GROUNDWATER SENSITIVITY AND SOLID WASTE DISPOSAL IN MINNESOTA

Transcription

1 GROUNDWATER SENSITIVITY AND SOLID WASTE DISPOSAL IN MINNESOTA Stuart Grubb, PG E. Calvin Alexander, Jr. PhD University of Minnesota May 7, 2009 Final Draft Approved for general distribution May be cited with attribution Friends of Washington County 2009 swrule1-01

2 Groundwater Sensitivity and Solid Waste Disposal in Minnesota Contents Executive Summary Introduction and Background Definitions Groundwater and Aquifers Groundwater Quality and Groundwater Contamination Sensitivity Objectives Groundwater Flow Unsaturated Flow Saturated Flow through Porous Media Aquifers Fracture Flow Aquifers Fractured Rock Aquifers Karst Aquifers Minnesota Geology and Groundwater Flow General Aquifer Characteristics Igneous and Metamorphic Sedimentary Karst Unconsolidated Glacial Unconsolidated Non-Glacial Regional Groundwater DNR Groundwater Provinces Groundwater Profiles Extent of Karst Areas Contaminant Transport Advection, Dispersion, and Diffusion Sorption Contaminant Reactions and Degradation Parameters, Measurements, and Testing Groundwater Flow Unsaturated Flow...27 Groundwater Sensitivity and Solid Waste Disposal 2

3 7.1.2 Saturated Porous Media Saturated Fracture Flow and Karst Isotopes and Other Environmental Tracers Modeling Time of Travel Contaminant Monitoring Groundwater Sensitivity Classifications Methodology Range of Sensitivity Evaluating Aquifer Materials Evaluating Travel Times Excluding Landfill Sites Based on Hydrogeologic Conditions Methodology Determining the Siting Region Identifying Target Areas Screening Candidate Sites Conclusions and Recommendations References...39 The Friends of Washington County 41 Figures Figure 1. Minnesota Bedrock Aquifers...16 Figure 2. Minnesota Karst Lands...18 Figure 3. Minnesota Glacial Deposits...19 Figure 4. Minnesota Groundwater Provinces...20 Figure 5. Idealized Contaminant Plume...27 Tables Table 1. Groundwater Sensitivity Ranking...33 Table 2. Classification of Unconsolidated Materials...34 Groundwater Sensitivity and Solid Waste Disposal 3

4 Executive Summary Introduction and Background The 2008 Legislature charged the MPCA to review current rules and policies, and to develop new rules that will provide the MPCA with a process to include groundwater sensitivity and financial assurance to reduce risks to groundwater. By January 15, 2010, the MPCA is to deliver a report to the Legislature on proposed rules. This paper proposes new methods of hydrogeologic site investigation for siting a landfill. It proposes a fundamental shift in the methods for determining site suitability. Currently a site is selected, and then an investigation is conducted to determine if the site meets certain minimum characteristics. With the proposed methods, a regional landfill siting area is identified, and then the best landfill site is selected from the areas that are least sensitive to groundwater contamination. Definitions MDNR (1991) defined groundwater sensitivity as time of travel, the time required for a contaminant to move vertically from the land surface to an aquifer This paper uses the same definition of sensitivity, with one key exception. MDNR (1991) and other publications consider only water flow vertically downward to the first aquifer. This paper also considers lateral migration of groundwater and contaminants to and in the saturated zone. Objectives We have based our recommendations on the assumption that at least one landfill constructed in Minnesota in the future will release contaminants into the subsurface. Proactive siting of that landfill (and all others) in the best location available will minimize the threats to human health and the environment. The best location will have subsurface characteristics that slow the migration of contaminants to water supplies or to surface waters. Groundwater Sensitivity Classifications The time of travel for contaminants moving through the vadose zone and aquifer is the primary factor that should be used to determine groundwater sensitivity. Time of travel is a good surrogate when accounting for other factors that might reduce contaminant concentrations and groundwater sensitivity, such as sorption and chemical reactions. In general, as time of travel increases due to the presence of silt and clay in the aquifer, the potential for sorption and favorable chemical reactions also increases. Range of Sensitivity Table 1 shows a landfill site s sensitivity to groundwater contamination based on hydrogeologic characteristics. The range of sensitivity is given on a scale of 10 (high sensitivity) to 1 (low sensitivity). Groundwater Sensitivity and Solid Waste Disposal 4

