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HomeMy WebLinkAbout10 MSS CAP Part 2_Appx G_FINALThis page intentionally left blank TECHNICAL MEMORANDUM ChZM Evaluation of Potential Groundwater Remedial Alternatives for the Marshall Steam Station Ash Basin Site PREPARED FOR: HDR Engineering PREPARED BY: CH2M HILL Engineers, Inc. (CH2M) DATE: March 3, 2016 PROJECT NUMBER: 667077.02. MA Introduction This memorandum summarizes a remedial technology screening evaluation that was performed for groundwater at the Marshall Steam Station (MSS) site, which is owned and operated by Duke Energy. The site is adjacent to Lake Norman in Catawba County, near the town of Terrell, North Carolina. MSS began operations in 1965 as a coal-fired generating station and currently comprises four coal-fired units. Coal combustion residue (CCR) consisting of bottom and fly ash material was disposed of in the station's ash basin, which consists of a single cell impounded by an earthen dike located on the southeast end of the ash basin and north of the power plant. Ash has also been placed in six other areas of the MSS site including the dry ash landfill units (Phases I and II) and Industrial Landfill No. 1. Fly ash was also used as a component of the photovoltaic farm structural fill and as structural fill beneath portions of the industrial landfill (HDR, 2015a). Since 1984, fly ash has mainly been disposed of in the onsite dry ash landfills, while bottom ash has continued to be sluiced to the ash basin (HDR, 2016a). The Coal Ash Management Act of 2014 (CAMA) directed owners of CCR surface impoundments to conduct groundwater monitoring and assessment. Beginning in July 2015, groundwater sampling was conducted to meet CAMA requirements. Analysis of samples collected by HDR from monitoring wells constructed in the shallow groundwater at the site shows varying levels of ash -related constituents, some of which exceed North Carolina groundwater quality (2L) standards or Interim Maximum Allowable Concentration (IMAC) standards. Also, several constituents of interest (COls) were reported in groundwater between the ash basin dam and Lake Norman, and water samples collected from a point in the unnamed stream (SW-6) that flows to Lake Norman contained COls at levels that exceed the 2B standards. In accordance with CAMA, Duke Energy is required to implement closure of the MSS ash basin; the closure requirements will depend on the final risk classification assigned by the North Carolina Department of Environmental Quality (NCDEQ). This memorandum discusses potential remedial alternatives to address the groundwater COls that appear to be related to historical ash deposition and that are predicted to have the potential to exceed the standards at the compliance boundary. Background The MSS site is located along the shore of Lake Norman, which is part of the Catawba River watershed. The MSS comprehensive site assessment (CSA) report (HDR, 2015b) indicates that no imminent hazard to human health or the environment is present at the site as a result of groundwater migration from the ash basin or ash storage areas. This is largely because there are no water supply wells located between EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FORTH E MARSHALL STEAM STATION ASH BASIN SITE the ash disposal areas and Lake Norman, which serves as a downgradient hydrologic boundary for groundwater at the MSS site. Bedrock appears to bound the impacted groundwater vertically (HDR, 2016a). Based on monitoring to date, there are no known standard exceedances beyond the current MSS site boundary (HDR, 2015a). Background groundwater quality was compared to the 2L standards or IMAC standards (based on availability) as part of the CSA. Eight COls were identified in at least one background well at concentrations exceeding standards: arsenic, barium, cobalt, chromium, iron, lead, manganese, and vanadium. The groundwater results from within the MSS site potentially impacted by ash storage and basin areas was also compared to the 2L standards or IMAC. Based on the CSA results, concentrations of COls at the MSS site appear to be more prevalent beneath the dry ash landfill (Phase 11) and downgradient and east of the ash basin and dry ash landfill (Phase 1) than beneath the ash basin (HDR, 2015b). This could, however, reflect the limited number of wells within the ash basin area, which is due to limited safe access. COI transport is generally to the southeast toward Lake Norman and east toward the unnamed tributary that flows to Lake Norman. Surface water results exceeding the 2B standards were identified in the unnamed tributary downgradient of the dry ash landfill (HDR, 2015b). The University of North Carolina at Charlotte (UNCC) modeled the COI fate and transport component of the groundwater by using conservative assumptions to account for data limitations. The results of their modeling efforts indicate that there is potential for some of the COIs, i.e., antimony, beryllium, boron, cobalt, chromium, sulfate, thallium, and vanadium, to exceed their applicable 2L or IMAC standards at the MSS site compliance boundary for at least the