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The presence of methane (CH4) indicates that a number of the wells had progressed into <br /> methogenesis <br /> The levels of electron acceptors present and the presence of the reaction products, carbon dioxide, <br /> methane and ferrous iron indicate that the bacteria in the soil and the compounds in the groundwater <br /> have the capability to consume a significant amount of hydrocarbons <br /> The gasoline range hydrocarbon plume associated with this site is declining in concentration and <br /> should continue to do so, since the hydrocarbons source has been removed <br /> Well screen intervals are currently below the top of groundwater (May 1998) but the wells are <br /> screened across the top of the aquifer sand which is capped by a silty clay insuring that <br /> representative water samples have been obtained <br /> RISK BASE CORRECTIVE ACTION— RBCA TIER 2 <br /> In order to help determine the risks associated with this site a Risk Base Corrective Action (RBCA) <br /> Tier 2 Rask Assessment was performed, see Appendix G of Update Status Report 453 <br /> During the Tier 2 Assessment, Site-Specific Target Levels (SSTLs) were calculated, using the <br /> RBCA Spread Sheet System, for the following compounds of concern (COCs) <br /> • Benzene in groundwater and subsurface soils <br /> using the current EPA Cancer SF for Benzene <br /> • Toluene in groundwater and subsurface soils <br /> • Ethylbenzene in groundwater and subsurface soils <br /> • Xylenes in groundwater and subsurface soils <br /> • MTBE in groundwater and subsurface soils <br /> • TPHg as Hexane in groundwater and subsurface soils <br /> Two potential transport pathways were considered during the Tier Two investigation of this site <br /> Vapor Inhalation and Groundwater Ingestion The third major pathway, soil ingestion, was not <br /> considered because there is no documented contamination in the upper three feet of soil and the site <br /> is paved over limiting the probability of the ingestion of contaminated soil <br /> Vapor Inhalation was divided into two categories <br /> 1 Volatilization to on-site inside air, this was not considered for this study because the <br /> j groundwater and soil plumes are not currently located beneath the station building and the <br /> modeling for inside air does not take this into account <br /> 9 R603 Update#54/CLOSURE 3 12/98 <br />