5 Table 1. Groundwater Sensitivity Ranking Water Table Aquifer - Most Sensitive Material Water Table Aquifer - Groundwater Age Dating Carbonate o coarse clasti overlain by a karst surfac Carbonate o coarse clasti without a karst surfac Other bedroc with fracture flow Sand and gravel Sandy silt tolean clay an clayey silt fat clay Other bedroc without fracture flow Less than 1 year to 10 years to 100 years More than 100 years Evaluating Aquifer Materials For purposes of groundwater sensitivity classification, unconsolidated aquifer materials are grouped into three categories, as shown in Table 2.. Table 2. Classification of Unconsolidated Materials Description High Permeability/ Sand and Gravel Moderate Permeability/ Sandy Loam and Silt Low Permeability/ Clay Hydraulic conductivity Unified Soil Classification System >10-3 cm/s Clayey gravel (GC), Silty gravel (GM), Poorly graded gravel (GP), Well graded gravel (GW), Poorly graded sand (SP), Well graded sand (SW) 10-3 to 10-6 cm/s Silty sand (SM), Clayey sand (SC) <10-6 cm/s Fat clay (CH), Lean clay (CL) Four categories are proposed for consolidated bedrock aquifers. From most sensitive to least sensitive, they are: Carbonate and coarse clastic aquifers overlain by a karst surface. Carbonate and coarse clastic aquifers without a karst surface. Other bedrock with fracture flow. Other bedrock without fracture flow. Groundwater Sensitivity and Solid Waste Disposal 5

6 These bedrock aquifer categories were selected on two basic premises: 1. Contaminant transport through fractures and solution channels is faster and therefore more significant than contaminant transport through porous media. 2. Solution cavities and karst features are common in carbonates, even if they are not encountered in a few boreholes at a site. Evaluating Travel Times A key recommendation of this paper is that groundwater travel times from the surface or near surface to the water table aquifer must be directly measured using the methods described in Section and However, it may not be possible to directly measure the groundwater age at each of the many locations being considered for siting a particular landfill. Regulators and landfill proposers need to work together to develop conceptual models of travel time based on the geologic materials found in the area. The conceptual models would need to be calibrated and validated using actual field test results. Excluding Landfill Sites Based on Hydrogeologic Conditions Methodology MPCA should adopt a landfill siting method based on the premise that landfills should be sited in the most favorable locations with the lowest groundwater sensitivity within a designated search area. Landfills should be excluded from less favorable locations. That is, a potential landfill site with a higher groundwater sensitivity ranking (according to Table 1) should be excluded if a site with a lower groundwater sensitivity ranking exists within the search area. MPCA should require tracer testing as described in Sections and for all sites being considered for landfill siting. While it is probably not possible to exclude sites based on certain values or concentrations of tracers, the data should be used to compare potential landfill sites. Landfill sites with tracer tests indicating older groundwater or slower recharge should be given preference over sites with younger, more sensitive groundwater. Siting Regions If taken to an illogical extreme, the proposed method could lead to all landfills in Minnesota being sited within one very small area with optimal hydrogeologic conditions. To prevent this, each proposed landfill should be assigned a large siting region within which waste transport and disposal would be practical. The siting region would be different for various waste types and various parts of the state. The proposed method is based solely on a site s geologic and hydrogeologic conditions. The socalled three-legged stool concept that incorporates waste segregation, engineered barriers and systems, and hydrogeologic conditions is not relevant to the siting process. Waste segregation and engineered barriers and systems certainly have a role in landfill design, construction, operation, and closure, but not in site selection. MPCA should also prohibit all landfills on all sites with high Groundwater Sensitivity Rankings shown on Table 1.. For example, landfills should be prohibited on sites with a groundwater sensitivity ranking of 8, 9, or 10. Groundwater Sensitivity and Solid Waste Disposal 6

7 Each proposed landfill would have its own siting region based on the location and the type of waste. The determination should be part of the scoping/eaw process, subject to public input and regulatory review. The Responsible Governmental Unit (RGU) would delineate the siting region. The size of the siting region would be partly based on type of waste and the environmental risk associated with disposal. Low-volume, high-risk wastes such as radioactive wastes would have the largest siting regions, possibly extending outside the state. Industrial waste and mixed municipal solid waste would have a siting region that might cover several counties. The siting region might also be extended to avoid karst areas. High-volume, low-risk wastes such as construction and demolition fill would have the smallest siting region. Identifying Target Areas After the siting region has been established, the siting process should identify the areas within the siting region that have the best hydrogeologic characteristics (the target areas ). First, areas with higher groundwater sensitivity, as indicated in Table 1, will be disqualified in favor of target areas with the lowest groundwater sensitivity. The goal of this process is to identify the target areas within the siting region with the lowest groundwater sensitivity. The target areas identification process will likely use existing regional hydrogeologic data to identify the target areas. It is important to use the most recent and best available data. The siting process also should exclude areas from the target area that do not meet current regulatory standards for landfill sites. Minnesota Rules (i.e ) and agency policies prohibit landfill siting near critical areas such as water resources, roads, occupied dwellings, etc. Screening Candidate Sites The next step in the siting process is for the landfill proposer to identify and screen candidate sites within the acceptable target areas. The landfill proposer can select specific proposed sites based on secondary criteria such as land cost, existing infrastructure, transportation costs, construction and operating costs, etc. The landfill proposer may submit one or more sites for regulatory permitting. Extensive site-specfic geologic and hydrogeologic testing will be required for the permits. This includes groundwater age dating and confirmation that the groundwater sensitivity ranking is valid. Conclusions and Recommendations MPCA should adopt a landfill siting method based on the premise that landfills should be sited in the most favorable locations with the lowest groundwater sensitivity within a region considered for landfill siting. Determining the region considered for landfill siting should be part of the scoping/eaw process, subject to public input and regulatory review. The Responsible Governmental Unit (RGU) would determine the region considered for landfill siting. The time of travel for contaminants moving through the vadose zone and aquifer is the primary factor that should be used to determine groundwater sensitivity. Groundwater Sensitivity and Solid Waste Disposal 7