next 100 years (HDR, 2016a).The model indicates that some COls may already be exceeding the standards at the compliance boundary (HDR, 2016a). It should be noted that neither iron nor manganese were included in the model predictions. Consequently, natural COI removal may have been underestimated by the groundwater model, since it did not consider the effects of adsorption by precipitated COls (principally iron hydroxide). As indicated by Miller (2015), iron hydroxide in particular has the potential capacity to lower many of the COls to below the appropriate standards. Miller's assessment found that arsenic, barium, beryllium, boron, chromium, cobalt, lead, thallium, and vanadium were all attenuating in groundwater at the MSS site. Geochemical modeling (HDR, 2016) supports Miller's findings, showing that the precipitated iron hydroxide at the MSS site potentially provides a large amount of adsorptive surface area, though actual adsorption depends on the pH and redox conditions. Tests to better estimate the site specific adsorption and site - specific capacity potential are needed (Miller, 2015). Manganese was the only C01 that exceeded its 2B standard in surface water samples collected from Lake Norman and exceed its 2L standard or IMAC standard in groundwater monitoring wells located between the ash basin and Lake Norman. Three other COls exceeded their respective 2B standards in Lake Norman surface water samples, but did not exceed their 2L standards or IMAC standards in groundwater monitoring wells located between the ash basin and Lake Norman. HDR ran a mixing model to predict potential water quality exceedances from impacted groundwater in Lake Norman. The mixing model used conservative assumptions, such as assuming a relatively low amount of dilution and using the predictions of the groundwater model rather than actual water sample results from monitoring wells to calculate the anticipated COI concentrations in Lake Norman. This means that the predicted results of the mixing model likely predicted higher C01 concentrations in Lake Norman than will likely occur. The results of the mixing model indicate that COI concentrations will not exceed 2B standards at the edge of the mixing zones in Lake Norman (HDR, 2016). Development of Remedial Alternatives While Duke Energy plans to close the active ash basin, the actual details of the closure have not yet been developed. In order to complete an evaluation of applicable remedial alternatives, it was assumed that the active basin will be drained and capped as part of the closure activity. Potentially suitable remedial EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE MARSHALL STEAM STATION ASH BASIN SITE measures that could be used as part of a comprehensive site remedy or as a stand-alone remedy to address the residual levels of contamination in groundwater were evaluated on that basis. Technology Screening Potentially applicable measures are summarized below. The purpose of this section is to briefly define the technology and any general qualifying remarks related to the MSS site. This section identifies whether the technology is a feasible measure to apply to the MSS site. This screening was used to develop the site remedial alternatives. Source Controls Description. Groundwater quality is improved by restricting ash contact with groundwater and surface water. Recharge through the ash basin can be reduced by placing an impermeable cap or cover over the ash. Impermeable caps help shed surface water and can be designed to direct this rain water to specific locations, where infiltration would be more desirable. If the ash basin intersects the water table and some of the ash is submerged, remedial options to control the leaching of COls from submerged ash can include ash solidification. In situ solidification/stabilization (ISS) involves mixing the ash and impacted soils with approximately 8-12 percent by weight of pozzolanic materials, such as portland cement or blast furnace slag, to reduce or eliminate leaching of COls from the source zones and underlying impacted soil. Blending pozzolans with ash or impacted soil can also reduce COI mobility, as the matrix either solidifies or chemically binds the COIs. The net impact of applying ISS to the site is generally to improve ash strength, reduce the leachability of COIs, and reduce hydraulic conductivity, which reduces groundwater contact with the COIs. Adding a pozzolan can change the local redox conditions or pH, so the overall chemical stability of the pozzolan addition should be explored at a bench -top scale. Applicability to MSS. Duke Energy has agreed to close the active ash basin, but the actual details of the closure design have not yet been developed. It has been assumed that the active basin will be drained and capped as part of the closure activity, leaving any submerged ash in contact with groundwater. Based on the selected closure approach ISS may be further considered to reduce leachability of any ash that may remain in contact with groundwater under lying the site. Groundwater Remediation Monitored Natural Attenuation Description. While model predictions