, to evaluate sensitivity of cover performance to design variables (e.g., meteorological input, soil properties, cover configuration, vegetation characteristics, etc.), to compare the performance of a proposed WB cover to that of a conventional cover, to predict performance of the WB cover relative to a particular design criterion, or to conduct what if? analyses that address specific issues relevant to a site or its stakeholders. The anticipated outcome of the modeling exercise is a refined conceptual understanding of the mechanisms important to cover performance and a realistic quantitative assessment of performance that has a high degree of reliability. Thus, the hydrological processes occurring in the field must be simulated as realistically as practical. Model Attributes Computer models used for simulating the hydrological performance of WB covers must properly account for the physics controlling water movement within the simulated soil profile and at the boundaries of the profile with the atmosphere and the underlying waste. This is particularly important in sensitivity analyses when a model is used to discern the importance of small changes in a design parameter. Models must also account for water removal by roots within the soil profile. In this section, important attributes of computer models used to simulate WB covers are described. 100

113 Albright / Benson / Waugh - Chapter 6 The most important attribute is that the model must be based on a solution of the fundamental partial differential equation governing water flow in unsaturated soil, namely Richards equation: θ K ψ = K S t z z z (6.1) where θ is volumetric water content, z is the vertical coordinate, t is time, ψ is matric suction, K is the hydraulic conductivity at suction ψ, and S is root water uptake at depth z and time t. Eq. 6.1 is shown in one-dimensional format for convenience, but can be written in two or three dimensions, and can be expanded to include other mechanisms such as water flow in the vapor phase or thermally driven flows. Inclusion of a term for root water uptake is particularly important. Removal of water throughout the soil profile via plant transpiration is a key mechanism affecting the water balance of most WB covers and must be simulated with a reasonable degree of realism. Applying a surface boundary that accounts for evaporation and transpiration in lieu of a direct mechanism for root water uptake is not sufficient. The computer model must also include a surface boundary that simulates interactions between the soil, plants, and atmosphere (often called an atmospheric boundary condition ). This boundary must explicitly account for precipitation, infiltration, evaporation, and runoff, and should be driven by site-specific meteorological data input by the user. In addition, a variety of lower boundaries should be available to account for different types of interactions between the cover and the underlying waste (Benson 2007). A suitable computer model should also report engineering quantities of interest as output. The most commonly used quantities, and those most important to design, are runoff (R), evaporation (E), transpiration (T), soil water storage (S), and percolation (P r ). Water content and suction as a function of depth and time are also useful outputs for interpreting the water balance quantities that are predicted, and for checking the reasonableness of a simulation. A list of models that meet these criteria is in Table 6.1. This list is not exclusive, but it does represent the models typically used in North American practice that have the aforementioned attributes. Noticeably absent from this list is the HELP model, which was developed by US EPA to simulate landfill hydrology (Schroeder et al. 1984). HELP is useful for water balance evaluations of landfills and for evaluating how various mechanisms affect the 101

114 Albright / Benson / Waugh - Chapter 6 landfill water balance. However, HELP does not simulate hydrological processes within covers with sufficient realism or accuracy to be useful for design of WB covers (Khire et al. 1997). Each of the computer models listed in Table 6.1 employs a numerical method to obtain a very precise, but approximate solution to Eq The finite difference (FD) and finite element (FE) methods are the most common numerical methods employed to solve Eq The soil profile is divided into a series of nodes (FD) or elements (FE) as shown in Figure 6.1, and the derivatives in Eq. 6.1 are approximated by slopes between adjacent points. A solution [θ(z, t)] is obtained at each node, and conditions between the nodes are obtained by interpolation. Configurations with more nodes (or smaller elements) generally provide more accurate solutions because approximations are made over smaller intervals, which permit a more realistic representation of non-linearity. Figure 6.1. Example of spatial discretization used in a numerical model. All of the models listed in Table 6.1 provide output that appears very realistic. For example, water balance quantities predicted with the LEACHM model are shown in Figure 6.2 for a WB cover at a landfill in northern California. The soil water storage record has a pattern similar to that present in field data (e.g., Figure 2.5), with many irregular changes in slope in response to precipitation events. The evapotranspiration curve also varies seasonally as observed in the field. Despite this realistic appearance, the water balance quantities shown in Figure 6.2 are not real. They are just a prediction from a computer model. This important difference must always be kept in mind when using computer models to simulate the hydrology of WB covers (or other engineering systems). Computer models provide predictions that reflect the assumed physics, the mathematical simplifications employed in the underlying algorithms, and the input 102