8 MPCA should prohibit all landfills on all sites with Groundwater Sensitivity Rankings of 8, 9, or 10 shown on Table 1.. MPCA should require tracer testing as described in Sections and for all sites being considered for landfill siting. MPCA should develop standards for the type and number of tests required for a evaluating a potential landfill site under this system. Groundwater Sensitivity and Solid Waste Disposal 8

9 Groundwater Sensitivity and Solid Waste Disposal in Minnesota 1 Introduction and Background Minnesota Statutes (Subdivision 4) states that: The rules for the disposal of solid waste shall include site-specific criteria to prohibit solid waste disposal based on the area's sensitivity to groundwater contamination, including sitespecific testing. The 2008 Legislature also imposed a moratorium on the siting of most new landfills. It directed the MPCA to reexamine its methods for determining groundwater sensitivity to pollution from solid waste management facilities. Specifically, it charged the MPCA to review current rules and policies, and to develop new rules that will provide the MPCA with a process to include groundwater sensitivity and financial assurance to reduce risks to groundwater. By January 15, 2010, the MPCA is to deliver a report to the Legislature on proposed rules. A report produced by Construction and Demolition and Industrial Solid Waste Landfill (CDIL) Work Group (MPCA Work Group, 2009) concluded: Current Minnesota Rules, part , subpart 3, require a detailed investigation to determine hydrogeologic characteristics for siting and expanding mixed municipal solid waste (MMSW) and combustor ash landfills. This investigation is used to develop a conceptual model of groundwater flow, which serves as the basis for design of a groundwater monitoring network to detect and intercept a contaminant release. These same rules are currently applied by policy for construction and demolition (C&D) and industrial landfill siting and expansion, although the scope of the investigation may be reduced based upon the presumed level of risk posed by the specific waste type to be landfilled. Under current practice, results from hydrogeologic investigations at a site are used primarily for determining if a site is monitorable and remediable, and for design of the groundwater monitoring system. The hydrogeologic investigation is not explicitly used to determine general suitability of a landfill site or to establish engineered control requirements (except for depth to the water table). Guidance should be developed, as a transition to rules, that addresses the scope and use of a hydrogeologic site investigation in determining site suitability and engineered control requirements as part of a risk-based environmental performance evaluation. This paper proposes new methods of hydrogeologic site investigation for siting a landfill. It proposes a fundamental shift in the methods for determining site suitability. Currently a site is selected, and then an investigation is conducted to determine if the site meets certain minimum characteristics. With the proposed methods, a regional landfill siting area is identified, and then the best landfill site is selected from the areas that are least sensitive to groundwater contamination. Groundwater Sensitivity and Solid Waste Disposal 9

10 The purpose of this paper is to: Provide common definitions to groundwater-related terms and concepts to facilitate better communication Examine the concept of groundwater sensitivity in the context of landfill siting and Minnesota s geology Report on advancements in investigation and analysis techniques that should be utilized in landfill siting Recommend a methodology for determining groundwater sensitivity when siting landfills Recommend criteria for excluding landfill sites from consideration based on hydrogeologic characteristics 2 Definitions 2.1 Groundwater and Aquifers The definition of groundwater is crucial in regulatory matters. The simplest definition, which is useful in many contexts, is any water that is below ground surface. When considering landfill siting, this definition leads to many practical problems that make it unusable. For example, water in the unsaturated zone just below the ground surface is found everywhere and will be highly sensitive to pollution. The definition of groundwater often differs according to the problem being considered. When considering a detailed water budget for a site or a region, water in the unsaturated zone and the effects of evapotranspiration can be very significant. When considering how much water can be pumped from regional aquifers, groundwater in the unsaturated zone or small areas of perched groundwater are no longer included under the practical definition of groundwater. The State of Minnesota defines groundwater different ways for different purposes: Minnesota Statutes, Section Subd. 6 and Minnesota Rules Section "Groundwater" means water contained below the surface of the earth in the saturated zone including, without limitation, all waters whether under confined, unconfined, or perched conditions, in near-surface unconsolidated sediment or regolith, or in rock formations deeper underground. Minnesota Administrative Rules, Definitions, Subp. 11. "Groundwater" means subsurface water in the saturated zone. The saturated zone may contain water under atmospheric pressure (water table condition), or greater than atmospheric pressure (artesian condition). Groundwater Sensitivity and Solid Waste Disposal 10

