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HomeMy WebLinkAboutNC0005088_10. CSS CAP Part 2_Appx G_FINAL_20160212This page intentionally left blank TECHNICAL MEMORANDUM C:12M,- Evaluation of Potential Groundwater Remedial Alternatives for the Cliffside Steam Station Ash Basin Site PREPARED FOR: HDR Engineering PREPARED BY: CH2M HILL Engineers, Inc. (CH2M) DATE: February 3, 2016 Introduction This memorandum summarizes a remedial technology screening evaluation that was performed for groundwater at Duke Energy's Cliffside Steam Station (CSS) site located near Mooresboro, in Rutherford and Cleveland counties, North Carolina. CSS began operations in 1940 with Units 1-4. Unit 5 began operations in 1972, followed by Unit 6 in 2012. Units 1-4 were retired from service in 2011 as part of Duke Energy's decommissioning and demolition program, and were imploded in October 2015. Currently, only Units 5 and 6 are in operation. Source areas at CSS are defined as the active ash basin, ash storage area, Units 1-4 inactive ash basin, and Unit 5 inactive ash basin. In accordance with the North Carolina Coal Ash Management Act of 2014 (CAMA), Duke Energy Carolinas, LLC (Duke Energy) is required to close the CSS ash basins. Duke Energy has begun removing ash from the Units 1-4 inactive ash basin. Prior to the commencement of ash removal, groundwater samples were collected and analyzed by HDR. Analysis of water samples collected by HDR from shallow, deep (transition zone), and bedrock groundwater monitoring wells shows various levels of ash -related constituents of interest (COI), some of which exceed their respective Title 15A (T15A) North Carolina Administrative Code (NCAC) 02L .0202 standards (2L Standards) or Interim Maximum Allowable Concentrations (IMACs). Some of the areas of wetness (AOW) sampled also contained COls at concentrations that exceeded the 2L Standards. However, the water quality data also indicate that the ash -related constituents do not exceed the applicable North Carolina Surface Water Quality (213) Standards for samples from Suck Creek and the Broad River (HDR, 2015a). AOWs will be addressed in a separate work scope. This memorandum discusses potential remedial alternatives applicable to restoring groundwater quality, which appears to have been adversely affected by the ash leachate in several areas across the CSS site, although not substantively in the compliance monitoring wells. Background The CSS Comprehensive Site Assessment (CSA) (HDR, 2015a) indicates that no water supply wells or springs within a 0.5-mile radius of the Compliance Boundary are impacted by the CSS site ash basin system, and there is no imminent hazard to human health or the environment as a result of groundwater migration from the ash basin or ash storage areas. Groundwater in the shallow aquifer under the CSS site ash basin and beneath the ash storage areas discharges to the Broad River either directly or via Suck Creek. The Broad River serves as a hydrologic boundary for groundwater within the shallow flow layer, prohibiting shallow groundwater flow from the ash basins to properties across the EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FORTH E CLIFFSIDE STEAM STATION ASH BASIN SITE Broad River north of the CSS site. There are no water supply wells between the ash basins or ash storage areas and the Broad River (HDR, 2015a). Exceedances of 2L Standards and IMACs were observed in nearly all of the monitoring wells across the site, including those at the outermost extent of the monitoring well system, but in most cases, the exceedances observed in the outermost wells appear to be related to background water quality. Antimony, cobalt, chromium, iron, manganese, and vanadium are background constituents that occur naturally in regional groundwater at concentrations that sometimes exceed their respective water quality standards (HDR, 2015a). Based on site monitoring to date, there are no known site -related exceedances of the 2L Standards beyond the CSS property boundary (HDR, 2015a). Some COls, such as arsenic, boron, beryllium, lead, and nickel, were reported at concentrations above the 2L Standards in one or more monitoring wells beneath or adjacent to the ash basins and ash storage areas and not in upgradient or background locations (HDR, 2015b). Other COls, such as chromium, cobalt, iron, and manganese, and vanadium that have been observed at concentrations above the 2L Standards in the background wells, were sometimes observed at significantly higher concentrations in the groundwater monitoring wells beneath or downgradient of the ash basins and ash storage areas. Some of these COls, such as beryllium, lead, and nickel, were identified only in shallow monitoring wells at isolated locations (HDR, 2015a). Similarly, only a few monitoring wells had arsenic levels that exceeded the 2L Standards, and there were no indications of significant migration (HDR, 2015a). Boron and sulfate rarely exceeded their respective 2L Standards across the CSS site: boron exceedances are