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Weston Solutions, Inc. Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California PREVIOUS INVESTIGATIONS collected from boring BH-14 at a depth of 111 feet bgs and contained 6,900 µg/L TPH-g and 81 µg/L benzene. The BH-14 boring location is shown on Figure 2-2. In June 2008, an undocumented UST was discovered at the planned location of a deep groundwater monitoring well (MW-10) in the sidewalk along South Sutter Street. The UST was discovered while clearing the proposed well location of utilities using air-vacuuming excavating. Therefore, the installation of the well was postponed. In July 2008, a geophysical survey was conducted along the South Sutter Street sidewalk; however, interference from rebar and wire mesh in the sidewalk limited the effectiveness of the survey. Air vacuum borings were advanced in six locations along the sidewalk, and one additional UST was identified in the sidewalk along South Sutter Street in a boring located south of the previously discovered UST. Native soil was observed at the other air-vacuum boring locations. In January 2010, seven additional air-vacuum borings were advanced in the area of the two newly discovered USTs to determine the size and number of the tanks and their location with respect to the street and HSA building. Four USTs were determined to be located beneath the sidewalk. In September 2010, GEMS Environmental Management Services, Inc., removed four 600- gallon, single-walled, steel USTs that were partially filled with sand. Only one of eight soil samples collected from beneath the USTs contained petroleum hydrocarbons(3.2 mg/kg TPH- diesel [TPH-d]). The 2010 UST removal sampling locations are shown on Figure 2-1. In October 2010, monitoring wells MW-4, MW-5, and MW-8 were removed from the monitoring program and abandoned by pressure grouting. In December 2010, deep monitoring well MW-10 was installed near well MW-2 and screened between 94 and 104 feet bgs. Groundwater monitoring wells are shown on Figure 1-2. In 2011, four quarterly groundwater monitoring events were conducted for the newly installed well(MW-10) and wells MW-2,MW-2A,MW-6, and MW-7. Groundwater samples collected from the MW-10 contained relatively low concentrations of TPH-g and benzene. 2-4 February 2021