11 Minnesota Administrative Rules (Department of Agriculture, Pest and Disease Control). "Groundwater" means the water in the zone of saturation in which all of the pore spaces of the subsurface material are filled with water. The water that supplies springs and wells is groundwater. Minnesota Administrative Rules (Department of Health, Wells and Borings). "Groundwater" has the meaning given in Minnesota Statutes, section , subdivision 6, and does not include water in an artificially created basin, such as a tank excavation, that is not hydrologically connected to the earth outside the basin. Minnesota Statutes, Section 103B.305 Subd. 4. "Groundwater systems" means the 14 principal aquifers of the state as defined by the United States Geological Survey in the Water-Resources Investigations 81-51, entitled "Designation of Principal Water Supply Aquifers in Minnesota" (August 1981), and its revisions. When considering groundwater issues on a larger scale, the term aquifer is often more descriptive and useful. Minnesota has definitions for different types of aquifers: Minnesota Administrative Rules (Department of Health, Wells and Borings) and (Pollution Control Agency, Solid Waste). "Aquifer" means a stratum of saturated, permeable bedrock or unconsolidated material having a recognizable water table or potentiometric surface which is capable of producing water to supply a well. Minnesota Administrative Rules, Definitions, Subp. 2. "Aquifer" means any water-bearing bed or stratum of earth or rock capable of yielding groundwater in sufficient quantities that can be extracted. Minnesota Administrative Rules, Definitions, Subp. 4. Artesian aquifer or confined aquifer means a water body or overlain by a layer of material of less permeability than the aquifer. The water is under sufficient pressure so that when it is penetrated by a well, the water will rise above the top of the aquifer. A flowing artesian condition exists when the water flow is at or above the land surface. Minnesota Administrative Rules, Definitions, Subp. 17. Water table aquifer or unconfined aquifer means an aquifer where groundwater is under atmospheric pressure. A perched aquifer (or perched groundwater) is usually defined as groundwater in a saturated zone with a confining layer and unsaturated material below. We did not find a regulatory definition. In Minnesota, some perched aquifers can be extensive, but many are only a few inches thick and have very limited extent. We recommend using the following definitions for groundwater and aquifer: Groundwater - Subsurface water in the saturated zone(s). Aquifer - A body of rock or sediment that is can yield significant or economic quantities of groundwater to wells and springs. Groundwater Sensitivity and Solid Waste Disposal 11

12 The definition of aquifer is intended to exclude small areas of perched groundwater. In this paper (and in most other similar publications and rules), the terms groundwater and aquifer are used interchangeably. In particular, when the terms groundwater sensitivity and groundwater protection are discussed, the reader may assume they mean the same as aquifer sensitivity and aquifer protection. Although that is a useful equivalence, groundwater is the water while aquifer is the container and that distinction can be significant. 2.2 Groundwater Quality and Groundwater Contamination Quantifying groundwater quality and groundwater contamination is beyond the scope of this paper. Any substance that emanates from a landfill to the subsurface is considered a contaminant. Any time one of these substances reaches the groundwater/aquifer, the groundwater/aquifer is considered to be degraded. 2.3 Sensitivity MDNR (1991) defined sensitivity as follows: The term sensitivity is commonly used to describe the general potential for an aquifer to be contaminated. One aquifer is said to be more sensitive than another aquifer if it has a higher potential to be contaminated. However, this definition of sensitivity is unsatisfactory because potential is vague and difficult, if not impossible to measure. Instead of trying to use an unmeasurable term such as potential to define relative sensitivity, this report uses the concept of time of travel, the time required for a contaminant to move vertically from the land surface to an aquifer. This interpretation is preferred as being specific and measurable. The factors mentioned above can make it very difficult to determine the travel time for a contaminant to reach an aquifer. Therefore certain simplifying assumptions have been adopted. In particular, any factors that may change over time, such as land use and seasonal effects, are not considered. Since contaminants are so variable in their behavior, contaminants are assumed to be inert and conservative and to behave the same as water. This paper uses the same definition of sensitivity, with one key exception. MDNR (1991) and other publications consider only water flow vertically downward to the first aquifer. This paper also considers lateral migration of groundwater and contaminants to and in the saturated zone. For example, one aquifer/landfill pair is more sensitive than a second aquifer/landfill pair if contaminants reaching the aquifer below the landfill at one site will migrate faster to the landfill boundary or a well than at the other site. In karst and fractured aquifers the contaminant advection in the aquifer can be significantly faster than the movement in the unsaturated zone. Groundwater Sensitivity and Solid Waste Disposal 12