isolated to shallow and deep monitoring wells near or within the active ash basin and ash storage areas. Cobalt, iron, manganese, and vanadium were the primary constituents detected in both shallow and deep groundwater at concentrations that exceeded the background concentrations and 2L Standards. In deeper (bedrock) wells, iron, manganese, and vanadium exceeded their associated 2L Standards or IMACs, but concentrations of these constituents were generally similar to upgradient and background levels and to those in areas likely to be impacted by the ash basin or ash storage areas. Seven COls (antimony, barium, beryllium, cobalt, chromium, sulfate, and TDS) identified in the bedrock groundwater were only found at several isolated locations (HDR, 2015a). Water samples collected from AOWs at the toes of the embankment dams that form the Unit 5 inactive ash basin, the Units 1-4 inactive ash basin, ash storage area, and the active ash basin, which drain towards the Broad River, identified the following COls: aluminum, arsenic, beryllium, cadmium, chloride, chromium, cobalt, lead, thallium, sulfate, and TDS. Water samples collected from the AOWs at the toe of the active ash basin upstream dam that drains to Suck Creek identified the following COls: aluminum, arsenic, barium, beryllium, cadmium, chromium, cobalt, lead, and thallium. Development of Remedial Alternatives Ash removal began at the Units 1-4 inactive ash basin in October 2015. Approximately 423,600 tons of ash will be removed from the basin overtime; the ash is being relocated to a lined on -site landfill. CH2M was asked to identify potentially suitable remedial measures that could be used in part or as stand-alone means to reduce the residual COI concentrations (not attributed to background concentrations) in the groundwater to the 2L Standards. After a review of the groundwater data from the previous two rounds of sampling (summer and fall 2015) and in consideration of some of the recent geochemical modeling results, a disparity in trends was identified. In back-to-back sampling rounds, some areas showed consistent exceedances of 2L Standards whereas others did not demonstrate a trend. For this reason, HDR and CH2M recommend that additional groundwater sampling rounds be conducted in order to adequately evaluate applicable alternatives for remediating residual groundwater impacts over time. This memorandum outlines various remedial measures that could be considered for addressing groundwater impacts once additional groundwater data has been collected and a better understanding EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REM EDIALALTERNATIVES FORTH CLIFFS IDE STEAM STATION ASH BASIN SITE of the groundwater conditions underlying the site is established. This memorandum presents an overview of remedial technologies that should be considered once additional data has been obtained should some active form of remediation be necessary. 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 site. This section also identifies whether the technology is a feasible measure to apply to the CSS site. This screening was used to develop the site remedial alternatives. Source Controls There are various methods to improve groundwater quality at a site including restricting the dissolution of COIs. Potential at -source control measures that could be implemented at ash -disposal sites include the following. Ash Removal Removing the ash and placing it in a lined landfill, as is being implemented for a portion of this site, is generally the most effective source control technology, but it is very costly and may not be required to demonstrate compliance with applicable standards in the future. Unless physical circumstances dictate, it is often not a cost-effective remedial measure. At the CSS site, ash is being removed from the Units 1- 4 inactive ash basins. Ash is being left in place at the Unit 5 inactive ash basin, the active ash basin, and the ash storage area. Hydraulic Controls Where removal of the ash is not technically or economically feasible, recharge through the ash can be reduced by placing an engineered cap or cover over it and/or divert groundwater around it and they may further depress the water table, which will also limit groundwater contact with the ash. Engineered caps can be constructed of natural (clay) or manufactured geosynthetic materials. Caps are commonly constructed on prepared grades to promote surface water flow off the area being addressed and to ensure the low -permeability layer can be constructed with requisite integrity. A drainage layer is commonly placed over the low -permeability area to facilitate shedding of surface water; a vegetative layer is often constructed over the drainage layer to control soil loss and promote evaporation. To divert groundwater from flowing through ash that exists beneath the water table, 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 the trench. Reinforcement is then lowered in, and the trench is filled, typically with a soil-bentonite or cement-bentonite