SITE INFORMATION AND CORRESPONDENCE

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December 30, 1994 • • Page 2 populations which would sustain continued biological activity if aerobic conditions were provided. Additionally, the biofeasibility study indicated that the soils currently contain adequate concentrations of nitrogen and phosphorus to support biological activity, and no inhibitory concentrations of toxic,compounds were identified. In this work plan, remedial objectives for the site will be presented and remedial alternatives will be evaluated. Next, a proposed scope of work will be developed for the preferred alternative, and finally an implementation schedule will be presented for the preferred. alternative. REMEDIAL OBJECTIVES In order to identify the recommended remedial action, remedial objectives were identified in the RAP. These objectives are summarized here to provide a basis for a comparison of the proposed alternatives. The objectives are distinguished as soil and groundwater based. • Soil: To the extent economically feasible, and using established technology, prevent the migration of petroleum hydrocarbons from the soil beneath the referenced site. • Groundwater: Within the physical and economic constraints: (1) eliminate the potential for exposure to groundwater with petroleum hydrocarbon concentrations in excess of acceptable compound levels, (2) eliminate hazardous conditions associated with the presence of petroleum hydrocarbon compounds in the subsurface environment, (3) protect the potential use of groundwater from the shallow water-bearing zone as domestic and municipal supply, and (4) initiate attenuation of petroleum hydrocarbon concentrations. The emphasis of the soil objective is to address soils in the vadose zone immediately above the piezometric surface. As shown on Figure 3, residual petroleum hydrocarbons in the vadose zone appear in greatest concentrations in the vicinity of Wells U-1, U-2, U-5, U-A, U-C, and Test Pits TP-2, TP-7, TP-8, TP-10 and TP-11. Petroleum hydrocarbon concentrations in soil that do not pose a threat to groundwater will constitute the numerical cleanup goal for soil. Acceptable petroleum hydrocarbon concentrations in soil will be determined by using the environmental attenuation factor method proposed by Marshack(Marshack,�1989). The groundwater-based remedial objective applies to groundwater within a boundary delineation defined by acceptable compound levels. In this way, beneficial uses of groundwater outside the boundary will be protected, along with public health and safety. The potential for hydrocarbon migration along groundwater flow lines to water-supply wells defines the primary exposure pathway. The associated potential exposure route is the ingestion of impacted,groundwater. Beneficial water use and,public health protection require eliminating(reducing)the potential for exposure to groundwater with concentrations in excess of 1 ppb,benzene, 680 ppb ethylbenzene, 1000 ppb toluene, and 1,750 ppb xylenes. These are 3100011 b/wkpin