13 3 Objectives Minnesota Statutes 103H.001 states the following degradation prevention goal: It is the goal of the state that groundwater be maintained in its natural condition, free from any degradation caused by human activities. It is recognized that for some human activities this degradation prevention goal cannot be practicably achieved. However, where prevention is practicable, it is intended that it be achieved. Where it is not currently practicable, the development of methods and technology that will make prevention practicable is encouraged. We have based our recommendations on the assumption that at least one landfill constructed in Minnesota in the future will release contaminants into the subsurface. Proactive siting of that landfill (and all others) in the best location available will minimize the threats to human health and the environment. The best location will have subsurface characteristics that slow the migration of contaminants to water supplies or to surface waters. These optimal subsurface materials and characteristics will help achieve the following basic objectives: 1. Minimize contaminant release. Low-permeability materials will help contain contaminants that escape the landfill liner, leachate collection and other engineered protection systems. 2. Contain contamination. Once contaminants are released, low-permeability materials will restrict contaminant migration away from the landfill. Rapid and unpredictable flow conditions (i.e. fracture or conduit flow) should especially be avoided. 3. Remediate contamination. Contaminants that have not been contained must first be detected by a monitoring system. After a contaminant release has been detected, it may be possible to remediate the problem by removing the contaminants from the subsurface or degrading them in situ. It is generally easier to perform this type of monitoring and remediation in isotropic, homogeneous, high-permeability materials than in low-permeability or heterogeneous materials. Ironically, this could lead to the mistaken conclusion that landfills should be sited in areas with high-permeability subsurface materials, which would contradict the first two objectives above. Landfill remediation has proven to be extremely expensive and ineffective in many cases. Therefore, monitoring and remediation is a much less preferable approach than is minimizing and containing contamination. Facilitating monitoring and remediation should not be considered as a landfill siting criterion. 4. Minimize impacts. In the event that the first three objectives are not met, then is desirable to have the landfill sited in an area that will lead to minimal impacts to human health and the environment. This usually means picking locations where contaminants migrating from the landfill will not encounter a drinking water supply or surface waters. Groundwater Sensitivity and Solid Waste Disposal 13

14 4 Groundwater Flow Our understanding of groundwater flow continues to evolve. Some advancements in the past 10 to 20 years have a bearing on landfill siting, but have not yet been incorporated into solid waste regulations. The discussion below is a basic introduction to some groundwater flow concepts with an emphasis on landfill issues and some common misconceptions. More detailed discussion is presented in several textbooks including Fetter (2001) and Freeze and Cherry (1979). 4.1 Unsaturated Flow Unsaturated flow is the movement of water through pore spaces that are not completely water filled or saturated. In most applications, only vertical movement (both up and down) is considered but horizontal flows can be significant. In landfill studies, unsaturated flow describes the movement of water or contaminants from below the landfill liner to the water table. Unsaturated flow is also important in understanding recharge of the water table aquifer by rainfall or surface water away from the landfill. Unsaturated flow is more complex and difficult to quantify than saturated flow. The factors that can significantly affect unsaturated flow include: Water content of the soil Pressure head of water in the soil Hydraulic conductivity (discussed more in the next section) Water content and pressure head may not be continuous across the site. That is, both parameters can vary significantly over short distances and over time. Water content and pressure head are difficult to measure accurately in the field. Finally, the mathematical equations that describe unsaturated flow are more difficult to solve than the equations describing saturated flow. Natural processes such as evaporation, transpiration, and capillary action affect unsaturated groundwater flow. Man-made activities including land use and changes to soil properties (i.e. compaction) must also be considered. Because unsaturated flow is so complex, simplifying assumptions are often made for landfill engineering and design purposes. For example, monitoring systems may assume that leachate emanating from the landfill flows vertically downward and immediately enters the water table. Direct measurements of the travel times through the unsaturated zone with tracers are an efficient ways of determining the travel times directly, without having to explain and quantify all of the factors that influence unsaturated flow. 4.2 Saturated Flow through Porous Media Aquifers Porous media aquifers have interconnected pore spaces between grains that can transport groundwater. The porous media can be either consolidated (i.e. sandstone) or unconsolidated (i.e. glacial outwash). Where a porous media aquifer is saturated, groundwater flow can be quantified and described using Darcy s Law: Q = K*i*A Groundwater Sensitivity and Solid Waste Disposal 14

15 Where: Q = Volume of groundwater flow over time (i.e. gal/min) K = Hydraulic conductivity i = Hydraulic pressure gradient A = Cross-sectional area of the aquifer. Hydraulic conductivity is an empirical parameter that incorporates many features of the porous medium. That is, hydraulic conductivity must be measured in laboratory or field tests rather than calculated from other physical parameters of the soil or rock. We know the simple relationship between some soil properties and hydraulic conductivity: Larger grain size Higher hydraulic conductivity Well rounded grains Higher hydraulic conductivity Variety of grain sizes mixed together Lower hydraulic conductivity Scientists and engineers also frequently make simplifying assumptions regarding hydraulic conductivity, including: Hydraulic conductivity does not change over time Hydraulic conductivity does not change over the range of groundwater velocities considered. (Darcy s Law does not apply to rapid, turbulent flow.) Hydraulic gradient is the change in pressure over distance, both horizontal and vertical. This is usually measured by installing a well in the aquifer and measuring how high the water in the well rises. Groundwater flows from areas of high hydraulic pressure to areas of lower hydraulic pressure. Pumping wells and natural groundwater discharge points such as springs or rivers can create areas of lower hydraulic pressure. In water table aquifers, a common simplifying assumption is that there is no hydraulic gradient in the vertical direction. Quantifying and predicting changes to groundwater flow is often accomplished by applying mathematical models and equations. Mathematical models range from simple analytical solutions to complex numerical models solved using computer programs. All models require assumptions, and the simpler, more convenient models require more simplifying assumptions, such as: The aquifer is homogenous and isotropic (same hydraulic conductivity in all directions). The aquifer is infinite in horizontal extent, or has impermeable no-flow boundaries. There is no vertical flow across confining layers at the top or bottom of the aquifer. The problem with this concept of groundwater flow is that it seldom occurs in nature. It is useful in some applications, such as designing a water supply well. But saturated porous media models often fail to accurately predict the direction of contaminant transport and underestimate contaminant transport velocities by factors of 10 to 10,000. The reason is that in most natural aquifers are not homogenous. Even subtle variations in seemingly homogenous materials can cause significant variations in hydraulic conductivity and hydraulic gradients. Preferential flow pathways can develop, and contaminants can be transported more quickly than predicted to various parts of the aquifer. Groundwater Sensitivity and Solid Waste Disposal 15

