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HomeMy WebLinkAboutNC0004961_10. RBSS_CAP Part 2 Appx G_FINAL_20160212 Appendix G Evaluation of Potential Groundwater Remedial Alternatives This page intentionally left blank TECHNICAL MEMORANDUM EN1009151049DEN 1 Evaluation of Potential Groundwater Remedial Alternatives for the Riverbend Steam Station Ash Basin Site PREPARED FOR: HDR Engineering PREPARED BY: CH2M HILL Engineers, Inc. (CH2M) DATE: February 8, 2016 Introduction This technical memorandum summarizes a remedial technology screening evaluation that was performed for groundwater at Duke Energy’s Riverbend Steam Station (RBSS) Ash Basin site near Mount Holly, in Gaston County, North Carolina. RBSS began operation as a coal-fired generating station in 1929 and was retired from service in April 2013. Decommissioning of RBSS is ongoing. Duke Energy has agreed to remove all of the ash in the ash basin and ash and cinder storage areas by excavating and transporting it to offsite lined landfills because the RBSS facility is located near a drinking water reservoir (Mountain Island Lake). Analysis of samples collected by HDR Engineering (HDR) at monitoring wells in the groundwater shows various levels of ash-related constituents (also known as constituents of interest, or COIs), some of which exceed Title 15A North Carolina Administrative Code 02L .0202 Standards (2L Standards) for groundwater or the Interim Maximum Allowable Concentrations (IMACs). Water quality data from upstream and downstream samples collected from the adjacent Catawba River (Mountain Island Lake) indicate that COIs that were sampled do not exceed the applicable North Carolina surface water quality (2B) Standards (HDR, 2015a). The following groundwater COIs for the RBSS site were retained in corrective action plan (CAP) Part 1 (HDR, 2015b): antimony, arsenic, boron, chromium, hexavalent chromium, cobalt, iron, manganese, sulfate, thallium, total dissolved solids, and vanadium have been identified as COIs. Of these, it is likely that chromium, cobalt, iron, manganese, and vanadium are in part related to natural background conditions and will be evaluated further pending installation and subsequent sampling of additional background wells in early 2016. In accordance with North Carolina Coal Ash Management Act of 2014 (CAMA) requirements, Duke Energy will permanently close the RBSS ash basin and storage areas. This memo discusses potential remedial alternatives for the site’s groundwater, which appears to have been affected by the ash leachate and will remain after the ash is removed. The remedial action objective is understood to be preventing occurrence of COI concentrations exceeding 2L and 2B Standards at the compliance boundary or property line, whichever is closer to the waste boundary. Background The RBSS Comprehensive Site Assessment (CSA) Report (HDR, 2015a) indicates that no imminent hazard to human health or the environment is present at the RBSS site as a result of the presence of ash-related constituents in groundwater. There are no water supply wells located between the ash basins and the Catawba River. The Catawba River and Mountain Island Lake serve as lower hydrologic boundaries for EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE 2 EN1009151049DEN regional groundwater in the vicinity of the RBSS site. Mountain Island Lake is a drinking water reservoir. Intakes for the drinking water reservoir are located approximately 7 miles downstream of the facility. The CSA identified constituents that were present in groundwater at one or more locations at concentrations that exceed the background and either the applicable 2L Standards or IMACs. A CSA supplement (HDR, 2016) includes results from a second round of groundwater sampling; however, it should be noted that due to ongoing ash removal actions, analytical results may not represent equilibrium conditions. As noted in the CAP Part 2 Report (HDR, 2016), background groundwater quality relative to the RBSS site boundary are represented by locations south of the facility and south of Horseshoe Bend Beach Road. The monitoring wells installed as background wells during the CSA, and the existing National Pollutant Discharge Elimination System ash basin background compliance well, are not installed in locations that represent true upgradient, background groundwater conditions. Refinement of the proposed provisional background concentrations is expected following the installation of new background wells. Following establishment of background concentrations of COIs and determination of post-ash-removal equilibrium conditions both for site hydrology and COI concentrations, the recommendations presented in this memorandum should be reviewed. The University of North Carolina at Charlotte (UNCC) has attempted to model 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—antimony, cobalt, thallium, and vanadium—to exceed the 2L Standard/IMAC at Mountain Island Lake. As discussed below, COI removal/attenuation from the groundwater as it migrated downgradient