can simulate long-term natural attenuation using a soil -water partitioning coefficient to estimate attenuation, natural conditions will dictate local sorption of COIs. Natural attenuation mechanisms include adsorption of COls onto soil particles and mineral precipitates, ion exchange, the formation of precipitated minerals that contain the COIs, and dilution from recharge. A key aspect of the monitored natural attenuation (MNA) approach is long-term groundwater monitoring to evaluate naturally occurring adsorption over time. Real-time data are the best indicator of natural attenuation mechanisms. The monitoring results will verify the degree to which natural attenuation is occurring and verify that the footprint of site -related impacts is not increasing. Applicability to MSS. It is reasonable to assume that COls remaining in groundwater after site closure will decrease in concentration over time as recharge flushes non -impacted water through the aquifer. The results of the groundwater model (HDR, 2016a) indicate some COI reductions, though the extents to which concentrations are reduced vary among the different COIs, which is in part due to attenuation factors. Attenuation is assessed in accordance with "tiers." For example, USEPA guidance defines Tier I as "Demonstration that the ground -water plume is not expanding and that sorption of the contaminant onto aquifer solids is occurring where immobilization is the predominant attenuation process" (USEPA, 2007). EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FORTH E MARSHALL STEAM STATION ASH BASIN SITE The groundwater model (HDR, 2016a) did not allow for removal of COls via coprecipitation with, and adsorption onto, freshly precipitated iron oxides, which likely resulted in an overprediction of COI transport and higher predicted COI concentrations at the compliance boundary. Miller (2015) documented strong arsenic and vanadium attenuation at the MSS site and, as part of a Tier II analysis, proposed a conceptual model for COI attenuation involving reversible and irreversible interaction with clay minerals, metal oxides, and organic matter. His most significant finding was that COls were concentrated in samples that had been exposed to groundwater containing higher concentrations of COIs, most likely indicating that the precipitating iron, and possibly also manganese, was removing other COls through coprecipitation and adsorption, thus confirming that attenuation was occurring (Miller, 2015). Based on the foregoing, it appears that attenuation, sorption, and precipitation are all occurring without active controls. However, more data and a Tier III analysis are needed to verify whether this is happening at a sufficient rate to adequately mitigate COI concentrations following site closure. Enhanced Recharge/Flushing Description. It is possible to increase the rate of groundwater quality improvement by increasing infiltration of uncontaminated water into the portions of a site that are beneath or downgradient of ash deposits, thereby flushing, diluting, and attenuating the remnant COI concentrations. There are various ways to do this, ranging from short-term or temporary methods (for example, surface irrigation using mechanical sprayers) to the creation of groundwater infiltration galleries or ponds or wetlands with a permeable bottom, which could be temporary or permanent. Where a continuing source is present, permanent infiltration galleries would be needed. Applicability to MSS. CH2M evaluated the MSS site to determine whether there exists a potential location for an infiltration basin. The best potential location identified on the MSS site for enhanced recharge is to the east of the ash basin, where there are dry ash landfills at present. However, unless the ash in these landfills is moved, this technology is not considered appropriate for the MSS site. In Situ Sorption or In Situ Chemical Fixation Description. Various measures can be taken to enhance adsorptive removal of COls by blending materials that have a high adsorptive capacity, such as clays, peat moss, and zeolites, into the contaminated soils. Contaminated groundwater can also be treated in situ using chemical fixation by adjusting the pH and/or redox state of the groundwater by, for example, enhancing the precipitation of iron and manganese oxide and hydroxide minerals in the groundwater. Enhanced formation of these minerals does more than remove iron and manganese from the groundwater because these minerals effectively coprecipitate and adsorb other COIs. Redox conditions can be adjusted either through adding one or more reagents (in situ chemical fixation, or ISCF) or through air sparging. Bench -scale treatability testing and/or pilot -scale tests are usually required to verify the effectiveness of this technology at a specific site prior to full-scale application, and to select the most appropriate reagent and dosage. It should be noted, that attenuation of boron may be limited (Goldberg et al., 1993) and that total dissolved solids and chloride concentrations will probably not be decreased using this approach. Preliminary geochemical modeling