16 4.3 Fracture Flow Aquifers Fractured Rock Aquifers Groundwater flow occurs through fractures, cracks, joints, faults, etc. in rock aquifers. The pore spaces between individual grains are the primary porosity. The porosity in fractures and cracks is called secondary porosity. Groundwater flow through the secondary porosity can be very rapid compared to groundwater flow through primary porosity. Darcy s Law is often not valid. Fractures and cracks are typically only partially interconnected. Consequently, it can be very difficult or impossible to quantify and predict groundwater flow patterns through fractured rock aquifers. An assumption that is often made for regional groundwater studies is that fractured rock behaves as a equivalent porous medium on a large scale. That is, if a large enough aquifer area is considered, flow through individual fractures is not so important, and the aquifer responds similarly to a saturated porous medium. At the scale of landfill siting and design, this assumption does not adequately describe the groundwater flow Karst Aquifers Karst aquifers are triple porosity aquifers. That is, they have porous matrix flow components, fracture flow components and conduit flow components in the same volume of aquifer. The conduits are self-organized into continuous flow systems by solution and mechanical erosion process. The conduits function to move water from recharge areas to discharge points (springs or wells). In Minnesota karst aquifer conduits, flow velocities of miles per day are common. Most of the water transport in karst aquifers is via the conduits. The matrix flow components serve as the storage components of karst aquifers. Most of the water stored in karst aquifers between recharge events is in the matrix. Water moves short distances from the conduits to the matrix during recharge events and from the matrix to the conduits during recession periods between recharge events. The water in the matrix components can have very long residence times. The fracture flow components function to connect the conduit and matrix components. 5 Minnesota Geology and Groundwater Flow Minnesota s diverse geology and hydrogeology create challenges for developing state-wide environmental policy. Simple terms and concepts like bedrock fracture and groundwater can be very different in different parts of the state. The challenges are compounded by the fact that boundaries between different hydrogeologic regions cannot be precisely delineated, individual aquifers with different characteristics often overlap and interact, and the hydrogeologic boundaries never follow established political boundaries. This section provides context for later discussions about testing methods and groundwater classification. 5.1 General Aquifer Characteristics Bedrock aquifers can be separated into two groups igneous and metamorphic aquifers and sedimentary aquifers. Karst aquifers develop in sedimentary aquifers and have different characteristics than other aquifers, so they are discussed separately and in more detail. Groundwater Sensitivity and Solid Waste Disposal 16

17 Unconsolidated aquifers have a broad spectrum of characteristics, but it is useful to consider two groups glacial aquifers and non-glacial aquifers Igneous and Metamorphic The general areas where igneous and metamorphic aquifers are important sources of drinking water are shown on Figure 1. Igneous and metamorphic aquifers have very little primary porosity (interconnected voids between individual grains in the rock), but may have considerable secondary porosity in partings, joints, fractures, and dissolution features. As a result, groundwater flow is often very fast. Groundwater flow often does not follow a straight line, either horizontally or vertically. It instead follows a complex path of preferential flowpaths. Regional studies can often identify general aquifer characteristics and flow patterns (e.g. flow towards an open mine pit), but localized flow patterns can be unpredictable Sedimentary Sedimentary aquifers are found generally in the southeast third of the state (Figure 1). Sedimentary aquifers were traditionally grouped as sandstone, limestone, and shale aquifers. Recent work at the Minnesota Geological Survey (Runkel et al., 2003, RI 61) demonstrates that a more realistic and useful classification groups the aquifers as Coarse Clastic, Fine Clastic and Carbonate Aquifers: Coarse Clastics (some, but far from all, sandstones) often have considerable primary porosity. However, groundwater flow (both volume and velocity) is often much greater through secondary porosity features (partings, joints, fractures and dissolution features) than through primary porosity. Coarse Clastics in Minnesota function as aquifers. Fine Clastics have lower primary porosity and can function as aquitards. Figure 1. Minnesota Bedrock Aquifers Carbonates (limestones and dolomites) in Minnesota typically have very low primary porosity but typically have significant secondary porosity and permeability via extensive joints and bedding plane partings that connect to well-developed solution conduits. Carbonates in Minnesota function as aquifers. When considering regional groundwater resources, it is customary to model sedimentary aquifers as homogenous porous media, but these models and assumptions often do not adequately describe groundwater flow at individual landfill sites. It is also important to recognize that sedimentary formations have different hydrogeologic characteristics in different areas. In the Twin Cities area, the Franconia Sandstone Formation is notorious for having very different hydraulic conductivity in different areas. The St. Lawrence Formation is often considered a confining layer when analyzing regional groundwater resources, but many residential wells are completed in the St. Lawrence Formation and dye traces have recently demonstrated rapid horizontal groundwater flows in the St. Lawrence Formation. Groundwater Sensitivity and Solid Waste Disposal 17