from the basins may have been underestimated, since the model did not consider the effects of adsorption by a precipitated COI (that is, iron hydroxide). Development of Remedial Alternatives Excavation and offsite disposal of ash is underway and expected to be completed by August 2019 (HDR, 2015b). CH2M was asked to identify potentially suitable remedial measures that could be used to address impacted groundwater that will remain on site after the ash has been removed. The sections below provide brief descriptions of each of the remedial measures that were identified as potentially suitable for addressing groundwater at an ash disposal site. These measures were then used to develop a set of remedial alternatives for the RBSS site groundwater. The three remedial alternatives are described briefly below and are listed and compared with respect to their anticipated effectiveness, implementability, and associated uncertainties in Table 1, at the end of this document. 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 RBBS site. This section identifies whether the technology is a feasible measure to apply to the RBSS site. This screening process is used in developing the site remedial alternatives. 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 COIs 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. Empirical data are the best indicator of EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE EN1009151049DEN 3 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 the RBSS Site. Given that the ash will be removed, it is reasonable to assume that COIs remaining in groundwater would continually decrease in concentration over time as recharge flushes non-impacted water through the aquifer. As documented in Appendix F of the CAP Part 2 Report (HDR, 2016), the groundwater COIs for the RBSS site are antimony, arsenic, boron, chromium, cobalt, iron, lead, manganese, sulfate, thallium, and total dissolved solids (TDS). 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, p. 6). A Tier I analysis was completed indicating that of the groundwater COIs for the RBSS site, arsenic, boron, chromium, selenium, and thallium should be carried through to a Tier II evaluation. The objective of the Tier II evaluation is to determine the mechanism and rate of attenuation. A conceptual model for COI attenuation involving reversible and irreversible interaction with clay minerals, metal oxides, and organic matter was proposed. A Tier II demonstration based on that conceptual model was partially executed. The most significant finding was that COIs 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 some of the manganese, was removing other COIs through coprecipitation and adsorption, thus confirming that attenuation was occurring. Clay minerals and Fe-Mn-Al oxides were found in all samples. Additional data collection is necessary to complete the Tier II assessment to determine the specific attenuation mechanisms for each COI and to determine the rate of attenuation such that it may be included in groundwater modeling and advancement to a Tier III MNA assessment. The Tier III assessment determines whether the capacity and permanence of the mechanism will be sufficient to achieve remedial goals. The groundwater model did not allow for removal of COI via coprecipitation with iron oxides, which likely resulted in an overprediction of COI transport, causing some of the COIs to exceed the 2L Standards at the compliance boundary in the model output. Completion of the Tier II tests described in Appendix F of the CAP Part 2 Report will address this issue. It is feasible to consider that MNA can be used partially or entirely to remediate the RBSS site. Enhanced Recharge/Flushing Description. It is possible to increase the rate of groundwater quality improvement by increasing infiltration of uncontaminated water into portions of a reclaimed site, thus flushing, diluting, and attenuating the remnant concentrations of boron, sulfate, TDS, and other COIs. 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 the RBSS Site. Following completion of ash removal activities, ponds could be designed in the reclaimed area to increase recharge at the site. Modeling would be required to predict the locations at which the ponds should be placed, based on predicted changes in COI concentrations with increased recharge. The time to achieve remedial objectives will depend upon the infiltration rate and the hydraulic gradients generated. As constituents would be flushed into the Catawba River, monitoring and detailed modeling will be needed during design to address the risk of exceeding 2B Standards in surface water as a result of the changes to the site hydrology. The groundwater models used recharge rates of 6.5 inches per year in the area outside of the primary and secondary cells and 21 inches per year within the ash basins based on anticipated factors influencing recharge. CH2M evaluated the RBSS site to determine if there exists a potential location for EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE 