SITE INFORMATION AND CORRESPONDENCE

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.7303 Pacific Ave:, Stockton -- --'2 ' - 19 August 1998 San Joaquin County Contamination associated with the second generatiori'of USTs was also overexcavated. The tank pit overexcavation activities were completed to the extent of the excavation equipment, (26 feet bgs), in the tank pit. Confirmation soil samples collected from the center of the tank pit at 26 feet bgs revealed 24 - ppm TPHg and 0.77 ppm benzene. Four groundwater monitoring wells were installed on-site between 10 and 12 October 1990. The top of the perforations are located approximately 48 to 49 feet bgs. In every sampling event since March of '1995, groundwater has been above the top of the well perforations for each of the four wells on-site. Groundwater samples collected from these wells have shown non-detectable or low concentrations of contaminants. Groundwater was not investigated in relation to the second generation of USTs. . Monitoring welllMW-2, installed to assess the groundwater impact related to the first generation of USTs, is'located down gradient to'cross-gradient of the second generation tank location. This well may provide a downgradient assessment of the impact, or lack of impact to groundwater, associated with the new tanks. A regional well survey was completed,�for ail known wells�within a 1/4 mile radiusof the site. One well, Inglewood Station, a municipal water supply well, is located approximately 114-mile due east of the site. The groundwater flow direction has below the Former Unocal station has been shown to be to the east. The Inglewood Station well is completed to a total depth of 317 feet bgs. Diesel fuel constituents were detected in soil samples at 1.3 ppm. Groundwater samples have also evidenced_ TPHd concentrations as high as 0.280 ppm. Diesel fuel was never dispensed at this location. SJCEHD believes this contamination is related to the former septic system and disposal field. Remediation: , Approximately 3,180 cubic yards (cu, yds.) of contaminated soil was removed from the two generations of UST excavations. Of this soil, 1175 cu. yds. of soil of soil was transported to and disposed of at Redwood Landfill in Novato, 1,375 cu, yds. of soil was disposed of by Laidlaw Environmental, and 630 cu. yds.was hauled to Forward Landfill. 1 �w...YK.y. `1cY S.�'t �-�i'}Y��1 a.w-C.�_i+Y• }F_}.-..-�� -r`a a1C4 MAx--.Ma,4a ��.�awr��#��.X.rYri,4 a„air�w�-- - - 'I Summary: e Significant overexcavation activities have taken place in both generations of the former tank*locations. A concerted effort.was made to delineate the extent of soil and'groundwater contamination associated with the first generation of USTs. Residual soil contamination does remain in the subsurface in the vicinity of MW-3 at approximately 17 feet bgs..The mass and specific distribution of the contaminated subsurface has not determined. • . . ' " - Soil contamination associated with the second generation of USTs has been adequately defined and. I Z , excavated to the extent feasible. Based on the site specific geology, and the fact that the source (USTs), and source contaminated soil has been overexcavated and removed from the site, to the extent feasible, significant impact to groundwater is unlikely. Based on the information provided, the risk to groundwater from this site is reasonably low. I suggest that we concur with the SJCEHD decision of no further action with regard to the UST activities at the site. IL - r ,