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1 <br />' Closure Report May 21 1993 <br /> Southland Site No 20680, 9110 Thornton Road, Stockton, CA 3 <br /> unconsolidated alluvial deposits of the Victor Formation, Arroyo Seca Gravel, and the Laguna <br /> Formation are approximately 1,100 feet thick (GSA Bulletin No 1470) Groundwater in this zone is <br />' generally of good quality and is present as an unconfined aquifer Below the Mehrten aquitard, <br /> saline water exists in the confined aquifer of the semi-consolidated Mehrten and Valley Springs <br />' Formations <br />' The unconfined aquifer above the Mehrten aquitard is primarily used for public water supply and Is <br /> also pumped privately for domestic and agricultural uses The water in the confined aqulfer typically <br /> is too saline for agricultural use During drilling at the site, soils encountered to a depth of 50 feet <br /> were predominantly sands with layers of clay, clayey silt, and silty sand Figure 2 illustrates the <br /> location of cross sections A-A' and B-B' Cross sections are included to illustrate the subsurface <br />' lithology (Figures 5 and 6) <br /> 22 Site Investigation <br /> Assessment activities included the following <br /> ■ Between 1986 and 1990, 24 groundwater monitoring wells, 4 recovery wells, 1 soil <br /> boring, and 4 vapor recovery wells were installed to delineate the lateral and vertical <br /> extent of petroleum hydrocarbons in soil and groundwater (Figure 2) Results of <br /> groundwater monitoring and sampling confirmed the presence of separate-phase <br />' and dissolved hydrocarbons in the groundwater <br /> ■ In September 1991, two off-site and one on-site groundwater wells (MW-18, MW-19, <br /> I and MW-20) were installed to monitor groundwater quality Results of sampling <br /> Indicated the soil samples collected from MW-20 at 41 5 feet below grade reported <br /> 9 parts per million (ppm) of total petroleum hydrocarbons-as-gasoline (TPH-G) <br /> I Groundwater results of samples collected from monitoring wells MW-18, MW-19, and <br /> MW-20 reported TPH-G and benzene concentrations below the method detection <br /> limits (MDLs) <br /> I <br /> I <br /> 1211 R013 <br /> GROUNDWATER <br /> TECHNOLOGY, INC <br />