ATTACHMENT B EVALUATION OF POTENTIAL LEAKAGE THROUGH CATEGORY 1 LANDFILL, CALL 4 LINING SYSTEM

Transcription

1 ATTACHMENT B EVALUATION OF POTENTIAL LEAKAGE THROUGH CATEGORY 1 LANDFILL, CALL 4 LINING SYSTEM

2 Texas Custodial Trust 2301 West Paisano Drive El Paso, Texas Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System July Report Prepared for Project Navigator, LTD (Trustee) By: Malcolm Pirnie, Inc. 211 N. Florence St. Suite 202 El Paso, TX

3 SEALED FOR SECTIONS 1-3 & 5 GEOSYNTEC CONSULTANTS, INC. TEXAS ENG. FIRM REGIST. #1182 (SECTIONS 1 & 2) i

4 Table of Contents Contents 1. Introduction Landfill Operations Leakage Rate Estimate Conceptual Site Model HYDRUS-1D Modeling HYDRUS-1D Code Description Key Model Input Parameters Model Simulation Results Model Sensitivity Analysis Conclusions References 6-1 Tables Table 1 Table 2 Table 3 Estimated Leakage Rates Summary Using Giroud Equations HYDRUS-1D Input Parameter Summary HYDRUS-1D Model Simulation Results Figures Figure 1 Site Location Map Figure 2 Cell 4 Footprint and Geology Figure 3 Cross-Section A-A Figure 4 Cross-Section B-B Figure 5 Landfill Sump Test Pit 01 Figure 6 Landfill Bottom Figure 7 Schematic Conceptual Site Model for the Landfill Sump Area Figure 8 Schematic Conceptual Site Model for the Landfill Bottom Figure 9 Parker Brothers Arroyo Boring Material Sieve Analysis Figure 10 Concentration Profiles for the Landfill Sump Area (Base Case) Figure 11 Concentration Profiles for the Landfill Bottom (Base Case) Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System i

5 Table of Contents Figure 12 Concentration Profiles for the Landfill Sump Area (Scenario 1) Figure 13 Concentration Profiles for the Landfill Sump Area (Scenario 2) Figure 14 Concentration Profiles for the Landfill Bottom (Scenario 1) Appendices Appendix A Leakage Rate Estimates Using Giroud Equations Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System ii

6 1. Introduction Remedial action at the Former ASARCO (ASARCO) El Paso Smelter (the Site; see Figure 1) is being conducted by the Texas Custodial Trust (Trust), the property Trustee, on behalf of the Trust s beneficiaries, the State of Texas represented by the Texas Commission on Environmental Quality (TCEQ) and the United States of America represented by the United States Environmental Protection Agency (USEPA). This report presents the results of an evaluation to assess whether possible leakage through the Category 1 Landfill, Cell 4 liner has the potential to affect groundwater at the Site. The landfill composite liner proposed for Cell 4 is composed of a geomembrane (60 mil [0.06 inch] textured high density polyethylene [HDPE]) underlain by a reinforced geosynthetic clay liner (GCL) placed on top of a 6-inch thick prepared subgrade. On the landfill side slope, any leachate that percolates through the waste to the liner will be conveyed to the bottom of the landfill in the 2-foot thick protective soil layer overlying the liner (Detail 4 on Sheet 6 of Engineering Drawings [Geosyntec, 2012]). On the landfill bottom (including sump), a double-sided geocomposite drainage layer will be placed between the protective cover layer and geomembrane to convey any percolated leachate to the leachate collection and removal system (LCRS) sump for subsequent removal from the landfill (Detail 3 in Sheet 6 and Section E in Sheet 7 of Engineering Drawings [Geosyntec, 2012]). Leachate migration through the liner system can occur if there are sustained leachate heads over defects in the geomembrane that occur during installation and are not identified and repaired after construction quality assurance. If there are significant heads of leachate over these geomembrane defects for enough time, a sufficient volume of leachate could be flow into and through the GCL component of the liner, into the vadose zone, and towards groundwater. However, in the arid climate of the Site, little or no leachate production is expected. Furthermore, as described in Section 1.1, operational measures will be implemented to manage leachate and preclude the development of significant leachate heads on the liner. Given the landfill design and proposed operating procedures, the hydraulic performance of Cell 1 is expected to be very good. The very good performance of geomembrane/gcl composite liners in operating landfills was clearly demonstrated in the published EPA report "Assessment and Recommendations for Improving the Performance of Waste Containment Systems" (Bonaparte et al., 2002). Two types of areas of the proposed landfill liner system were evaluated for the potential migration of leachate to groundwater: the sump area and the bottom of the landfill. The landfill sump area, located at the southeast corner of the landfill (see Figure 2), is considered the most critical area with respect to the potential for leachate migration due Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