mixture or with concrete, which displaces the slurry. Grout curtains are thin, vertical walls installed in the ground 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 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 can be vibrated into the ground in a manner similar to sheet pile, provided the overburden soils do not have too many obstructions that would complicate construction. Site -specific aspects, such as the required depth, anticipated groundwater pressure, and nature of the subsurface determine which approach is appropriate at a specific site. In -Situ Solidification/Stabilization (ISS) involves mixing the ash and contaminated soils with pozzolanic materials, generally at proportions of 8-12 percent. Common pozzolans are portland cement and blast furnace slag, and the net effect is to reduce or eliminate leaching of COls from the source zones. The net impact of applying ISS to the site is generally to improve ash strength, reduce the leachability of COI, and EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FORTH E CLIFFSIDE STEAM STATION ASH BASIN SITE reduce hydraulic conductivity, which reduces groundwater contact with the COIs. Adding a pozzolan can change the local redox conditions or pH, so the overall impact on COI mobility throughout the water column should be evaluated prior to final selection of this measure. 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. Empirical 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 CSS. Given that some of the ash will be removed, it is reasonable to assume that COls remaining in groundwater downgradient of the basins where ash is being removed would continually decrease in concentration over time. As documented in Appendix F of Corrective Action Plan (CAP) Part 2 (HDR, 2016), the groundwater COls for CSS potentially include antimony, arsenic, barium, beryllium, boron, chromium, cobalt, iron, lead, manganese, mercury, nickel, sulfate, thallium, TDS, and vanadium. Chromium, cobalt, iron, manganese, and vanadium occur naturally in regional groundwater. Tier I analysis indicates that arsenic, barium, beryllium, boron, chromium, cobalt, lead, thallium, and vanadium should be advanced to Tier II determinations of mechanism. A conceptual model for COI attenuation involving reversible and irreversible interaction with clay minerals, metal oxides, and organic matter was proposed. Adsorption to iron oxides and hydroxides is demonstrated for antimony, arsenic, barium, boron, cadmium, chromium, cobalt, iron oxide, lead, magnesium, mercury, nickel, selenium, sulfate, vanadium, and zinc (Dzombak and Morel, 1990). Soil chemistry results at the CSS site show abundant Fe2O3 and manganese oxide values in soils from the CSS site (HDR, 2015a, Table 6-2) and a strong potential for adsorption. A Tier II demonstration based on that conceptual model was partially executed. 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. The groundwater model did not allow for removal of COls via co -precipitation with iron oxides, which likely resulted in an over -prediction of COI transport. Completion of the Tier II tests described in Appendix H of CAP Part 2 (HDR, 2016) will address this issue. It is feasible to consider that MNA can be used partially or entirely to remediate the CSS site, including areas downgradient of Unit 5 inactive ash basin as well as the ash storage areas and active ash basin where ash will remain in place. It is noted that the Broad River is located immediately downgradient of the ash areas and as a result, 2B Standards will apply to groundwater discharging to surface water. 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 site, thereby flushing, diluting, and attenuating the remnant concentrations of COIs. There are various ways to do this, ranging from short-term or temporary methods (e.g., 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 CSS. CH2M evaluated the CSS site to determine if a potential location exists for an infiltration basin, given that remaining ash in the active ash basin may be consolidated on -site. The use 4 EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REM EDIALALTERNATIVES FORTH CLIFFS IDE STEAM STATION ASH BASIN SITE of this technology may be applicable as noted, if areas of the existing active ash basin are clean closed and become available. Directly behind the berm in the active ash basin, there appears to be a limited amount of accumulated ash, which facilitates use of this area for enhanced recharge/flushing. It appears that to some degree, existing conditions have been providing for flushing of surface water through the dam and into the flow layers at the base of it where elevated concentrations of COls have been detected. The structure provides some opportunity for optimization and potentially improvement on existing conditions. As noted above, the Broad River forms the hydrologic boundary to the north and will dictate compliance standards. In -Situ Sorption or In -Situ Chemical Fixation Description. Various measures can be taken to enhance