18 Finally, the Minnesota Geological Survey (Runkel et al., 2003, RI 61) has shown that the hydraulic characteristics of all of the sedimentary aquifers become much more transmissive when those rocks are within a few hundred feet vertically or horizontally of the present or former land surface. The impermeability of confining layers drops dramatically in the near surface environment. Almost all potential landfill sites will be in this surface or near surface zone Karst In Minnesota karst aquifers have developed in soluble rocks composed of, containing beds of, or containing cements of limestone and dolomite. Karst aquifers are also developed in sandstones. Major parts of the karst aquifers in Minnesota are covered by glacial deposits. It has proven useful to divide the karst aquifers into the regions with less than 50 feet of glacial sediment cover, feet of glacial cover and more than 100 feet of glacial cover. The regional scale distribution of karst aquifers is shown in Figure 2. Figure 2. Minnesota Karst Lands The word karst was originally defined by geographers as a land surface developed over soluble bedrock, which is characterized by sinkholes, caves, springs, sinking streams, etc. Current usage of the phrase karst aquifer includes all aquifers in which solution porosity and permeability play a significant hydraulic role in the aquifer. All karst surfaces overly karst aquifers, but most karst aquifers in Minnesota do not have classically defined karst surfaces over them currently. Glacial on other processes have obscured or obliterated the karst surfaces but the karst aquifers remain. Groundwater Sensitivity and Solid Waste Disposal 18

19 A paucity or absence of surface karst feature is not evidence that the underlying aquifer can be correctly modeled, regulated, or managed as a porous media aquifer Unconsolidated Glacial All of Minnesota has been shaped by glacial activity at some time in the past. The lasting effects of the glaciation vary widely across the state. Glacial terrains can include: Areas where soils are thin or absent because glaciers removed them. Thick, dense, clay-rich tills that originated at the base of large glaciers (basal tills). Variable and unpredictable assemblages of sand, silt, and clay that create hummocky terrain (lateral and terminal moraines). Well-sorted deposits of sand and gravel that were deposited in cracks or rivers through the glacial ice (kames and eskers). Expansive areas of sand deposited by water running off a melting glacier (outwash deposits). Expansive areas of silt deposited by wind blowing across a bare landscape after a glacier recedes (loess). Silt and sand deposited at the edge or in the middle of lakes that developed on top of or adjacent to glaciers. Glacial deposits of sand, silt, and clay can vary in thickness zero feet to over three hundred feet in thickness. The deposits can also vary significantly over horizontal distances of a few feet. The variations create the need for extensive and detailed characterizations of potential landfill sites. Glacial deposits in Minnesota are generally classified as clayey, sandy, and thin or absent, as shown on Figure 3. Coarse-grained glacial deposits are often tapped for water supplies by residential, municipal, and industrial wells. Groundwater flow through some glacial aquifers can sometimes be approximated as a homogenous and isotropic. However, care must taken to identify common areas of anisotropy (especially in the vertical direction) and preferential flowpaths. Figure 3. Minnesota Glacial Deposits Unconsolidated Non-Glacial Areas of sand and silt deposited by rivers are found throughout Minnesota. Many have limited areal extent, although some deposits associated with major rivers are significantly larger. The coarser deposits can be important water supplies, especially in areas where bedrock aquifers have low yields and poor water quality. Because of their limited extent, generally high hydraulic conductivity, and importance as local water supplies, river deposits should almost always be excluded as landfill sites. Groundwater Sensitivity and Solid Waste Disposal 19

20 5.2 Regional Groundwater DNR and MPCA have developed classification systems for groundwater throughout the state. They are very similar. Both have advantages for various different applications. Both systems are presented here to help with discussions about regional landfill issues. These classification systems and maps cannot be substituted for detailed site studies or other area investigations such as a county hydrogeologic atlas DNR Groundwater Provinces DNR developed a classification of six groundwater provinces based on the basic geologic information discussed above (MDNR, 2009). Combining the regions of the two general geology settings in Figure 1 (bedrock) and Figure 3 (glacial deposits) creates the groundwater provinces shown in Figure 4. Provinces 1 and 4 (metro and central, respectively) are characterized by buried sand aquifers and relatively extensive surficial sand plains as part of a thick layer of unconsolidated sediments deposited by glaciers overlying the bedrock. Province 1 is underlain by sedimentary bedrock that has good aquifer properties, but in Province 4 the glacial sediments are thick, sand and gravel aquifers are common, and the deeper fractured bedrock is rarely used as an aquifer. The unconsolidated glacial sediments of Provinces 2 and 5 (southcentral and western, respectively) are typically clayey and may contain limited extent surficial and buried sand aquifers. In Province 2 the sedimentary bedrock aquifers are commonly used, but in Province 5 the fractured bedrock is usually buried deeply beneath glacial sediments and is only locally used as an aquifer. Figure 4. Minnesota The unconsolidated sediments in Provinces 3 (southeastern) and 6 Groundwater Provinces (arrowhead) are thin or absent and, therefore, not used or relatively unimportant, except in major river valleys where sediment thickness is greater. However Province 3 is underlain by productive bedrock aquifers, but Province 6 is underlain by hard fractured bedrock that typically has limited ground-water yield Groundwater Profiles An Interagency Ground Water Coordination Group developed profiles of nine groundwater regions (MPCA 1995). For landfill siting discussions, the regional profiles are often more useful than the DNR province classifications. The regions are smaller and are described by county, a governmental unit commonly involved in solid waste issues. Groundwater Sensitivity and Solid Waste Disposal 20

