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Work Plan for Groundwater Monitoring Well Installation, Soil Gas and Groundwater Investigation, and Groundwater Monitoring Army Aviation Support Facility, Stockton, California 2.1 UST REMOVALS The three 5,000-gallon USTs and associated piping were excavated and removed in 1990 by Wallace-Kuhl. Five soil samples collected during the removal of the USTs indicated that TPH-g and TPH as jet fuel (TPH-j) constituents were present in the soil beneath the former USTs at concentrations greater than 100 milligrams per kilogram (mg/kg). Two of the five samples collected contained concentrations of TPH-d and TPH-j up to 3,500 mg/kg and 5,100 mg/kg, respectively. An Unauthorized Release Form (no. 90-Ul1) was completed and submitted to the San Joaquin County Environmental Health Department (SJCEHD) on January 31, 1990 (Versar, 2004; URS, 2008). The two 20,000-gallon USTs were reportedly removed in the mid-1990's (URS, 2008). As of this �— date, no documentation has been identified regarding the collection of soil or groundwater samples to assess the potential for contamination beneath the USTs following their removals. 2.2 PREVIOUS GROUNDWATER MONITORING Following removal of the three 5,000-gallon USTs, additional investigations were conducted to further assess and characterize the magnitude of contamination beneath the former USTs. These investigations determined that the petroleum impacts in soil were limited to the former UST area and warranted the installation of three groundwater monitoring wells (MW-1 to MW-3; Figure 3) in 1993 to assess impacts to groundwater(URS, 2008). These wells were monitored six times between February 1993 and March 2000. Over the course of sampling, TPH-g had been detected just once, in MW-2 at a concentration of 149 pg/L (May 1996). TPH-g has not been historically detected in wells MW-1 and MW-3. BTEX constituents and methyl tertiary butyl ether(MTBE), a fuel oxygenate, have been detected sporadically in wells MW-1 and MW-2, at trace concentrations. Only benzene has been detected above its MCL of 1.0 pg/L. 2.3 FEBRUARY 2004 SITE INVESTIGATION In February 2004, three additional borings (B-1, B-2, and B-28; Figure 3) were drilled to groundwater at the site, with soil and grab groundwater samples collected and analyzed for TPH-g, TPH-d, TPH-j, TPH as oil (TPH-o), BTEX, fuel oxygenates, and lead. Results associated with this investigation were presented in the Preliminary Assessment/Site Investigation Update Report, California Army National Guard Facility(Versar, 2004). Boreholes B-1 and B-2 were advanced to groundwater adjacent to the former 20,000-gallon USTs to the immediate southeast. No TPH fractions were detected in soil samples from either boring. MTBE was detected at a concentration of 0.9 mg/kg in the sample collected from ten feet bgs in boring B-1. Lead was detected in soil at low concentrations (<10 mg/kg) with the exception of at 10 feet bgs in B-1 (52.3 mg/kg). Grab groundwater samples from both borings indicated no detections at the method reporting limits for all contaminants. Boring B-28 was advanced to a total depth of 46 feet bgs to assess the impacts to soil and groundwater at approximate location of the surface spill south of the former USTs (Figure 3). Four soil samples and a grab groundwater sample were collected and analyzed for TPH-g, TPH-d, TPH-j, TPH-o, BTEX, and fuel oxygenates. Shallow soil samples collected at 2 and 5 feet bgs each reported TPH-d and TPH-j at concentrations of 130 mg/kg and 420 mg/kg, respectively (Versar, 2004). Wells MW-1 to MW-3 were also sampled on March 8, 2004. TPH-g was detected in MW-2 for only the second time historically at the site at a concentration of 110 pg/L. No other contaminants were detected in groundwater during this sampling event (Versar, 2004). OTIE 6