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0 • Work Plan for Groundwater Investigation and Groundwater Monitoring Well Installation Field Maintenance Shop #24, Stockton, California 2.0 PREVIOUS ENVIRONMENTAL INVESTIGATIONS The primary source area for petroleum hydrocarbons and related compounds 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). The subsections below further describe the UST removals and the subsequent environmental investigations/activities that have occurred at the site, to date. 2.1. UST REMOVALS The USTs and associated piping were excavated and removed in 1996. Soil samples collected during the removal of the USTs indicated that TPH-g, TPH-d and BTEX contaminants were present in the soil. In March 2000, soil samples were collected at depths ranging from 5 to 25 feet bgs in the vicinity of the former USTs, and were analyzed for gasoline, diesel, jet fuel, and oil hydrocarbon constituents. Sampling results indicated TPH-g and TPH-d were present in soil at concentrations up to 25.4 milligrams per kilogram (mg/kg) and 209 mg/kg, respectively (Versar, 2004). 2.2. FEBRUARY 2004 SITE INVESTIGATION (VERSAR) In February 2004, five additional borings were drilled at the FMS (1330-1334; Figure 3). Three of the borings (1330-1332) were drilled to groundwater in the immediate vicinity of the former USTs and pump island, and 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). Sampling results indicated the highest concentrations of TPH-d (260 mg/kg), ethylbenzene (18 mg/kg), and xylenes (50.4 mg/kg) were reported at 35 feet bgs in boring B31, located approximately 75 feet south of the former diesel UST. The TPH-d concentration was above the 100 mg/kg Tri -Regional Board Guideline for soil (CVRWQCB, 1990). The noted detections of ethylbenzene, xylenes, and other BTEX compounds were below preliminary remediation goals (PRGs) established by the United States Environmental Protection Agency (USEPA) Region 91. The maximum TPH-g concentration (47 mg/kg) was detected in boring B30 at a depth of 40 feet bgs, below the 100 mg/kg Tri -Regional Board Guideline. All detected concentrations of benzene and toluene in soil were less than their respective residential PRGs. 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 PRGs. Groundwater grab samples were collected from the three boreholes (1330-1332) and analyzed for TPH-g, TPH-d, BTEX and fuel oxygenates, and for lead by USEPA Methods 8015G and D, 8260B, and 6010B, respectively. Groundwater analytical results indicated that the maximum concentrations of TPH-d (1,400 micrograms per liter [pg/L]) and TPH-g (74,000 pg/L) in boring B30, located east of the former UST locations (Figure 3), exceeded taste and odor thresholds of 100 pg/L and 5 pg/L, respectively (CVRWQCB, 2004). The highest concentrations of benzene (up to 24,000 pg/L) and ethylbenzene (up to 2,300 pg/L) were reported in boring B32 and ' Preliminary Remediation Goals (PRGs) established by the USEPA Region 9 have since been combined with other risk-based screening levels into what are currently known as the Regional Screening Levels (RSLs). 5 OTIE