Independent Technical Review of the X-740 Groundwater Remedy, Portsmouth, Ohio: Technical Evaluation and Recommendations

Two major remedial campaigns have been applied to a plume of trichloroethene (TCE) contaminated groundwater near the former X-740 facility at the Portsmouth Gaseous Diffusion Plant in Piketon Ohio. The two selected technologies, phytoremediation using a stand of hybrid poplar trees from 1999-2007 and in situ chemical oxidation using modified Fenton's Reagent from 2008-2009, have proven ineffective in achieving remedial action objectives (RAOs). The 'poor' performance of these technologies is a direct result of site specific conditions and the local contaminant hydrogeology. Key among these challenges is the highly heterogeneous subsurface geology with a thin contaminated aquifer zone (the Gallia) - the behavior of the contamination in the Gallia is currently dominated by slow release of TCE from the clay of the overlying Minford formation, from the sandstone of the underlying Berea formation, and from clayey layers within the Gallia itself. In response to the remediation challenges for the X-740 plume, the Portsmouth team (including the US Department of Energy (DOE), the site contractor (CDM), and the Ohio Environmental Protection Agency (OEPA)) is evaluating the feasibility of remediation at this site and identifying specific alternatives that are well matched to site conditions and that would maximize the potential for achieving RAOs. To support this evaluation, the DOE Office of Groundwater and Soil Remediation (EM-32) assembled a team of experts to serve as a resource and provide input and recommendations to Portsmouth. Despite the challenging site conditions and the failure of the previous two remediation campaigns to adequately move the site toward RAOs, the review team was unanimous in the conclusion that an effective combination of cost effective technologies can be identified. Further, the team expressed optimism that RAOs can be achieved if realistic timeframes are accepted by all parties. The initial efforts of the review team focused on reviewing the site history and data and organizing the information into a conceptual model and findings to assist in evaluating the potential of alternative remediation technologies. Examples of the key conceptual findings of the EM-32 review team were: (1) The Gallia represents the most practical target for deployment of in situ remediation treatment reagents - injection/extraction focused in this zone would provide maximum lateral impacts with minimal potential risk of failure or adverse collateral impacts. (2) The slow release of TCE from clay and sandstone into the Gallia represent a long term source of TCE that can re-contaminate the Gallia in the future - technologies that effectively treat the permeable portions of the Gallia, but do not leave residual treatment capacity in the system are unlikely to achieve long term remedial action objectives. CDM, the site contractor, provided important and useful information documenting the status and preliminary results of the on-site technology alternative evaluation. In the CDM evaluation, potential technologies were either retained (or screened out) in two preliminary evaluation phases and a detailed evaluation was performed on the five alternatives that were retained into the final 'detailed analysis' phase. The five alternatives that were included in the detailed analysis were: (1) hydraulic fracturing with EHC (a solid bioremediation amendment), (2) enhanced anaerobic bioremediation, (3) in situ chemical oxidation, (4) electrical resistance heating, and (5) reactive barriers. In several cases, two or three variants were separately evaluated. The review team found the CDM effort to be generally credible and reasonable. Thus, the review team focused on providing additional considerations and inputs to Portsmouth and on amending and refining the alternatives in ways that might improve performance and/or reduce costs. The Department of Energy Portsmouth Paducah Project Office requested assistance from Department of Energy Office of Environmental Management (EM-32) to provide an independent technical panel to review previous and ongoing remedial activities designed to address TCE contamination in groundwater at X-740. In 1999, a corrective action was implemented at X-740 to address contamination in the groundwater. The DOE planted poplar trees over 2.6 acres of the groundwater plume. After several years of groundwater monitoring, the results indicated that the benefits of phytoremediation were limited and insufficient to meet remedial objectives. In 2007, an alternative remedial strategy based on in situ oxidative treatment was negotiated with Ohio EPA. During FY2008, three treatment phases of chemical oxidation were completed. The general charter for the independent technical review team is to review the results of s previous and ongoing remedial actions to determine whether the current strategy will be effective in meeting the end points specified in the regulatory work plan