7 Section 1 Introduction to possible increased heads after significant storm events during the potential 3-year operation period. The landfill bottom requires separate analysis because (1) the floor of the landfill occupies a larger area and is more likely to have a higher number of defects than the smaller sump; and (2) the floor and sump areas are underlain by different geologic units. The leachate evaluation was carried out by calculating a leakage rate through hypothetical holes in the liner. Calculations were completed following procedures described in the technical paper by Giroud (1997). The leakage rate was then input into a vadose zone model, which was developed using HYDRUS-1D (Version 4.08) (Šimůnek, et al., 2009) to simulate the transport of constituents of concern (COCs) in the unsaturated zone. The evaluation was carried out for 53 years, including the 3-year operation period and a 50-year postclosure period. HYDRUS -1D was selected for modeling the vadose zone as it is a public-domain model and is considered a standard tool in both research and industrial applications. It is conservative for evaluating the potential for leakage through the liner to reach ground water because it is onedimensional and neglects lateral spreading that would be simulated using HYDRUS-2D and -3D. Two analyses were carried out as part of the evaluation presented in this report. The first analysis, called the base case condition, was evaluated using conservative assumptions for landfill operations at an arid site (see Sections 2, 4.3, and 4.4). In the second analysis, two sets of sensitivity analyses were conducted to determine under what hypothetical operating conditions leachate from Cell 4 would reach the groundwater table (see Section 4.5) Landfill Operations It is important to take the operation plan for Cell 4 into consideration in this potential leakage evaluation to assure that the analysis presented here is consistent with the anticipated conditions. During the anticipated 3-year operation period, impacted materials will be placed in Cell 4 with protective cover soil being placed over the slopes as the fill is raised. Procedures that will be carried out carefully during the 3-year operation period to minimize the generation of leachate and head accumulation on top of the liner system are described as below: Waste materials will be placed relatively dry; water will only be added to control dust. If rain events are forecasted, the placement area will be sloped to a low point forming a temporary sump where runoff water will be collected. The temporary Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

8 sump will be lined with low permeability soil, a geomembrane or other impervious material. Section 1 Introduction Ponded water collected in the temporary sump from rain events will be promptly removed and properly managed. The LCRS sump will be monitored monthly and after significant daily rain events (i.e., 0.5 inch) during the 3-year operation period. Any accumulated leachate will be promptly pumped from the LCRS sump and properly managed. Clean soil cover will be placed and compacted on slopes as waste is accumulated to prevent infiltration. The final cover system will be installed after Cell 4 has been filled to capacity (closure). The final cover system is designed to essentially prevent infiltration into the waste material. The LCRS sump will be operated until no liquids are observed in the sump for four consecutive quarters after closure and will be monitored thereafter during annual cover inspections. After closure, the cover system and LCRS will also be monitored and inspected after significant rain events (e.g., greater than 1.25-inches of rainfall in 1-hour or the 10-year storm events). Additional inspections may be completed if inclement weather or other events occur that may have caused damage to the containment systems. Necessary repairs to the Cell 4 cover system and LCRS monitoring will be scheduled and completed. Provisions that require any future owners of Cell 4 to continue the long-term monitoring and maintenance will be included in deed restrictions. Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