4 EN1009151049DEN an infiltration basin, given that ash removal will be conducted onsite. It was concluded that the primary and secondary basins are so close to the river that implementation of infiltration would likely cause downgradient seepage and, potentially, slope instability. Further, there are exceedances of the 2L Standards upgradient of the basins, which could then migrate around the area of increased infiltration and reach the Catawba River. Consequently, it was decided that there was no practical place at this site to apply this technology. Chemical Fixation Description. Various measures can be taken to enhance adsorptive removal of COIs by soil blending with materials that have a high COI adsorptive capacity, such as clays, peat moss, and zeolites, into the contaminated material or affected groundwater. Groundwater containing COIs can also be treated in- situ using chemical fixation by adjusting the pH and/or redox state of the groundwater, for example, by 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 addition of one or more reagents 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. Applicability to the RBSS Site. Reagent addition (or air sparging) could accelerate the precipitation of COIs by changing the redox conditions over targeted areas. Natural attenuation could be enhanced in this manner using either of two technologies: in-situ chemical fixation (ISCF) or air sparging. While ISCF involves the injection of a chemical oxidant, such as potassium permanganate, air sparging simply involves pumping air into the targeted saturated zone. Although it would appear that air sparging would always be less expensive, since there are no chemical costs, lifetime costs may be comparable (that is, air sparging has operations and maintenance costs that may or may not outweigh the cost of chemicals and possible reinjection events). Therefore, if this technology is part of a selected alternative, it is recommended that both approaches be tested with materials obtained from the RBSS site (bench scale) or onsite (pilot test) during to see which works better, since a myriad of variables affect their comparable performance. Pilot tests would likely need to be performed following completion of excavation after the site hydrology stabilizes. Due to the wide distribution of COIs (that is, along approximately 4,800 feet of shoreline) and in the shallow/deep, and bedrock zones), the application of ISCF technology would require a very extensive application of reagents. For example, at location GWA-2S/2BRU/2BR, downgradient (west) of the ash basins, COIs are present above the 2L Standard or IMAC in all three collocated wells, with screens ranging between 35 feet below ground surface (bgs) to 120 feet bgs. Applying chemical fixation to the RBSS site is most likely to be cost effective for key limited areas of concern, rather than an extensive area. For example, ISCF or air sparging could be applied in the gate portion of a funnel-and-gate configuration to treat a larger area of COI exceedances in groundwater while focusing treatment on a smaller area. With any funnel-and-gate system, water level monitoring would be needed to ensure that the gate is permeable enough and has sufficient treatment capacity to minimize COI migration around or through the system. Periodic reinjection (or air sparging) would be necessary to maintain the desired redox conditions as additional groundwater enters the treatment area. Monitoring would be required to determine the frequency of reinjection, groundwater geochemical modeling would be recommended to evaluate whether the precipitation of COIs will be effective using the recommended reagent(s), and groundwater flow modeling would be required to design the configuration of the funnel-and-gate system. Another potential application of chemical fixation would be the treatment of select areas with a greater mass of COIs to reduce the flux of COIs through the treatment gate, regardless of the selected material EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE EN1009151049DEN 5 that will form the gate. Application of the technology for this purpose could be considered in the future following the reevaluation of site conditions following completion of source removal. Permeable Reactive Barrier Description. Permeable reactive barriers (PRBs) are a passive form of in-situ groundwater treatment that can be constructed to remove both organic and inorganic contaminants. They are typically constructed by excavating a trench that penetrates the saturated zone and then backfilling the trench with media such as limestone aggregate that will react with COIs in the groundwater and thereby remove them in- situ. Specialized equipment can be used to trench and backfill reactive media simultaneously to hasten construction and potentially construct the PRB in locations that would not be feasible using conventional trenching. A funnel-and-gate system can also be used to channel the contaminant plume into a gate that contains the reactive material. The simplest