21 South Central Region Location: Blue Earth, Faribault, LeSueur, Martin, Nicollet, Waseca, and Watonwan Counties Hydrogeology: The distribution of aquifers in this area is transitional between those having abundant, high-quality groundwater to the east and those with groundwater that is scarce and poor in quality to the west. Glacial aquifers are limited in extent and yield. Quantity Issues: Quality Issues Occasional well interference problems are noted. Nitrate contamination may affect the Prairie du Chien-Jordan aquifer as well as surficial aquifers. Agricultural drainage wells, where present, pollute deeper ground water. Thick, clay-rich till is fairly protective. There are no high-priority problems for public water supplies in till areas. Proper well construction practices are critical when penetrating confining layers Southwest Region Location: Brown, Cottonwood, Jackson, Lac Qui Parle, Lincoln, Lyon, Murray, Nobles, Pipestone, Redwood, Rock and Yellow Medicine Counties Hydrogeology: Scattered, shallow alluvial sands and limited, buried sand aquifers are present. Low-yield crystalline bedrock is vulnerable to contamination at or near the surface. The highest yielding aquifers in this region are mostly narrow, channel outwash deposits. Quantity Issues: Aquifers located here tend to be low yielding and not as well defined as elsewhere in the state. The Sioux Quartzite aquifer is near the surface in much of the region and is known for its low yield and high vulnerability to contamination. Groundwater Sensitivity and Solid Waste Disposal 21

22 Quality Issues Wells completed in the buried sand and gravel and Cretaceous aquifers often yield water of poor natural quality (high sulfate and total dissolved solids). Channel aquifers are highly susceptible to contaminants including nitrate from feedlots, agriculture, and human wastewater North Woods Region Location: Aitkin, Beltrami, Clearwater, Itasca, Kanabec, Koochiching, Lake of the Woods, Mahnomen, Mille Lacs and Pine Counties Hydrogeology: Quality Issues: The region is dominated by shallow glacial aquifers with numerous connections to wetlands. Availability of ground water varies because of low-yielding crystalline bedrock is present near the surface in many areas. The presence and level of natural contaminants (trace metals) should be better defined to determine long-term health impacts Central Sands Region Location: Benton, Cass, Crow Wing, Hubbard, Morrison, Stearns, Todd and Wadena Counties Hydrogeology: Extensive high-yield, sand-plain aquifers characterize much of the region. Some areas with crystalline bedrock near the surface offer little potential as aquifers. Quantity Issues: Quality Issues Some areas must rely on low-yielding, glacial and bedrock aquifers. Sand aquifers are highly susceptible to contamination by land-use activity such as irrigated agriculture, septic systems, lakeshore development, and commercial and industrial development that lacks a proper sewer system. Deeper aquifers may have higher levels of dissolved solids and trace metals than water-table aquifers. Groundwater Sensitivity and Solid Waste Disposal 22

23 West Central Region Location: Becker, Big Stone, Chippewa, Douglas, Grant, Kandiyohi, McLeod, Meeker, Otter Tail, Pope, Renville, Sibley, Stevens, Swift and Traverse Counties Hydrogeology: High-yield, surficial, sand aquifers are present in large parts of this area. Deeper glacial aquifers are limited in areal extent. Bedrock does not yield large amounts of ground water in this region. Quantity Issues: Quality Issues Some well interference problems exist. There is adequate observation-well density. Where present, surficial sands are susceptible to contamination. Arsenic and other elevated trace metals are associated with the geology of the region. Agricultural practices and domestic land uses may impact ground-water quality with the increased presence of nitrates and dissolved solids Southeast Region Location: Dodge, Fillmore, Freeborn, Goodhue, Houston, Mower, Olmstead, Rice, Steele, Wabasha and Winona Counties Hydrogeology: Quality Issues Layered sandstone and carbonate bedrock aquifer systems are highly productive and of high natural quality. Extensive near-surface karst areas result in aquifers that are highly vulnerable to contamination. Glacial aquifers are not widely present and are often of moderate to poor yield. Near-surface, karst aquifers have potential for contamination and major problems with landuse management, including siting of industrial, municipal, and agricultural facilities. Pesticides occur in older, shallow wells constructed in karst aquifers. Nitrate contamination in near-surface aquifers is widespread. Groundwater Sensitivity and Solid Waste Disposal 23