9 2. Leakage Rate Estimate The Giroud equation (Giroud, 1997), provides an analytical means of calculating the rate of leachate migration through composite liners. The equation was used to estimate the leakage rate and its associated wetted area to estimate the volume of soil that could be affected due to possible geomembrane defects. If there is a defect in the geomembrane component of a composite liner, the liquid first passes through the geomembrane defect, then flows laterally for a distance between the geomembrane and the low-permeability soil (GCL in Cell 4) and finally infiltrates into and through the low-permeability GCL. The modeled lining system is illustrated in the figure below (Giroud, 1997). Flow in the space between the geomembrane and the low-permeability GCL is called interface flow, and the area covered by the interface flow is called the wetted area. Note: The space between the geomembrane and the low-permeability soil is exaggerated to show the interface flow. The flow in the soil is vertical and R is the radius of the wetted area. (Giroud, 1997) In this evaluation, it was assumed that the geomembrane defect is circular. Therefore, the Giroud equations for rate of leachate migration and its associated wetted area estimated for the case of a circular defect were used, as described below (Giroud, 1997): where: Q : rate of liquid migration through geomembrane defect, or the leakage rate (m 3 /s) C : contact quality factor (dimensionless) for a circular hole qo Q C h h qo[ 1 0.1( ) ] a h k t s s a h k s A Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

10 Section 2 Leakage Rate Estimate h : head of liquid on top of the GCL (m) t s: thickness of GCL (m) a: surface area of geomembrane circular defect (m 2 ) k s : hydraulic conductivity of GCL (m/s) A : wetted area (m 2 ) h The factors affecting the rate of leachate migration through a composite liner are: the quality of the contact between the geomembrane and the GCL, the subgrade preparation, the size of the defect, the hydraulic conductivity of the GCL, and the depth of liquid on top of the geomembrane. It was assumed that the contact quality of the geomembrane/ GCL liner would be classified as good contact conditions, which means that the geomembrane is installed, with as few wrinkles as possible, on top of a prepared subgrade that has been adequately compacted and graded, as defined by Giroud and Bonaparte (1989). These conditions correspond to the contact quality factor of 0.21 used in the calculation (Giroud, 1997). In addition, it was assumed that the geomembrane defect would be a circular hole occurring at the standard design rate of 2.5 holes per hectare (one hole per acre) and an area of 10 square millimeters (mm 2 ). The hydraulic conductivity of a GCL used in a landfill liner application is typically between and meters per second (m/s). The hydraulic conductivity of the GCL at the Site was assumed to be m/s (Thiel and Criley, 2005). Leakage calculations are provided in Appendix A. In the base case condition in the LCRS sump, the head was assumed to be 0.9 meter (m) (3.0 feet [ft]), equal to the sump depth for the entire 3-year operation period. This is a conservative assumption considering that the sump will be inspected on a monthly basis and after significant precipitation events, and any fluids found will be evacuated as part of the landfill operations plan. During the 50-year postclosure period, the sump will be operated until no liquids are observed in the sump for four consecutive quarters to assure all liquids have been removed from the system. To add conservatism in the evaluation, a sustained head of 0.09 m (0.3 ft) was assumed to account for any potential residual water in the sump for the 50-year postclosure period. In the base case condition, the head on the geomembrane/gcl liner over the bottom of the landfill outside of the sump was assumed to be m (0.02 ft), equal to the thickness of the geocomposite drainage layer during the 3-year operation period. During the 50-year postclosure period, no sustained head was applied on the landfill bottom given that a cover system will be installed that essentially prevents infiltration. The calculated hypothetical leakage rates are summarized in Table 1. The geomembrane in the sump area was assumed to have one hole, and the leakage rate through this hole during the landfill operation period was calculated to be cubic meters per Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

11 Section 2 Leakage Rate Estimate second (m 3 /s), equivalent to 0.5 gallon per day (gpd). The associated wetted area is 29.1 square meters (m 2 ) or square feet (ft 2 ). The leakage rate was calculated to be m 3 /s (0.011 gpd) with a wetted area of 3.7 m 2 (39.4 ft 2 ) for the postclosure period. The geomembrane at the landfill bottom was assumed to have four holes, with each hole having a calculated leakage rate of m 3 /s ( gpd) and wetted area of 0.3 m 2 (2.9 ft 2 ) during the operational period. The total leakage rate through the four holes during the 3-year operation period is m 3 /s ( gpd) with a wetted area of 1.1 m 2 (11.7 ft 2 ). Assuming no sustained head on the landfill bottom during the postclosure period, zero leakage was calculated for the landfill bottom area during the postclosure period. Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