suggests that iron and manganese are already precipitating beneath and downgradient of the ash basin and dry ash landfill and storage areas limiting the mobility of other COls in the process, but this process is limited by the amount of available oxygen in the groundwater (HDR, 2015b, 2016). Natural attenuation could be enhanced by injecting an oxidant —such as potassium permanganate or air —to enhance oxidation and precipitation of the iron already present in the water in specifically targeted areas. This would add additional sorbent capacity (Evanko and Dzombak, 1997; Ilayskj, 2008; Twidwell and Williams -Beam, 2002). Periodic reinjection (or air sparging) would be necessary to maintain the desired redox conditions as upgradient groundwater enters the treatment area, until post -closure monitoring demonstrates that the groundwater has been adequately improved. 4 EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE MARSHALL STEAM STATION ASH BASIN SITE Applicability to MSS. The only portion of the MSS site where the use of this approach was considered is east of the ash basin to prevent COls from entering the unnamed stream that flows to Lake Norman. However, boron is one of the COls that exceeded groundwater standards at this location, and this approach has a limited ability to attenuate boron. Should post -closure monitoring indicate that 213 standards are being exceeded in Lake Norman, in situ chemical fixation might be implemented by drilling injection wells between the ash basin and the lake, and beneath the ash basin, after the ash basin has been dewatered. However, modeling indicates that this is unlikely to be warranted (HDR, 2016). Permeable Reactive Barrier Description. A permeable reactive barrier (PRB) is a passive form of in situ water treatment that removes COls in a subsurface zone using media that react with COls to remove them from groundwater as it flows through the barrier. PRBs are typically constructed by excavating a trench that fully penetrates the saturated zone of the unconsolidated aquifer and placing material in the trench to treat the groundwater. Depending on the required depth, specialized equipment can be used to trench and place media simultaneously. There are multiple types of media that are used for in situ treatment, and they are selected based on the contaminants required for removal. Some media are difficult to deploy at depth, so installation depth limitations may be determined by the media required to treat the COI. A funnel -and -gate system can be used to channel the contaminant plume into a gate that contains the media that will treat the COIs. The simplest design of this system consists of a single permeable area (the gate) containing appropriate media to remove the COIs, with impermeable walls extending from both sides (the funnel). The main advantage of the funnel -and -gate system is that a smaller reactive region can be used to treat the plume, reducing costs. In addition, if the treatment media has to be renewed or replaced, it is easier because there is less material. PRBs generally have a limited lifespan, depending on the sorption characteristics and flow rate, as the reactive media are consumed or become less effective. Long-term remediation effectiveness may require the periodic replacement of PRB media. Applicability to MSS. There are many successful examples of PRBs having been implemented at sites with a wide range of constituents, but only limited testing with water containing ash -related COls (EPRI, 2006; ITRC, 2005). The principle reason is that conventional PRB media have not been shown to effectively reduce boron concentrations. However, some materials, such as iron hydroxide, have been shown to absorb boron in the laboratory (Goldberg et al., 1993; Man et al., 2012). Laboratory tests would have to be conducted and then scaled up to a pilot -scale level to confirm how well these materials would perform in a PRB and whether additives would be necessary to maintain the required level of permeability. Removal of inorganics has been accomplished using a range of materials, including apatite, zero-valent iron, carbon, and other media. Site -specific media should be evaluated with a range of adsorbents to best determine the type and blend ratio to effectively remove COls while maintaining hydraulic conductivity. Near the unnamed stream east of the ash basin, where the depth to bedrock is shallow, a PRB could be constructed, if warranted. However, boron concentrations exceed the 2L Standard in this location, so pilot -scale tests of the adsorbents shown to remove boron in the laboratory would be necessary. PRB construction can be an expensive remedial approach relative to both natural attenuation and in situ treatment and should be considered a fallback downgradient provision at the MSS site. However, if increasing COI trends are identified following the completion of source control measures, the potential applicability of PRB placement should be reevaluated. Groundwater Treatment Description. As an alternative to in situ groundwater treatment methods, as discussed above, groundwater can be treated above grade. Impacted