adsorptive removal of COls by blending soil with materials that have a high adsorptive capacity, such as clays, peat moss, and zeolites, into the contaminated material or affected groundwater. Contaminated groundwater can also be treated in -situ using chemical fixation by adjusting the pH and/or redox state of the groundwater (e.g., 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 co -precipitate 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 CSS. Reagent addition (or air sparging) could encourage the precipitation of Cols by changing the redox conditions over targeted areas. Enhancing natural attenuation in this manner could be effected 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 be less expensive since there are no chemical costs, lifetime costs may be comparable (i.e., air sparging has operation 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 on -site during a pilot study to see which works better, since a myriad of site - specific variables affect their comparable performance. Due to the wide distribution of COls and in the shallow and deep zones, the application of ISCF technology would require an extensive application of wells and reagents. Given the extent of the area, it may not be feasible to use this technology to manage the limited exceedances of 2L Standards, particularly when 2B Standards may be applicable for compliance. It may be feasible, however, to use chemical fixation for the treatment of select areas with a greater mass of COls to reduce the flux of COls through the aquifer to reduce loading for MNA treatment. Application of the technology for this purpose could be considered in the future following re-evaluation of CSS site conditions following partial source removal and additional monitoring rounds. Permeable Reactive Barrier Description. A permeable reactive barrier (PRB) is a passive form of in -situ water treatment that removes COls in a reactive subsurface zone (ITRC, 2005). PRBs are typically constructed by excavating a trench that fully penetrates the saturated zone of the unconsolidated aquifer and places reactive material in the trench to treat the groundwater. Reactive material may be media that absorbs or adsorbs COls or potentially forms precipitates with COls to reduce dissolved concentrations. Specialized equipment (for example, http://www.dewindonepasstrenching.com) can be used to trench and place reactive 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, and installation depth may be determined by the media required to treat the COIs. EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FORTH E CLIFFSIDE STEAM STATION ASH BASIN SITE A funnel -and -gate system can also be used to channel the contaminant plume into a gate that contains the reactive material. The funnels are non -permeable, and the simplest design consists of a single gate with walls extending from both sides. The main advantage of the funnel -and -gate system is that a smaller reactive region can be used to treat the plume, which can reduce costs. In addition, if the reactive media need to be replaced, it is much easier to do so because there is less material to replace. Removal of inorganics has been accomplished using a range of reactive materials, including apatite, zero-valent iron, carbon, and other media. Site -specific media should be evaluated with a range of reactive adsorbents to best determine the type and blend ratio to effectively remove COls while maintaining hydraulic conductivity. The PRB lifespan is a function of the COI concentration and the media removal characteristics. PRBs may be placed as an interim or long-term measure. The lifespan is generally proportional to cost as the effectiveness generally increases with more media. Due to uncertainty and cost factors, it is common to evaluate PRB design life in terms of decades; therefore, if it is anticipated that the COls will continue to persist in groundwater for multiple decades, long-term remediation may require periodic replacement of the PRB's reactive media. Applicability to CSS. 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). Based on a review of the data, it appears that the PRB could comprise a 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 precipitation of the inorganics, and potentially zero-valent iron to help promote and sustain the reducing conditions. Since impacted groundwater has moved beyond the inactive ash basin berms and active ash basin dam, installing a PRB along the Broad River and base of the dam to intercept the impacted water could be appropriate, but the feasibility of doing so may be limited along the dam by its structural integrity. Furthermore, the Compliance Boundary is the Broad River, and COI concentrations may not be generating a 2B Standard exceedance. Beneath the inactive ash basin for Units 1-4, the grading of the ash berms may allow access for PRB installation; however, once again, the Compliance Boundary will be the surface water body in this vicinity. Near the inactive ash basin for Unit 5, there are multiple locations where the PRB could be constructed based on access; however, it is not clear from existing data if active remediation is required now or in the future. Groundwater Treatment Description. As an alternative to in -situ groundwater treatment methods discussed above, groundwater can be treated above grade. Impacted groundwater would be pumped to the surface (pump -and -treat) or captured at surface AOWs 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 (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 a much larger land area than active systems, 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 TDS. 