design consists of a single gate with non-permeable walls (the funnel) extending from both sides. The main advantage of this system is that a smaller reactive region can be used to treat the plume, which can reduce costs. In addition, if the reactive media have to be replaced, it is much easier to do so because there is less material to replace. The reactive material either removes the COIs or transforms them into less problematic forms, depending on the COI and the media (ITRC, 2005). The materials composing a PRB must be carefully selected to remove the COIs under site-specific conditions. The design of a PRB can involve the use of multiple types of reactive material depending on the compatibility of each of the COIs to certain PRB media and whether pretreatment is required to enhance the intended removal mechanisms effectiveness. Multiple types of reactive material may be placed simultaneously to create a single reactive zone or sequentially to create multiple reactive zones. The PRB lifespan is a function of the COI concentration and the media removal characteristics, which may be influenced by site-specific geochemical conditions and other competing constituents. PRBs may be placed as an interim or a long-term measure. Lifespan is generally proportional to cost, as the effectiveness generally increases with more media. Due to uncertainty and cost factors, it is common to look at PRB design life in terms of decades; therefore, if it is anticipated that the COIs will continue to persist in groundwater for multiple decades, long-term remediation may require the periodic replacement of the PRB’s reactive media. Removal of inorganics has been accomplished using a range of reactive materials, including apatite, zero-valent iron (ZVI), carbon, and other media. Site-specific media should be evaluated against a range of reactive adsorbents to best determine the type and blend ratio to effectively remove COIs while maintaining hydraulic conductivity. Applicability to the RBSS Site. There have been many successful PRB remedies at sites with a wide range of constituents, but only limited testing with water containing the constituents in ash leachates (EPRI, 2006). Nonetheless, examples exist where each of the individual constituents has been addressed successfully, and these media could be combined to remove multiple constituents. Based on a review of the data, it appears that the PRB could be engineered to include combination of limestone aggregate (to provide PRB stability, transmissivity, and pH buffering) and organic materials (mulch, wood chips, etc.) to promote the reduction of sulfate to sulfide and the precipitation of the inorganics, along with (potentially) ZVI to help promote and sustain the reducing conditions. The location of the PRB, or PRBs, is ideally along the riverfront, just upgradient of the boundary with the river but at select intervals that are downgradient of source areas. A review of data suggests that both shallow and deep aquifer zones are or have the potential to be impacted with COIs and the bedrock zone may also be impacted. Therefore, additional data evaluation should be conducted following completion of removal actions and establishment of site-specific background concentrations of COIs. Additionally, the depth to bedrock is over 150 feet bgs at parts of the RBSS site and installation of a PRB EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE 6 EN1009151049DEN along the entire downgradient compliance boundary encompassing both shallow, deep and bedrock groundwater is not expected to be cost-effective or feasible. Applying a smaller PRB in key locations at which COIs exceed the 2L and/or 2B Standards, as applicable, after taking background concentrations into consideration, may be effective. Accurate groundwater modeling calibrated to post-ash-removal conditions would be critical to inform the need for this design effort. Ex Situ Groundwater Treatment Description. As an alternative to in-situ groundwater treatment methods, impacted groundwater can be extracted from the flow layers and treated above grade. Impacted groundwater would be pumped to the surface (pump-and-treat) or collected from surface areas of wetness (AOW) 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 electrical power and the continual addition of chemicals) or passive (systems that take 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 much larger land area than active systems, but it has the advantages of requiring less maintenance and having lower operating costs. Passive treatment systems, however, can be ineffective at removing select COIs, such as boron. 