groundwater would be pumped to the surface EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FORTH E MARSHALL STEAM STATION ASH BASIN SITE (pump -and -treat) or captured at surface seeps in order to provide hydraulic containment and prevent COI migration to sensitive receptors. Following treatment, the water may be discharged directly to a surface water body or reinjected underground, depending on the site conditions and permitting requirements. Water treatment can be active (requiring the continual addition of chemicals and, typically, electrical power) or passive (taking advantage of reactions that occur in nature, such as constructed wetlands or limestone beds to provide neutralization). The use of passive systems is generally restricted to smaller flows because the approach typically requires a much larger land area than active systems do but has the advantages of less maintenance and lower operating costs. Passive treatment systems, however, can be ineffective at removing select COIs, such as boron and total dissolved solids. Active treatment systems are generally costly to construct and operate but can be designed to effectively lower the concentrations of all COIs. Applicability to the MSS. The areal extent and depth of impacted groundwater across the site is widespread. While a zone of groundwater depression could be created to minimize migration of COIs, it is anticipated that such a system would have to operate a considerable time into the future until groundwater attained applicable standards. It may be feasible to consider applying this technology over localized portions of the site; however, the vast areal coverage of the site makes centralized treatment extremely challenging and costly to implement. Groundwater Diversion Description. Groundwater can be diverted from its natural path to prevent it from leaching COls from submerged ash or to control where the groundwater eventually comes to the surface. This typically involves the construction of cutoff walls. Cutoff walls can be constructed with soil-bentonite slurry, cement grout, or geosynthetic materials. Slurry wall construction requires the excavation of trenches, which are backfilled with slurry. The slurry prevents the trench from collapsing and prevents water from flowing into it. The trench is filled, typically with a soil-bentonite or cement-bentonite mixture or with concrete, displacing the slurry. Grout curtains are thin, vertical grout walls installed in the ground that are constructed by pressure - injecting grout directly into the soil at closely spaced intervals. The spacing is selected so that the grout forms a continuous wall, or curtain. Polymer grouts are usually used for barrier applications because they are impermeable to gases and liquids and resist acidic and alkaline environments. Grout curtains are similar to slurry walls but typically do not require trenching. Geosynthetic material similar to a sheet pile can be vibrated into the ground, provided the overburden soils do not have too many obstructions that would complicate construction. Site -specific aspects, such as the depth to bedrock, the anticipated groundwater pressure, and the nature of the subsurface, determine which approach is appropriate at a specific site. Applicability to MSS. Diversion of groundwater around the ash basin is not feasible at the MSS site due to the areal extent of the basin and depth to the confining layer. Diverting drainage water more locally, such as the groundwater that is believed to be flowing from the ash basin beneath the dry ash landfill toward the unnamed stream east of that area, appears to be feasible. This is an option that should be explored once a closure approach has been selected. Preliminary assessment is that it would be better to construct the cutoff wall closer to the ash basin than near the stream, possibly by using the haul road as an operational platform for either slurry wall construction or emplacement of a grout curtain. These options should be further explored once the ash basin is closed and the results of soil borings along the potential alignment are available. Furthermore, the likely effectiveness of a cutoff wall should be assessed by modeling. Assumptions The following assumptions were used when developing potential remedial alternatives for the MSS site: EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE MARSHALL STEAM STATION ASH BASIN SITE • All of the ash will be capped in place. Where feasible, berms will be removed or graded to reduce ponding of surface runoff. • The COI concentrations observed to date in the monitoring wells are representative of sitewide groundwater quality. The recommended remedial alternatives consider the results of the UNCC groundwater modeling (HDR, 2016a) of the MSS site, which has projected the nature of present and future groundwater quality downgradient of the ash cells and ash storage areas. However, given the areal extent of the ash basin and the anticipated uniformity of chemical constituents in the ash, the model's predictions of localized contaminated areas may be biased by the representativeness of the limited available