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 CSS. The area and depth of impacted groundwater across the site varies and some of the elevated COls are likely attributable to background. While a zone of depression could be created to capture groundwater exceeding background concentrations thereby minimizing off -site transfer of COIs, it is anticipated that the system would have to operate over a large area, in many cases adjacent to the Broad River, a considerable time into the future. It may be feasible to evaluate applying this technology 6 EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REM EDIALALTERNATIVES FORTH CLIFFS IDE STEAM STATION ASH BASIN SITE over portions of the CSS site; however, the areal extent of the CSS site makes centralized treatment more challenging. Assumptions The following general assumptions were used when developing the potential remedial alternatives for the CSS site: • Source removal at the Units 1-4 will occur prior to implementation of a groundwater remedy. Source removal will not occur at the active ash basin. • The COI concentrations observed to date in the monitoring wells are representative of the site -wide groundwater quality. If groundwater quality is observed to change significantly during subsequent monitoring, then the selected alternatives may have to be re-examined. It is noted that changes in groundwater quality have been observed and general concentrations of COI are used to determine appropriate remedial actions. • Groundwater modeling results are representative of site conditions and are sufficiently accurate to predict both hydraulic and COI transport under the scenarios modeled. The evaluation of remedial technologies is based on the apparent COI transport response to the modeled scenarios. • 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 report figures (HDR, 2015a, 2015b, 2016). Where boundaries occur upland of a surface water body, the compliance standard is 2L Standard or IMAC; where boundaries occur within a surface water body, the compliance standard is 2B Standard, and applied to observed concentrations (if available) or mixing zone modeled concentrations. Most compliance boundary designations downgradient of the basins are surface water bodies needing to meet the 2B Standard. The only locations where this is not the case is to the east of the active basin and to the northeast and northwest of inactive Unit 5. 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. Remedial Alternatives Development and Evaluation As noted above, the inconsistency of groundwater concentrations makes selection of remedial alternatives challenging. Furthermore, the general indication is that the COI concentrations, though exceeding 2L Standards at various locations, are not exceeding standards at the Compliance Boundary. For the purposes of this alternatives evaluation, therefore, only passive remediation is considered pending collection of additional data over the next year. A No Further Action alternative is also presented for purposes of comparison. The alternatives are described in the following subsections and evaluated in Table 1. Alternative 1—No Further Action The purpose of including a No Further Action alternative is to provide a baseline for comparison to other alternatives. With this approach, there would be no remedial actions conducted at the site to control or remove the source of the COls other than the ash removal already agreed to (Duke Energy, 2015) and no further remedial action would be taken for groundwater. This measure does not include long-term monitoring or institutional controls. EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE CLIFFSIDE STEAM STATION ASH BASIN SITE Alternative 2—Monitored Natural Attenuation (MNA) This alternative involves regular monitoring of select parameters to verify that concentrations of COls in the groundwater are decreasing through naturally occurring processes. Groundwater monitoring would be continued until remedial objectives are met (i.e., groundwater concentrations are at or below applicable standards). It is anticipated, based on the groundwater modeling results (HDR 2015b, 2016) that water quality improvement will occur over time downgradient of the Units 1-4 inactive ash basins following the removal or partial removal of source material from that area. COI attenuation will occur over time due to dilution from recharge to the shallow