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t <br /> and began remedial actions in the vicinity of the impacted soils in mid 1990 After demolition and <br /> disposal of the Maintenance shop, impacted soil was excavated and treated in an on-site <br /> bioremediation treatment cell In 1991, at the request of the San Joaquin PHS-EHD, Dames & <br /> Moore removed and decontaminated a pump island and associated product lines <br /> 3.0 GROUNDWATER <br /> 31 HYDROGEOLOGY <br /> Groundwater beneath the site is contained in the shallow, unconsolidated deposits of recent <br /> alluvium, the Victor Formation, and the underlying unconsolidated deposits of the Laguna <br /> Formation Groundwater in the San Joaquin Valley occurs under unconfined, confined, and <br /> semiconfined conditions In the portion of the valley underlying the site, groundwater in the Victor, <br /> Arroyo Seco Gravels, and Laguna Formations occurs under semiconfined conditions, and shallow <br /> groundwater is unconfined <br /> rDepth to shallow groundwater at the site was 61 feet 1n May, 1994. Depth to deeper <br /> groundwater (below 150 feet below ground surface (bgs)) was 72 feet in California Water Service <br /> (CWS)Well 77-01 located 3500 feet to the north of the site The groundwater gradient has remained <br /> relatively flat, varying from 0 0015 to 0 00056 feet per lmear foot <br /> The eastern San Joaquin County groundwater basin, which includesroundwater underlying <br /> g Y g <br /> the site area, is under a condition of overdraft, and water levels in deep water supply wells have been <br /> dropping significantly over the past few decades, according to the San Joaquin County Flood Control <br /> and Water Conservation District As a result, a regional cone of depression has developed, centered <br /> to the southeast of Stockton (Figure 4) <br /> The precipitation/evaporation balance-in the Stockton area is such that surface moisture has <br /> a limited potential to migrate downward and recharge the groundwater beneath the site. The mean <br /> annual precipitation at the site is approximately 15 inches, and the mean annual evaporation rate is <br /> 66 92 inches, which indicates net evaporation of approximately 50 inches of water per year <br /> 3 2 GROUNDWATER USE <br /> A well survey performed in 1988 (Dames & Moore, July 1988) identified 17 wells within <br /> a 1-mile radius of the site, excluding groundwater monitoring wells These wells are shown in <br /> Figure 5 and listed in Table 1 California Water Service (CWS) Company maintains six active water <br /> SAC116 07 4 AUGUST 1994 <br /> DAMES &MooRE <br />