data or the limitations of the model. Actual conditions may differ, warranting the collection of additional data and the further review of remedial alternatives. • No site inspection or other engineering assessment has been performed regarding the implementability of any option; therefore, concepts presented will need to undergo a constructability assessment (i.e., an evaluation of site conditions to determine whether site -specific conditions would preclude the construction of the technology in the proposed location). Further evaluation of remedial alternatives may be necessary if any of these conditions/assumptions change. CH2M recommends that the modeling be reviewed to determine whether additional calibration is needed before a final remedial alternative is selected. Evaluation of Remedial Alternatives for the MSS The screening above identified MNA, groundwater diversion (east of the ash basin), PRBs, and enhanced natural attenuation using ISCF (or air sparging) as potentially applicable technologies. These technologies were evaluated with respect to feasibility, anticipated benefits, uncertainties, and cost effectiveness. PRBs were determined to be inappropriate, except perhaps as a fallback provision, due to their unproven ability to reduce boron concentrations and relatively high costs. The remedial alternatives for addressing COls in groundwater that were determined to be potentially applicable to the MSS site are discussed below. The alternatives are summarized and compared in Table 1. Alternative 1—No Further Action The purpose of including No Further Action is to provide a baseline for comparison to other measures. With this approach, there would be no further remedial actions conducted at the site to control or remove the source of the COIs, and no further remedial action would be taken. This measure does not include long-term monitoring or institutional controls. Alternative 2—Monitored Natural Attenuation MNA involves regular monitoring for select parameters to ensure that concentrations of COls in the groundwater are attenuating. Monitoring would be maintained until water quality meets 2L Standards or IMACs at the compliance boundary (groundwater). It is anticipated that water quality improvement will occur over time due to natural processes. The monitoring framework would be selected based on modeling and historical results. As several COls are also measured in background wells, a network of background well locations would also be identified so that temporal changes that occur naturally can be monitored, and progress toward attainment of standards can be assessed. Most MNA programs require monitoring at least twice annually. Based on the groundwater model and its projections, it will take a number of years after implementation of site closure measures to attain applicable standards at the compliance boundary. However, MNA was examined in more detail (Miller, 2015), and it appears that precipitation, coprecipitation, and adsorption will greatly reduce the concentrations of at least some of the COIs. The Tier I and partial Tier II MNA demonstrations indicate that MNA is operable on a timescale comparable EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FORTH E MARSHALL STEAM STATION ASH BASIN SITE to active remedial technologies (Miller, 2015). After additional data are obtained, modeling should be reevaluated for the future migration of COls, and a Tier III analysis should be conducted to determine if natural processes are sufficient to address groundwater impacts and prevent COI exceedances at the groundwater compliance boundary. Alternative 3—Monitored Natural Attenuation and Construction of a Groundwater Cutoff Wall East of the Ash Basin Diverting drainage water that is believed to be flowing from the ash basin beneath the dry ash landfill toward the unnamed stream east of that area is an option that should be explored in terms of potential cost effectiveness. A preliminary assessment suggests that the haul road between the ash basin and the unnamed stream could be used as an operational platform for constructing either a slurry wall or a grout curtain. Slurry wall construction and a cutoff wall length of approximately 2,000 feet is projected; the actual required length of the cutoff wall should be determined through the collection of additional data and modeling to ensure proper hydraulic control can be achieved. In addition, it is assumed that water quality would continue to be monitored, as described for Alternative 2. Recommendations CH2M recommends that groundwater monitoring be continued until a comprehensive characterization of the groundwater conditions underlying the site established. Furthermore, groundwater monitoring should continue during and after the closure activities (e.g., Capping) have been completed. While this is going on, efforts should be made to update the groundwater model so that it can fully consider the effects of transitioning recharge as well as the potential effects of site geochemistry, such as COI removal through adsorption onto precipitated COls. Evidence supports Alternative 2 (MNA) as a viable remediation