12 3. Conceptual Site Model The geologic units that will be in direct contact with the composite liner system are shown in Figure 2. The landfill geomembrane/gcl/prepared subgrade liner is underlain by alluvial sediments at the sump area (primarily interbedded sand, silt and clay of Fort Hancock Formation locally), while the liner placed on the landfill bottom is generally underlain by weathered sandstone. Several areas in the eastern portion of the landfill were over-excavated to remove slag and associated impacted alluvium, and will be backfilled and compacted prior to the liner installation. Two cross-sections (A-A' and B-B') were prepared for the Site, with A-A' crossing the landfill sump (see Figure 3) and B-B crossing the landfill bottom (see Figure 4). Figure 3 illustrates that the landfill composite liner system at the sump is underlain by alluvial sediments, which overlay the shale and sandstone bedrock. One test pit (see Figure 2; Sump TP-1) was dug to a depth of 21 feet below ground surface (ft bgs) in the landfill sump area. A photo taken at Sump TP-1 is shown in Figure 5. Shale bedrock was observed at the test pit, and no groundwater was observed to the total depth of 21 ft. The B-B' cross-section (see Figure 4) demonstrates that the composite liner system on the bottom of the landfill is underlain primarily by weathered sandstone, which overlays the shale bedrock. A photo taken at the landfill bottom (see Figure 6), looking northwest, shows the weathered sandstone that will underlie the composite liner system. A conceptual site model (CSM) was prepared for both the sump area and landfill bottom. These schematic CSMs, along with their assumptions, are shown in Figures 7 and 8 for the sump and the bottom of the landfill, respectively. At the landfill sump, the alluvial sediments underlying the landfill composite liner mainly consist of particles with a United States Department of Agriculture classification as silts and clays (70-90% < 0.05 mm as extrapolated from the particle size distribution curves), as shown by the sieve analyses from nearby borings PBA-09 and PBA-11 shown in Figure 9 (Samples PBA- SB and PBA-SB ). These alluvial sediments are much finer grained and have much lower hydraulic conductivity than the alluvium south of the landfill where most groundwater flow occurs primarily through sand and gravels, as illustrated by the sieve analysis data from PBA-SB and PBA-SB located on the main channel of PBA (see Figure 9). As shown in Figures 3 and 7, in the landfill sump area, the estimated landfill liner elevation is 3,743 ft above mean sea level (amsl). Groundwater was not encountered in borings PBA-09 and PBA-11 or the Sump TP-1; however for this evaluation the water table was assumed to be approximately at 3,730 ft amsl based on current water levels Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

13 Section 3 Conceptual Site Model from PBA (The water level at EP-85 was measured at 3, ft amsl in February 2012 [Malcolm Pirnie, 2012]), resulting in a vadose zone minimum thickness of 13 ft. Based on shale tagged in PBA-11, the alluvium in the landfill sump is underlain by the shale and sandstone bedrock, at approximately 3,724 ft amsl. It was assumed that there would be one geomembrane defect in the sump which covers an area of 100 m 2 (see Figure 4). This assumption is conservative since this defect frequency is 40 times higher than the design defect frequency of 2.5 holes per hectare. The calculated leakage rate and its associated wetted area are also shown in the schematic figure (see Figure 7), along with the assumptions discussed in Section 2. The schematic CSM for the bottom of the landfill is shown in Figure 8. The landfill is underlain by weathered sandstone, which overlays shale bedrock. The representative liner bottom elevation is 3,750 ft amsl, and the water table was assumed to be at approximately 3,725 ft amsl (based on current water levels in PBA), resulting in a 25-ft thick vadose zone. The elevation of the shale and sandstone bedrock interface is interpolated to be approximately 3,705 ft amsl from shale elevations encountered in soil borings in the PBA channel. Based on the assumption of 2.5 holes per hectare, it was estimated that there would be a total of four geomembrane holes occurring at the landfill bottom (see Figure 8). The calculated leakage rate per hole and its associated wetted area are also shown in Figure 8, along with the assumptions discussed in Section 2. Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

