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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 1.2.2. Previous Environmental Investigations The area of environmental concern is the former location of the diesel and gasoline USTs (Figure 3). The former 10,000 -gallon gasoline and 5,000 -gallon diesel USTs, which were both single -walled steel tanks, were installed sometime in the early 1950s, when the FMS began site operations. As indicated above, the USTs were removed in 1996 (Versar, 2004). Details of historical site investigations have been presented in previous OTIE reports, and most recently the Work Plan (OTIE, 2011c). Findings presented in those documents are summarized below. Soil samples collected during the removal of the USTs indicated that total petroleum hydrocarbons as gasoline (TPH-g); TPH as diesel (TPH-d); and benzene, toluene, ethylbenzene, and total xylenes (collectively referred to as BTEX) were present in the soil. Follow-up soil sampling in March 2000 at depths ranging from 5 to 25 feet below ground surface (bgs) in the vicinity of the former USTs found TPH-g and TPH-d at elevated concentrations (Versar, 2004). In February 2004, five additional borings were drilled at the FMS, including borings B30 to B32 (Figure 3), which were advanced to groundwater in the immediate vicinity of the former USTs and pump island (Versar, 2004). A total of eight soil samples and three groundwater grab samples were collected and analyzed for TPH-g, TPH-d, BTEX and fuel oxygenates, and metals (Versar, 2004). Impacts from TPH-g, TPH-d, and BTEX constituents were detected at depths of up to 40 feet bgs. Analytical results indicated that soils beneath the area southeast and northeast of the previous diesel UST location have likely been impacted by leaks from the former diesel UST, and some contaminants in soil are present at concentrations exceeding Tri -Regional Board guidelines but less than residential preliminary remediation goals (PRGs) established by the United States Environmental Protection Agency (U.S. EPA) Region 9. Groundwater grab samples were collected from the three boreholes (1330 — B32) and analyzed for TPH-g, TPH-d, BTEX and fuel oxygenates, and for lead. Groundwater analytical results indicated that the maximum concentrations of TPH-d and TPH-g exceeded taste and odor thresholds established by the CVRWQCB (CVRWQCB, 2004). The highest concentrations of BTEX exceeded their respective primary maximum contaminant levels (MCLs). The noted impacts indicated that groundwater beneath the former diesel and gasoline UST locations was impacted by leaks from the former diesel and gasoline USTs, and occurred at concentrations greater than applicable taste and odor thresholds and/or primary MCLs. Contaminant concentrations exceeded primary MCLs in each of the borehole locations. The extent of contamination had not been fully defined by the Versar investigations. The site investigations conducted by URS in May and October 2007 were documented in the December 2007 Site Investigation Report (URS, 2007), and included the collection of soil, soil gas, and groundwater samples from five hollow stem auger (HSA) borings at the site (FMS24SB01 through FMS24SB05; Figure 3). Borings were placed in and around the location of the two former USTs and associated piping. Soil, soil gas, and groundwater samples were collected and analyzed for TPH-g and TPH-d, VOCs including methyl tert-butyl ether (MTBE)/BTEX/fuel oxygenates, and metals (URS, 2007). The investigation showed that soil, soil gas, and groundwater contamination extends in all directions from the former USTs, and that BTEX and TPH concentrations exceeded applicable regulatory clean-up levels. In addition, the lateral and vertical extent of contamination was not defined. Based on these results, a second phase of sampling was conducted by URS in October 2007, which included the collection of soil and groundwater grab samples from six HSA borings (FMS24SB06 through FMS24SB11) and four soil gas samples from shallow direct -push probes (FMS24DP01 through FMS24DP04) for analysis by an onsite laboratory. Sampling locations for the October 2007 investigation are OTIE