groundwater table and natural mineral precipitation and COI adsorption. The monitoring framework for MNA would be selected based on modeling and historical results. As several COls 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 progress toward attainment of standards 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 H), and it appears that precipitation, co -precipitation, and adsorption will greatly reduce the concentrations of at least some of the COIs. It was assumed that a subset of the existing and background wells along with 13 new wells —a total of 122 wells —would be monitored at least twice annually for COIs. While attenuation timeframes were not projected, the lifespans of this and all alternatives were fixed at a maximum of 30 years. The data would be compiled and reviewed for MNA annually, and a report would be issued. Recommendations As noted, there is a need to collect additional groundwater data in order to establish trends and identify areas that potentially require remediation. Based on the limited data, the following observations and approaches are outlined. Inactive Ash Basin for Units 1 4 Duke Energy has agreed to remove the ash from the inactive ash basin for Units 1-4 (Duke Energy, 2015; HDR, 2015b). This should, over time, improve the water quality downgradient of the basin. MNA should be implemented to observe that COls are decreasing following completion of the removal action. Compliance in this area is 2B Standards in the Broad River, which have been modeled and predicted to be compliant. Inactive Ash Basin for Unit 5 Ash in inactive basin Unit 5 will remain on -site; MNA alone may not be sufficient to achieve remedial goals; however, Tier III analysis should be completed to determine this. In the primary flow direction of the shallow and deep flow layers where impacts have been observed, an evaluation as to whether active remediation is needed to meet 2L Standards at the groundwater compliance boundary. Localized areas may be treated using ISCF or broader areas may require a more comprehensive approach in the direction of the primary flow path such as a PRB. Active Ash Basin and Ash Storage Area The active ash basin and ash storage area will remain on -site; MNA alone may not be sufficient to manage groundwater to meet standards; however, the Broad River receives recharge from groundwater immediately downgradient of these areas. Indications are that after mixing with surface water, surface water standards should be met. As a contingency, focused source treatment could be considered. The EN1009151049DEN EVALUATION OF POTENTIAL GROUNDWATER REM EDIALALTERNATIVES FORTH CLIFFS IDE STEAM STATION ASH BASIN SITE selection of a source treatment technology will require further data collection, cost -estimation, and bench -scale treatability testing. References Duke Energy. 2015. "Cliffside Steam Station at the Rogers Energy Complex, Coal Ash Excavation Plan for Units 1-4 and Unit 5 Inactive Ash Basins to Address Notices of Deficiency." http:/www.duke- energy.com/pdfs/Cliffside-Excavation-Plan_9-11-15.pdf Dzombak, D. A. and Morel, F. M. M. 1990. Surface complexation modeling: hydrous ferric oxide. New York: Wiley. xvii, 393. 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: 2006. 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 Cliffside Steam Station Ash Basin. HDR Engineering Inc. HDR. 2015b. Corrective Action Plan (CAP) Part 1 Cliffside Steam Station Ash Basin. HDR Engineering Inc. HDR. 2016. Corrective Action Plan (CAP) Part 2 Cliffside Steam Station Ash Basin. HDR Engineering Inc. EN1009151049DEN 9 This page intentionally left blank EVALUATION OF POTENTIAL GROUNDWATER REMEDIAL ALTERNATIVES FOR THE CLIFFSIDE STEAM STATION ASH BASIN SITE Table 1. Remedial Alternatives Remedial Alternatives Description Anticipated Effectiveness Implementability Uncertainties 1. No Further No action beyond the source removals already While COI concentrations may No action would be None. Action planned would occur. decrease following ash removal, taken. groundwater monitoring would stop, so reductions could not be verified. 2. MNA Monitoring of surface and groundwater to verify the At present, exceedances of 2L Easy to implement Ash removal from Units 1-4 should reduction of COI concentrations due to natural Standards are confined to within the using existing eliminate the source of the site -related processes after ash removal or due to natural site boundaries and the site is not monitoring wells and groundwater contamination in the vicinity processes alone for ash left in place. causing downgradient environmental sampling sites of the basins. Further, ash that remains in problems. MNA is expected to be place appears to be having a limited impact effective many areas of the site, if not on groundwater. It will take an unknown the entire site. amount of time for groundwater quality to meet remedial. Additional data will help address this uncertainty. This alternative requires regulatory acceptance of the remedial timeframe. 10 EN1009151049DEN