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Weston Solutions, Inc. <br /> Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California <br /> 2 PREVIOUS INVESTIGATIONS <br /> 2.1 SITE HISTORY, INVESTIGATIONS AND REMOVALS <br /> In April 1988, WESTON supervised the removal of nine underground storage tanks (USTs) <br /> from a former gasoline station located on the northwest corner of Washington and Sutter <br /> Streets. Petroleum hydrocarbons were detected in soil samples collected from beneath the <br /> tanks. <br /> In December 1988, an initial soil and groundwater investigation was performed at the Site. <br /> Soil borings (BH-1 through BH-12) were drilled, and groundwater monitoring wells MW-1 <br /> through MW-3 were installed. The soil investigation was conducted to determine the lateral <br /> and vertical extent of petroleum hydrocarbon contamination in soil above the groundwater <br /> table. Building construction activities were occurring at the Site at the same time as the <br /> investigation, and borings were drilled in excavated areas with variable ground surface <br /> elevations. The borings were drilled to depths ranging from 15 to 30 feet below ground surface <br /> (bgs). Samples collected from two of the borings (BH-3 and BH-4) in the area of the two <br /> former 10,000 gasoline gallon USTs contained total petroleum hydrocarbons (TPH) at <br /> maximum concentrations of 11,000 milligrams per kilogram (mg/kg). Benzene was detected <br /> in only one sample at a concentration of 240 mg/kg from boring BH-3. Petroleum <br /> hydrocarbons were noted to increase with depth with maximum concentrations in samples <br /> collected from just above groundwater at 25 feet bgs. <br /> In 1989, approximately 2,000 cubic yards of contaminated soil was removed from the Site in <br /> general accordance with an April 1989 Remedial Plan. Soil was excavated until groundwater <br /> was encountered at approximately 29 feet bgs. Soil samples collected from the southwest <br /> portion of the excavation indicated that a limited volume of soil in excess of the target cleanup <br /> level remained but could not be removed because of site constraints.Following the excavation, <br /> the pit was backfilled with clean soil (WESTON, 1989). The former USTs and excavation <br /> extents are shown on Figure 1-2. Excavation sampling locations and borings are shown on <br /> Figure 2-1. <br /> Dissolved hydrocarbons were detected in monitoring wells MW-2 and MW-3. The highest <br /> concentrations of petroleum hydrocarbons were detected in monitoring well MW-2. <br /> 2-1 Febraary 2021 <br />