alternative to address groundwater underlying the site; however, further data and analyses are needed to demonstrate this and to verify that the site -specific conditions demonstrate adequate attenuation due to naturally occurring reactions. There are areas onsite where adversely affected groundwater appears to have migrated from beyond where the ash was stored and to be approaching the compliance boundary. It is not clear how much of this effect is simply due to elevated background concentrations of the COIs; this should be investigated further. In addition, recognizing that capping the ash will affect recharge, and that subsequent surface rehabilitation can be designed to direct surface water and, to a lesser extent, groundwater flow in a desired direction, action should be taken during site closure activities to redirect surface and groundwater away from the compliance boundary and any potentially sensitive areas. Alternative 3 (Monitored Natural Attenuation and Construction of a Groundwater Cutoff Wall East of the Ash Basin) may be the most appropriate and cost-effective way to prevent COI migration to the unnamed stream east of the ash basin. References EPRI (Electric Power Research Institute). 2006. Groundwater Remediation of Inorganic Constituents at Coal Combustion Product Management Sites: Overview of Technologies, Focusing on Permeable Reactive Barriers. EPRI: Palo Alto, CA. Evanko, C.R., and D.A. Dzombak. 1997. Remediation of Metals -contaminated Soils and Groundwater. Technology Evaluation Report 97-01. Ground -water Remediation Technologies Evaluation Center, Pittsburgh, PA. Goldberg, S., H. S. Forster, and E. L. Heick. 1993. Boron adsorption mechanisms on oxides, clay minerals, and soils inferred from ionic strength effects. Soil Science Society of America Journal, vol. 57, pp. 704- 708. EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE MARSHALL STEAM STATION ASH BASIN SITE HDR. 2O15a. Comprehensive Site Assessment (CSA) Report for the Marshall Steam Station Ash Basin. HDR Engineering Inc. HDR. 2O15b. Corrective Action Plan (CAP) Part 1 Marshall Steam Station Ash Basin. HDR Engineering Inc. HDR. 2016. Groundwater Flow and Transport Model Marshall Steam Station, Catawba County, NC, prepared by HDR Engineering Inc. in conjunction with the University of North Carolina, Charlotte, NC,. HDR. 2O16b. Corrective Action Plan (CAP) Part2 Marshall Steam Station Ash Basin (draft). HDR Engineering Inc. Ilaysky, J. 2008. "Removal of Antimony from Water by Sorption Materials." Slovak Journal of Civil Engineering. pp. 1-6. http://www.svf.stuba.sk/docs/sjce/2008/2008_2/file3.pdf. ITRC (Interstate Technology and Regulatory Council). 2005. Permeable Reactive Barriers: Lessons Learned/New Directions. Interstate Technology and Regulatory Council, Permeable Reactive Barriers Team, PRBA Washington, D.C., available on the Internet at www.itrcweb.org. Miller, G.P. 2015. Technical memorandum to Mark Filardi, HDR Engineering, Inc. Twidwell, L. G., and C. Williams -Beam. 2002. "Potential Technologies for Removing Thallium from Mine and Process Wastewater: An Abbreviated Annotation of the Literature." European Journal of Mineral USEPA. 2007. Monitored Natural Attenuation of Inorganic Contaminants in Groundwater., EPA/600/R- 07/139. EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FORTH E MARSHALL STEAM STATION ASH BASIN SITE Table 1. Remedial Alternatives Remedial Alternatives Description Anticipated Effectiveness Implementability Uncertainties 1. No Further No remedial measures This alternative provides a baseline for No action would be taken Not applicable. Action implemented. comparison to other measures. to address groundwater COIs. 2. MINA Monitoring of surface water At present, the site is not causing any Demonstrated effectiveness At present, COI exceedances of regulatory criteria are and groundwater after ash significant downgradient at many sites and easy to not impacting sensitive receptors. Ash capping should capping. environmental problems. MINA is implement using existing reduce site -related water contamination, though it protective of human health and the monitoring wells and may take time for groundwater quality to meet environment because site -related sampling sites. remedial objectives. Evidence of MINA mechanisms Cols will normally attenuate overtime attenuating COI has been demonstrated; Tier III after appropriate site closure. evaluation of capacity and other site -specific factors is required. 3. MINA and Construction of a A properly designed and constructed The haul road east of the Subsurface characterization is necessary before construction of a groundwater cutoff wall to cutoff wall should prevent COI ash basin and dry ash construction. Modeling of groundwater flow with groundwater cutoff prevent COI migration from migration to the unnamed stream. landfill should allow cutoff walls of various lengths should be done to wall east of the ash the ash basin to the unnamed sufficient room for prevent groundwater from going around the barrier. basin stream. This alternative construction of a cutoff includes MNA for the rest of wall. the site. 10 EN1009151049DEN