14 4. HYDRUS-1D Modeling 4.1 HYDRUS-1D HYDRUS-1D is a public-domain software package for simulating water, heat, and solute movement in one-dimensional variably saturated media; it numerically solves Richards equation for variably-saturated water flow and the advection-dispersion type equations for heat and solute transport. HYDRUS-1D is considered applicable for evaluating the potential for leakage migration from Cell 4 because it includes the major processes relevant to the leachate transport analysis: advection and dispersion. Although HYDRUS-1D can be used to simulate retardation and degradation of leachate constituents, these processes were conservatively neglected. The one-dimensional analysis conducted with HYDRUS-1D is considered conservative for this evaluation because it neglects lateral spreading that would be simulated using HYDRUS -2D or 3D, which would further reduce the potential for leachate to percolate through the vadose zone to the water table. 4.2 Code Description HYDRUS-1D is a public domain code developed by J. Šimůnek and M. Th. van Genuchten, Department of Environmental Sciences, University of California Riverside, Riverside, California. It is considered a standard tool in both research and industrial applications. The governing flow and transport equations are solved numerically using Galerkin-type linear finite-element schemes. 4.3 Key Model Input Parameters The key HYDRUS-1D input parameters include the infiltration rate (the leakage rate through a hole across the wetted area), vadose zone thickness, and soil hydraulic parameters (residual water content, saturated water content, saturated hydraulic conductivity, and van Genuchten parameters). Site-specific data supplemented by literature values, when site-specific data were not available, were used in the evaluation of the potential leachate migration through the liner. Neither degradation nor retardation was applied in each simulation for the evaluation. The key HYDRUS-1D input parameters are summarized in Table 2. For the sump area, the applied net infiltration rate is 2.32 centimeters per year (cm/year) (0.91 inch per year [in/year]) for the 3-year operation period, which is equivalent to the 0.5 gpd associated with a wetted area of ft 2, as calculated in Table 1. The net infiltration rate for the postclosure was calculated to be 0.40 cm/year (0.16 in/year), which is equivalent to Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

15 Section 4 HYDRUS-1D Modeling gpd with a wetted area of 39.4 ft 2. As summarized in the CSM discussion, the vadose zone thickness is assumed to be 13 ft thick at the sump area. The residual water content, saturated water content, and saturated hydraulic conductivity were specified at 0.034, 0.46, and 0.20 feet per day (ft/day), respectively, based on the typical values for silt (Šimůnek, J., et al. 2009). The van Genuchten parameters ( and n) relating moisture content, matric potential and hydraulic conductivity are also presented in Table 2. For the landfill bottom the net infiltration rate is 0.17 cm/year (0.07 in/year) for the 3-year operation period, which is equivalent to gpd with a wetted area of 2.9 ft 2. The net infiltration rate for the 50-year postclosure period is 0.0 cm/year (0.0 in/year). Residual water content, saturated water content, and saturated hydraulic conductivity were specified at 0.04, 0.30, and 0.16 ft/day, respectively, based on literature values for weathered sandstone (Barker and Tellem, 2006). These values are representative of weathered bedrock and are considered reasonable when comparing them with the ranges for fine-grained and medium-grained weathered sandstone provided by Morris and Johnson (1967): to for saturated water content and to 29.4 ft/day for saturated hydraulic conductivity. 4.4 Model Simulation Results To facilitate the evaluation of potential leachate migration, a concentration of 1.0 mmols/cm 3 was assigned to all leachate migrating though the liner. COC concentration profiles were prepared for each simulation to illustrate leachate migration through the vadose zone and any potential impacts to groundwater. Model simulated COC migration depths for the base case are summarized in Table 3. Values determined by model simulations are shown in bold. Landfill Sump Figure 10 presents the model results for the landfill sump base case simulation. COC concentration profiles were prepared for years 1, 3, 20, 40 and 53. As shown in Figure 10, for the conservative hypothetical leakage scenario, leachate from the landfill sump would travel downward to a maximum depth of 200 cm (6.5 ft) by year 53, indicating that leachate would not reach groundwater below the landfill sump and, therefore, groundwater impacts from this leachate would not occur. To further demonstrate that leachate from the landfill sump will not reach groundwater, the total volume of leachate leaking through the liner was compared to the available storage volume in the vadose zone below the liner. It was estimated in the HYDRUS-1D simulation that the cumulative leachate infiltration volume during the 53-year period is 27.0 cubic centimeters per square centimeter (cm 3 /cm 2 ), while the available storage capacity is cm 3 /cm 2, indicating that the total leachate volume leaking through the liner accounts for only 21 percent of the available storage capacity in the vadose zone. Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

