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Treatability Study Report and Feasibility Evaluation for In Situ Petroleum Hydrocarbon Remediation Field Maintenance Shop #24, 8020 South Airport Way Stockton, California indicated in Figure 3. The borings were placed further from the former UST locations relative to previous borings to help define the lateral extent of contamination. Soil, soil gas, and groundwater grab samples were analyzed by the on-site lab for BTEX and TPH-g/TPH-d compounds. Direct -push shallow soil gas samples were collected at 8 feet bgs adjacent to four buildings in the vicinity of the USTs, to determine whether a potential health risk exists to workers within the buildings. No constituents of concern were detected in the October 2007 soil samples; however, TPH-g, TPH-d, BTEX constituents, and select chlorinated VOCs were detected in groundwater at concentrations exceeding applicable regulatory standards (URS, 2007). Based on results from the two phases of investigation in 2007, URS concluded that the UST source contamination had reached groundwater and the contamination plume has migrated northeast, but has not extended to the facility boundary (URS, 2007). The report further concluded that the lateral extent of contamination at the former UST site had been determined. The site investigation conducted by OTIE in March 2010 included advancing five soil borings that were subsequently converted to groundwater monitoring wells (FMS-MW1 to FMS-MW5; Figure 3). The borings were advanced using HSA and soil samples were collected at approximately 5 -foot intervals and analyzed for TPH-g, TPH-d, VOCs including BTEX and fuel oxygenates, and total lead. Groundwater samples collected from the monitoring wells after installation and development were analyzed for TPH-g, TPH-d, and VOCs including BTEX and fuel oxygenates. The key findings associated with implementation of the 2010 investigation were.. • Physical soil conditions were shown to be generally consistent with the findings from previous site investigations, as documented in the 2004 Versar and 2007 URS reports, and indicate that the shallow first water bearing zone (35-40 feet bgs) is dominated by interbedded sand units within fat clays and silty/sandy clays. • Although trace concentrations of petroleum hydrocarbon constituents and/or select VOCs were detected intermittently in samples from soil borings for each of the groundwater monitoring wells, only soil samples from the boring for well FMS-MW5 (within the immediate vicinity of the former USTs) indicated marked impacts from petroleum hydrocarbons. • The most significant impacts from petroleum hydrocarbon constituents documented in soil samples from the boring for well FMS-MW5 (160 mg/kg TPH-g at 30 feet) were generally in samples at or near the occurrence of first groundwater (30-45 feet bgs), and included exceedances of potentially applicable regulatory standards for TPH-d, TPH-g, benzene, and/or ethylbenzene. • Groundwater elevations resulting from surveyed well datum elevations indicate that the generally east/northeast direction of groundwater flow presumed in the 2007 URS report is correct, that the gradient is relatively low (0.0015), and that no evidence of separate phase hydrocarbons (SPH) exists on the groundwater. • Groundwater samples collected during the first monitoring event following well installation indicate that groundwater has been impacted with petroleum hydrocarbon constituents and related VOCs at concentrations that exceed potentially applicable regulatory standards. Subsequent groundwater monitoring performed by OTIE for Second Quarter 2010, Third Quarter 2010, and First Quarter 2011, as described above, confirmed that the overall direction of groundwater flow at the site is generally northeasterly, with a relatively flat gradient. No OTIE