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f W GE! _ 1386 E. REAMER STREET ROUTE #2 BOX 177 WESTERN GEO-ENGINEERS W00DLANo,CA 95662-9503 FAX(916)562-6273 CALIF.CONTRACTOR#513857 A CORPORATION (916) 662-4541 REGISTERED GEOLOGISTS INTRODUCTION Western Geo-Engineers completed a soil and ground water investigation July 25, 1988. This investigation indicated that the soil contamination in and around the underground tank areas has been alleviated during tank removal and excavation prior to installation of the upgraded tank and piping system. During this investigation three ground water monitor wells where placed on site (MW-1 north of the fuel tank excavation, MW-2 south of the office and southeast of the fuel tank excavation and MW-3 east of the pump islands and fuel tank excavation). Sampling of the monitor wells during the site investigation indicated that minor dissolved, gasoline range, ground water contamination exists near the northwest corner of the station property. This ground water ranges between eleven and thirteen feet below the surface. ]LOCATION Desert Petroleum Service Station #787, a Mobil Station, is located at 950 11th Street, Tracy, San Joaquin County, California, in Sec 29;T2S; R5E; MDB&M at approximately 50 feet above mean sea level, and in Assessor's book 231, parcel number 09-02 (see figures 1 and 2). i PURPOSE AND SCOPE 4 The purpose of this supplemental report is to substantiate the minor dissolved, gasoline range, ground water contamination that exists near the northwest corner of the station property and to document the trend of the ground water gradient two months after the closure of the excavations. On Juiy 7, 1988 the site visit included re-surveying the top of the casings of the monitor wells (post traffic box installation), depth to ground water measurements and ground water sampling. The ground water samples were preserved and chain-of-custody hand delivered to Eureka Laboratory of Sacramento, CA. Another site visit was conducted on August 9, 1988 at which time depth to ground water measurements and ground water samples were obtained. Also during this site visit all water stored in collection drums (from the purging of the monitor wells prior to sampling) was taken to the City of Tracy Water Treatment Plant for disposal and the site was cleared of containment drums. DISCUSSION AND SUMMARY The initial site assessment report (Western Geo-Engineers technical report dated July 25, 1988) showed that the excavation produced from the fuel tanks removal =— -------= - - --acted-as-a-recharge-and-discharge_area,_depe.ndent ui on local precipitation and weather conditions. This excavation was closed (nevi tanks and fill material) on June 1, 1988 and the subsequent ground water data obtained from the monitoring rounds conducted on July 5, and August 9, 1988 show the ground water gradient trended back to the northeast, which was the initial ground water gradient