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 RBSS Site. The area and depth of impacted groundwater across the RBSS site is widespread. While a zone of depression could be created using active pumping to minimize offsite transfer of the COIs, it is anticipated that the system would have to operate a considerable time until groundwater concentrations decrease to meet the applicable Standards. According to the CAP Part 2 (HDR, 2016), a simulation was performed using six wells pumping at a rate of 3 gallons per minute. The results of the simulation show that the modeled simulated supply well configuration and pumping rate would not adequately capture the groundwater in the shallow zone that has been impacted by the ash basin and other source areas. To conclude that containment of the COIs using active pumping is or is not feasible, a more detailed modeling analysis and pilot study would be needed to predict recovery rates and design an efficient recovery system. Currently, it is not expected that groundwater pumping to contain and treat COIs would be cost effective compared to a PRB; due to the proximity of the Catawba River to the COIs to be captured, groundwater pumping would likely be drawing water not only from the site but also from the river. Consequently, this technology is not considered further in the analysis for large-scale treatment. Ex-situ groundwater treatment may, however, have limited application for treatment at AOWs, where pumping is not required to capture the COIs. For example, COIs at which 2B Standards are exceeded may be treated through a constructed wetland prior to discharging at the Catawba River. Assumptions The following assumptions were made when developing the potential remedial alternatives for the RBSS site: • Complete source removal and subsequent seasonal groundwater monitoring will occur prior to selection and implementation of a groundwater remedy. AOWs will be addressed through basin closure activities. • Concentrations of COIs near monitoring wells GWA-3SA and GWA-3D are not associated with the ash basins/cinder storage and are not considered for the purposes of this preliminary alternatives evaluation. EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE EN1009151049DEN 7 • Background concentrations of COIs are being established; for the purposes of this evaluation, any exceedance of the 2L Standards in a monitoring well was considered an exceedance to be addressed by the remedial alternatives. • The COI concentrations observed to date in the monitoring wells are representative of the initial sitewide groundwater quality. If groundwater quality is observed to change significantly during subsequent monitoring following completion of the ash removal, then the selected alternatives will warrant reexamination. • The recommended remedial alternatives are based on the existing RBSS site data set and the UNCC groundwater modeling (Appendix A of the CAP Part 2), which has projected the nature of future groundwater quality downgradient of the ash cells and basins. 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 limited available data or the limitations of the model. Actual conditions may differ, warranting further evaluation of remedial alternatives. • Further evaluation of remedial alternatives may be necessary if any of the aforementioned conditions/assumptions change. CH2M recommends that the UNCC groundwater modeling be reviewed to determine whether additional calibration is needed before it is used for remedial design. • Compliance boundaries are as identified on the CAP Part 2 (HDR, 2016) report figures. Where boundaries occur upland of a surface water body, the compliance standard is 2L or IMAC; where boundaries occur within a surface water body, the compliance standard is 2B and applied to observed concentrations (if available) or mixing zone modeled concentrations. These values may change in the future with the calculation of site-specific background concentrations. • 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 (that is, an evaluation of site conditions following the removal action to determine whether any site-specific conditions would preclude the construction of the technology in the proposed location). Evaluation of Remedial Alternatives The screening above identified MNA, ISCF, PRBs, and limited application of ex-situ groundwater treatment as potentially applicable technologies. These technologies were evaluated with respect to feasibility, anticipated benefits, uncertainties, implementability, and cost effectiveness. The remedial alternatives for addressing COIs in groundwater following ash removal that were determined to be potentially applicable to RBSS are discussed below. Alternative 1—No Further Action The purpose of including No Further Action is to provide a baseline for comparison to other alternatives. With this approach, there would be no remedial actions conducted at the site other than the ash removal action already underway. No further remedial action would be taken for groundwater. This measure does not include long-term monitoring or institutional controls. Alternative 2—Monitored Natural Attenuation MNA involves regularly monitoring select parameters to ensure that groundwater concentrations of COIs in the groundwater are decreasing. Monitoring would be maintained until water quality meets 2L Standards or IMAC levels at the compliance boundary. It is anticipated that groundwater quality improvement will