16 Section 4 HYDRUS-1D Modeling This supports the conclusion drawn based on the COC concentration profiles that leachate from the landfill sump will not reach groundwater. Landfill Bottom Model simulation results for the landfill bottom base case are presented in Figure 11. The concentration profiles demonstrate that for the conservative hypothetical leakage scenario, leachate from the landfill bottom would travel downward up to 50 cm (1.6 ft) by the end of year 53, indicating that leachate would not reach groundwater below the landfill bottom and, therefore, groundwater impacts from this leachate would not occur. The total leachate volume leaking through the liner during the 53-year period and the available storage volume estimated in the HYDRUS-1D simulation are 0.5 cm 3 /cm 2, and 67.8 cm 3 /cm 2, respectively, indicating that the total leachate volume is negligible (0.7 percent) compared to the available storage capacity in the vadose zone. Therefore, leachate will not reach groundwater from the landfill bottom area. 4.5 Model Sensitivity Analysis The model simulations were conducted using a series of conservative assumptions as described in the previous sections. The results demonstrated that it is highly unlikely for any potential leachate that migrates through potential geomembrane defects in the landfill bottom or sump areas to reach the water table and impact groundwater. To further evaluate our results, a sensitivity analysis was conducted to determine under what hypothetical conditions leachate could affect groundwater. In the sensitivity analysis, iterative simulations were carried out in HYDRUS-1D to determine the sustained leachate head on a geomembrane hole required for the leachate to reach the water table, and to assess whether these conditions could reasonably occur under the current landfill design and operation plan. Landfill Sump At the landfill sump area, two hypothetical scenarios were analyzed. In Scenario 1, it was assumed that the base case head would remain at 0.09 m (0.3 ft) during the 50-year postclosure period. HYDRUS-1D simulations were conducted to estimate the head required that would result in leachate reaching the water table for the 3-year operation period. After multiple model simulations a minimum head of 9.3 m (30.5 ft) was estimated to be necessary for leachate to reach the water table, as shown by the concentration profiles (years 1, 3, 20, 40 and 53) in Figure 12. However, it is highly unlikely for conditions to develop that would result in a sustained head of 9.3 m (30.5 ft) during the 3-year operation period as the sump will be monitored monthly and after significant precipitation events and any liquids collected will be removed. Further, a leachate head this high could not occur as the resulting liquid levels in the sump would Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

17 Section 4 HYDRUS-1D Modeling exceed the height of the adjacent perimeter berm as well as the leachate riser pipe (which outlets approximately 18 ft above the bottom of the sump). In Scenario 2, it was assumed that the base case head of 0.9 m (3.0 ft) would persist during the 3-year operation period. HYDRUS-1D simulations were conducted to estimate the leachate head required during the 50-year postclosure period for the leachate to arrive at the groundwater table. The estimated required head was determined to be 0.6 m (2.0 ft) (see Figure 13). The estimated 0.6 m (2.0 ft) of head could hypothetically develop within the sump; however it is highly unlikely that it could be sustained for the 50-year postclosure period considering that long-term monitoring of the landfill and sump would prevent leachate accumulation for extended periods of time. Landfill Bottom A similar sensitivity analysis was conducted for the landfill bottom. Iterative HYDRUS- 1D simulations were carried out to determine a hypothetical head that would result in leachate reaching groundwater within the 3-year operation period. The simulation results indicated that the required head would be 7.8 m (25.6 ft) during the 3-year operation period (see Figure 14). This estimated head is also very unlikely to occur because the geocomposite drainage layer placed on top of the landfill liner will route leachate to the sump where it will be removed. Further, a leachate head this high could not occur as the resulting liquid levels in on the landfill bottom would exceed the minimum height of the perimeter berm above the landfill bottom (approximately 2 ft above the bottom of the landfill sump). Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

