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I ' • 0 1 Executive Summary Residual petroleum hydrocarbons have been found at the Transmission Shop site located at 515 West Charter Way in Stockton, California. Their presence is likely due to the historical use of the property as a retail service station. One 550 -gallon fuel underground storage tank (UST) was reportedly removed in 1996. There had been one other fuel UST and one waste oil UST on the property. It is believed that these other USTs were removed sometime between 1950 and 1971, although no records exist regarding their removal. Subsequent investigations indicated subsurface soil and groundwater at the site contained elevated levels of petroleum hydrocarbons. Primary chemicals of concern are gasoline -based fuel hydrocarbon and associated components; namely, benzene, ethyl benzene, toluene, and xylenes (alternatively referred to as BTEX). Previous remedial actions included the use of soil vapor extraction to remove the volatile petroleum hydrocarbons from vadose zone. After several years of operation, it was estimated that 3,200 pounds of the hydrocarbons had been removed. Recent onsite groundwater monitoring results showed the presence of the hydrocarbons below the groundwater table, with the bulk mass between 30 and 60 feet below ground surface (bgs). In July 2007, the California Regional Water Quality Control Board, Central Valley (CVRWQCB) requested that a feasibility study (FS) and further delineation of the plume extent be performed for the Transmission Shop site. In response to the request, Shaw Environmental, Inc. (Shaw) prepared this FS for the site, on behalf of Mr. Don Rodgers (the property owner). In this FS, four treatment alternatives were identified as potentially applicable: • Alternative 1 — Groundwater pump -and -treat • Alternative 2 — Air sparging with SVE • Alternative 3 — Biosparging • Alternative 4 — In-situ chemical oxidation These alternatives were further evaluated and compared based on technical effectiveness, implementability, and cost effectiveness. Because the groundwater beneath the site has a beneficial use for drinking water supply, the State of California maximum contaminant levels (MCL) is selected as the initial target cleanup goal for groundwater. However, the primary treatment objective will be to remove the bulk mass of the hydrocarbons from the soil and groundwater to the extent technically and economically feasible. Based on the results of comparison, biosparging is considered the most preferred treatment alternative for the site. Although adjustments to the recommendation may be required pending new findings from the plume extent delineation to be conducted in the near future, it is expected that biosparging will still be the preferred choice of treatment. Should biosparging be selected, FS -Onsite Petroleum Hydrocarbon Remediation.doc ES -1 Shaw Environmental, Inc.