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i Aeration GAC air toa,ar Iiror P*,:wng Olacnarge matanal Purnpa o — Clean wean out� E:aacdan well Infection well Water level i Contaminated ground wafer Uncontaminated groundwater INTERIM REMEDIAL MEASURE and EPA have evaluated four alternatives for groundwater cleanup at DDRW-Tracy. These four Phase I of the Interim Remedial Measure alternatives are discussed below. (IRM) has been constructed at the northeastern boundary of DDRW-Tracy. The objective of the TCE and PCE are the target chemicals of IRM is to reduce additional contamination from concern to be cleaned up. Other volatile organics migrating off depot. The IRM is a groundwater such as 1,1-dichloroethene, carbon tetrachloride, extraction and treatment system designed to collect and chloroform will be cleaned up along with the groundwater from the upper horizon of the con- TCE and PCE. Inorganics, such as boron and taminated aquifer (about 50-foot depth), treat the nitrate,found at elevated concentrations in the OU water by air stripping, treat volatile organic air #1 plume, and pesticides,such as dieldrin, are not emissions using a vapor-phase granular activated believed to have on-base sources.However,ground- carbon control unit, and dispose of the treated water extracted for treatment may have to be pre- groundwater by shallow well injection. treated for one or more of these compounds, if present in sufficiently high concentrations,to pre- At present,in the first phase of operation,the vent damage to the treatment system and impacts air stripping system operates at about 120 gallons on the reinjection aquifer groundwater quality. per minute(gpm)using an air stripper designed to treat 500 gpm,six extraction wells,three injection Alternative 1, termed "no-action," consists wells, and ten monitoring wells. of no physical cleanup, but monitors the ground- water plume using about 30 existing on-base and SUMMARY OF REMEDIATION off-base monitoring wells. Consideration of the ALTERNATIVES "no action"remedial alternative is required by the National Oil and Hazardous Substances Pollution The OU #1 groundwater FS for DDRW- Contingency Plan (NCP). It also serves as a Tracy was presented to EPA in October 1992. reference for comparison of the cost and non-cost DDRW-Tracy,the Central Valley Regional Water characteristics of the other remedial alternatives. Quality Control Board (RWQCB), California's This alternative would be implemented for 30 Department of Toxic Substances Control(DTSC), years. The estimated costs are shown below: 5