18 5. Conclusions Model simulations of hypothetical scenarios of leachate migration through geomembrane defects in the landfill liner and underlying GCL and into the vadose zone were conducted using conservative assumptions (e.g., one-dimensional flow without lateral spreading, conservative transport without attenuation mechanisms, thickness of the vadose zone, and assumed head conditions). The model results indicate that it is highly unlikely for any potential leachate from the landfill to reach the water table during the 53-year period analyzed. Additionally, a sensitivity analysis was conducted to estimate the heads required for leachate to reach the water table. The sensitivity analysis indicated that highly unlikely and unreasonable heads exceeding the elevations of the perimeter berm would have to occur and be sustained for extended periods of time as summarized in Table 3 for leachate to reach groundwater. These heads are not only very unlikely to occur and unsustainable, but would be prevented from developing in the landfill based on the landfill design (e.g., provision of a geocomposite drainage layer and sump, elevation of perimeter berm) and operations (e.g., regular sump monitoring and leachate removal). The vadose zone analysis indicates that leachate from the landfill is not likely to reach groundwater under a variety of conditions; therefore no additional analysis of groundwater flow and solute transport below the water table (such as Visual MODFLOW and MT3D) is deemed necessary. Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

19 6. References Barker, R.D. and Tellam J.H Fluid Flow and Solute Movement in Sandstones: The Onshore UK Permo-Triassic Red Bed Sequence, GSL Special Publication No. 263, p Bonaparte, R., Koerner, R.M., and Daniel, D.E Assessment and Recommendations for Improving the Performance of Waste Containment Systems, research report published by the U.S. Environmental Protection Agency, National Risk Management Research Laboratory, EPA/600/R-02/099, December. Geosyntec Consultants (Geosyntec) Design Drawings, Category I Landfill Cell 4, ASARCO El Paso Smelter Remediation Site, El Paso, Texas, June, 15 sheets. Giroud, J.P., Equations for Calculating the Rate of Liquid Migration Through Composite Liners Due to Geomembrane Defects, Geosynthetics International, Vol. 4, Nos. 3-4, pp Giroud, J.P. and Bonaparte, R Leakage Through Liner Constructed with Geomembranes. Part II: Composite Liners, Geotextiles and Geomembranes, Vol. 8, No. 2, pp Malcolm Pirnie, Inc Field Demonstration of Zerovalent Iron Treatment Technology in Parker Brothers Arroyo, El Paso, Texas. Morris, D.A. and A.I. Johnson Summary of Hydrologic and Physical Properties of Rock and Soil Materials as Analyzed by the Hydrologic Laboratory of the U.S. Geological Survey U.S. Geological Survey Water Supply Paper 1839-D. Šimůnek, J., et al The HYDRUS-1D Software Package for Simulating the One- Dimensional Movement of Water, Heat, and Multiple Solutes in Variably-Saturated Media, Department of Environmental Sciences, University of California Riverside, Riverside, California. Thiel, R.S., and Criley, K Hydraulic Conductivity of Partially Prehydrated GCLs Under High Effective Confining Stresses for the Three Real Leachates, GSP 142 Waste Containment and Remediation, pp Texas Custodial Trust Evaluation of Potential Leakage through Category 1 Landfill, Cell 4 Liner System

20 Table 1 Estimated Leakage Rates Summary Using Giroud Equations Former ASARCO Smelter Facility El Paso, Texas Landfill Sump Landfill Bottom Landfill Areas Leakage Rate Wetted Area m 3 /s gallons per day (gpd) m 2 ft 2 3-Year Operation Period 2.1E Year Postclosure Period 4.6E Year Operation Period (1 hole) 1.5E Year Operation Period (4 holes) 6.0E Tables LeachEvalRpt-V2.xlsxTable 1

21 Table 2 HYDRUS-1D Input Parameter Summary Former ASARCO Smelter Facility El Paso, Texas Property, Parameter or Dimension Landfill Sump Landfill Bottom Vadose thickness (cm) Vadose thickness (ft) Net infiltration rate (cm/year)-3-year operation period Net infiltration rate (cm/year)-50-year post-closure period c Residual water content (unitless) a 0.04 b Saturated water content (unitless) 0.46 a 0.30 b Saturated hydraulic conductivity (ft/day) 0.20 a 0.16 b Parameter a in the soil water retention function (cm -1 ) a 0.02 a Parameter n in the soil water retention function 1.37 a 2.2 a Tortuosity parameter in the hydraulic conductivity function 0.5 a 0.5 a Notes: a HYDRUS-1D default values for silt. b Literature values from Barker, R.D. and Tellam J.H. Fluid Flow and Solute Movement in Sandstones; GSL; c Infiltration rate is zero assuming no sustained head on the liner head during post-closure period. Tables LeachEvalRpt-V2.xlsxTable 2