occur over time due to natural processes once the COI source material has been removed at the RBSS site. The monitoring framework would initially be selected based on the existing EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE 8 EN1009151049DEN groundwater modeling and historical analytical results. As several COIs are suspected to be present in background groundwater in the area, a network of background well locations would also be identified so that temporal changes that occur naturally can be monitored, and background standards can be established. This requires that background concentrations be accurately determined so that true source COI concentrations can be assessed. Based on the groundwater model and its projections, COI concentrations would not be effectively reduced by ash removal. However, MNA was examined in more detail (CAP Part 2 Appendix F), and it appears that precipitation, coprecipitation, and adsorption capacity of the natural material will substantially, if not completely, attenuate the COIs. However, continued MNA assessment is warranted. In the assessment, it was assumed that a subset of the existing and background wells—a total of 25 wells—would be monitored at least twice annually for COIs. In addition, six new wells (three locations with two screened depths each) would be installed within the source areas following completion of ash removal. This alternative does not include costs associated with the Interim Monitoring Plan described in Section 9 of the CAP Part 2 (HDR, 2016), but begins upon closure of the ash basin, ash storage area, and cinder storage area. Costs also do not include monitoring activities associated with the AOWs, as these are being addressed by basin closure activities. The monitoring includes a network of monitoring wells assumed to consist of 19 downgradient, 6 source area, and 6 upgradient or background monitoring wells. Monitoring wells are assumed to be sampled semiannually; data would be compiled and reviewed for MNA annually, and a report would be issued. While attenuation timeframes were not projected, the lifespans of this and all alternatives were fixed at a maximum of 30 years. Based on future discussions with North Carolina Department of Environmental Quality , the sampling frequency and program may change; the assumptions herein have been made to support preliminary cost estimation. Alternative 3—Monitored Natural Attenuation and Permeable Reactive Barrier at Key Locations It is anticipated that removal of the ash, the presumed source of most of the COIs at the RBSS site, will resolve most of the groundwater contamination, although it will take time for groundwater concentrations in downgradient areas to decrease. This alternative has been prepared as a contingency only, to provide a comparison to the MNA alternative. If the COIs from the RBSS site exceed the 2L Standards at the compliance boundary or 2B Standards in the Catawba River following removal of the ash (and establishment of seasonal and background groundwater concentrations of COIs), a more active remedial technology could be applied. One potentially applicable technology, depending upon the specific conditions at the location of the exceedances, would be an appropriately constructed PRB between the source area(s) and the Catawba River to reduce contaminant loading to the subsurface. Design investigations would be required, both for optimal placement and selection of appropriate reactive or adsorptive media to incorporate into the PRB layout design. Laboratory tests would be required to confirm the effectiveness of the media on the RBSS site-specific COIs (EPRI, 2006) and geochemical conditions. This alternative includes the construction of a PRB, or segmented PRBs, along the Catawba River or compliance boundary only in key locations where post-ash-removal groundwater monitoring indicates COIs in excess of the applicable standards that are not expected to attenuate based on modeling calibrated for post-removal conditions and considering attenuation mechanisms. If it is determined that this targeted treatment is required, pre-design investigations, including bench scale and/or pilot scale tests, and consideration of the feasibility of air sparging or subsurface reagent injection in lieu of a PRB should be considered. For preliminary cost estimating purposes the following is assumed: • One PRB would be constructed using caissons to deliver reagents to a depth of 100 feet bgs, between the MW-4S/D/BR and MW-5S/B locations (approximately 550 linear feet). EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE EN1009151049DEN 9 • Results from compliance monitoring well MW-13 (located between MW-4S and MW-5S) were used to establish a preliminary media mix that could be tested for use in the PRB. That media mix includes a combination of limestone aggregate (to provide PRB stability, transmissivity, and pH buffering); organic materials (mulch, wood chips, etc.) to promote the reduction of sulfate to sulfide and precipitation of the inorganics; and potentially zero