22 Table 3 HYDRUS-1D Model Simulation Results Former ASARCO Smelter Facility El Paso, Texas Landfill Sump Scenario Period Head (m) COCs migration depth (cm) Base case 3-Year Operation Year Postclosure 0.09 Scenario 1 3-Year Operation Year Postclosure 0.09 Scenario 2 3-Year Operation Year Postclosure 0.6 Landfill Bottom Base case 3-Year Operation Year Postclosure 0 Scenario 1 3-Year Operation Year Postclosure 0 Note: COCs 200 Constituents of Concern Numbers in bold indicate that these numbers were estimated from HYDRUS-1D evaluation. Tables LeachEvalRpt-V2.xlsxTable 3

23 PROJECT AREA EL PASO SITE 375 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER SITE LOCATION MAP 1

24 GEOLOGY LEGEND Fill SL PBA-04 EX-3 LEGEND Qal Kal Coring 01 SL SL Qal SL Kal Qal Kal Qal SL SL Qal SL SL Qal Fill Kal Qal SL Fill SL FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER CELL 4 FOOTPRINT AND GEOLOGY 2

25 EXISTING GRADE EXISTING GRADE LANDFILL ROAD EXISTING GRADE AS OF FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER CROSS-SECTION A-A' 3

26 EXISTING GRADE EXISTING GRADE AS OF EXISTING GRADE LANDFILL ROAD FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER CROSS-SECTION B-B' 4

27 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER LANDFILL SUMP TEST PIT 01 5

28 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER LANDFILL BOTTOM 6

29 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER SCHEMATIC CONCEPTUAL SITE MODEL FOR THE LANDFILL SUMP AREA 7

30 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER SCHEMATIC CONCEPTUAL SITE MODEL FOR THE LANDFILL BOTTOM 8

31 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER PARKER BROTHERS ARROYO BORING MATERIAL SIEVE ANALYSIS 9

32 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER CONCENTRATION PROFILES FOR THE LANDFILL SUMP AREA (BASE CASE) 10

33 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER CONCENTRATION PROFILES FOR THE LANDFILL BOTTOM (BASE CASE) 11

34 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER CONCENTRATION PROFILES FOR THE LANDFILL SUMP AREA (SCENARIO 1) 12

35 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER CONCENTRATION PROFILES FOR THE LANDFILL SUMP AREA (SCENARIO 2) 13

36 FORMER EL PASO SMELTER SITE EL PASO, TEXAS EVALUATION OF POTENTIAL LEAKAGE THROUGH LANDFILL LINER CONCENTRATION PROFILES FOR THE LANDFILL BOTTOM (SCENARIO 1) 14

37 APPENDIX A LEAKAGE RATE ESTIMATES USING GIROUD EQUATIONS

38 Appendix A Leakage Rate Estimates Using Giroud Equations Former ASARCO Smelter Facility El Paso, Texas Giroud Equation: Q C h qo [ 1 0.1( ) ] a h k t s s Notes: C q0 h (m) t s (m) 2 a (m 2 ) k s (m/s) Q (m 3 /s) A h 0.212a 0.1 h 0.9 k 0.26 s the contact quality factor (dimensionless) for a circular hole (0.21 for the case of good contact conditions) the head of liquid on top of the GCL thickness of the low-permeability soil component of the composite liner defect circular surface area hydraulic conductivity of the low-permeability soil component of the composite liner the rate of liquid migration through geomembrane defect Landfill Sump Landfill Bottom 50-Year 50-Year Giroud Eq 3-year 3-year Postclosure Postclosur Operation Period Operation Period Period Period C q0 (unitless) h (m) t s (m) a (m 2 ) 1.0E E E E-05 k s (m/s) 50E11 5.0E-11 50E11 5.0E-11 50E11 5.0E-11 50E11 5.0E-11 Q (m 3 /s) 2.1E E E E+00 Q (gal/day) A (m 2 ) NA A (ft 2 ) NA Leakage rate (cm/year) G:\ENV\PROJ\1200\ MP - El Paso Smelter\LF Design Package\May 2012 LF Package\Final LF Package July 2012\Appendix A_V2.xlsx