valent iron to help promote and sustain the reducing conditions. • Piezometers would be constructed both upgradient and downgradient of the PRB to record head conditions, as an increasing head on the upgradient side of the PRB indicates potential clogging of the media. The goal would be to construct the PRB with sufficient media to treat COI for the period of time modeling predicts it will take for the COI to attenuate. If additional time is needed, the media life may be extended with injection wells into the existing trench using a carbon substrate and buffering agents (as needed). For costing, it was conservatively assumed that this reinjection would need to be repeated every 2 years. Continued groundwater monitoring as described for Alternative 2 is included in Alternative 3. Recommendations Given that Duke Energy has agreed to remove the ash that is the likely source of most of the documented surface and groundwater impacts, and that background concentrations of several COIs are being established, CH2M recommends the implementation of MNA (Alternative 2). Alternative 3 will likely not be required and, given its relatively high cost, should be considered only if it proves necessary based on monitoring results after ash removal. MNA data will be reviewed on an ongoing basis, and if statistical increasing trends are identified, feasible technologies should be reevaluated after conditions have stabilized. A course of action that may include implementation of Alternative 3 would then be proposed to North Carolina Department of Environmental Quality. 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. ITRC (Interstate Technology and Regulatory Council). 2005. Permeable Reactive Barriers: Lessons Learned/New Directions. Interstate Technology and Regulatory Council, Permeable Reactive Barriers Team, PRB-4, Washington, D.C. http://www.itrcweb.org. HDR. 2015a. Comprehensive Site Assessment Report for the Riverbend Steam Station Ash Basin. HDR Engineering, Inc. HDR. 2015b. Corrective Action Plan (CAP) Part 1 Riverbend Steam Station Ash Basin. HDR Engineering, Inc. HDR. 2016. Corrective Action Plan (CAP) Part 2 Riverbend Steam Station Ash Basin. HDR Engineering Inc. USEPA (United States Environmental Protection Agency). 2000. In Situ Treatment and Remediation of Soil and Groundwater Contaminated with Chromium—A Technical Resource Guide. EPA/625/005. Office of Research and Development. Washington, D.C. EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE 10 EN1009151049DEN USEPA (United States Environmental Protection Agency). 2007. Monitored Natural Attenuation of Inorganic Contaminants in Groundwater. Vol. 1: Technical Basis for Assessment. EPA/600/R-07/139. USEPA (United States Environmental Protection Agency). 2015. “Disposal of Coal Combustion Residuals from Electric Utilities” (Final Rule). EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE RIVERBEND STEAM STATION ASH BASIN SITE EN1009151049DEN 11 Table 1 Remedial Alternatives Remedial Alternatives Description Anticipated Effectiveness Implementability Uncertainties 1. No Further Action No remedial measures implemented. This alternative merely provides a baseline for comparison to other measures. No action would be taken to address groundwater COIs. Not applicable. 2. MNA Monitoring of surface and groundwater after ash removal is an assumed component of Alternatives 2 and 3. At present, the site is not causing any downgradient environmental problems. MNA is protective of human health and the environment because site-related COIs would attenuate over time from natural processes. Demonstrated effectiveness at many sites and easy to implement using existing monitoring wells and sampling sites. At present, COI exceedances of regulatory criteria are not impacting sensitive receptors. Ash removal should eliminate the source of the site-related water contamination, though it may take time for groundwater quality to meet remedial objectives. Evidence of MNA mechanisms attenuating COI has been demonstrated; Tier III evaluation on capacity and other site-specific factors needs to be performed. 3. MNA and PRB at Key Locations Ash removal will probably eliminate the COI problem. However, if water quality is not improved, and there is a concern that the State’s 2L and/or 2B Standards will be exceeded, then a PRB placed in a key location is a potential remedial option. A PRB tailored to the specific COIs of concern and properly emplaced between the source and the compliance boundary river should effectively prevent exceedances of the State’s standards. An appropriately designed PRB should be able to sufficiently reduce concentrations of all of the COIs. If COIs exceed 2L Standards after the other alternatives are implemented, a specialized PRB design may be necessary. However, if this alternative/remedy is required, it is likely that only one or two COIs will have to be removed, as others may have been attenuated, allowing for a more conventional PRB media selection. Bench-scale testing is recommended before full-scale construction.