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Site Investigation Report for the <br /> Former UST Locations at Stockton AASF <br /> California Army National Guard <br /> detected in boring B9 at concentrations of 1,840 and 156 mg/kg at depths of 30 and 35 feet bgs, <br /> respectively(exceeds Tri-Regional Board Guidelines) (RWQCB, 1990). Soil borings located north, <br /> northeast, and southeast of B9 contained no diesel; therefore, the lateral extent of diesel <br /> contamination in soils is defined at this site. <br /> 3.2.3 Groundwater. Groundwater samples were collected from all 12 soil borings at first <br /> groundwater encountered(approximately 40 feet bgs). The only contaminants exceeding action <br /> ' levels(primary maximum contaminant levels [MCLs])were benzene at 6 micrograms per liter <br /> (gg/L)(MCL= 1 gg/L) in boring B5 (located north of the former 5,000-gallon USTs) and benzene at <br /> 1.5 gg/L in boring B9 (located at the south side of the former 20,000-gallon USTs). Groundwater <br /> from boring B9 also contained CC14 at 16 gg/L (MCL=0.5 gg/L) and cis-1,2-DCE at 18 gg/L <br /> (MCL=6 gg/L). Gasoline was detected in groundwater from borings B4 and B5 at concentrations of <br /> 740 and 410 gg/L,respectively. Borings B4 and B5 are located adjacent to the former 5,000 gallon <br /> USTs.Diesel concentrations were detected in boring B9 at 11,000 gg/L; no other groundwater <br /> sample contained diesel. The only other contaminant exceeding MCLS was TCE, detected at boring <br /> B 13 at 8.3 gg/L (MCL=5 gg/L); no other groundwater sample had TCE reported. Boring B 13 boring <br /> located southeast of boring B9.The lateral extent of groundwater contamination is defined to the <br /> is <br /> north, east, and west of the former USTs,however, south of boring B9 (upgradient), no data have <br /> been collected. <br /> ' 3.3 Contaminant Migration Potential <br /> 3.3.1 To evaluate the rate of surface water infiltration and the rate of migration of possible contami- <br /> nants, additional quantitative data are needed regarding the physical properties of surface and vadose <br /> zone soils in the areas of the two former 20,000-gallon USTs and the three former 5,000-gallon <br /> USTs. Surface water runoff at the site flows toward the north. The potential for surface water <br /> ' infiltration is limited because approximately 90% of the site is paved.Therefore, any petroleum <br /> constituents present in subsurface soils could possibly continue to migrate to groundwater. <br /> ' 3.3.2 Potential migration pathways at the AASF former tank location include outdoor air, surface <br /> water, soil, and goundwater. The primary release mechanism is the former UST location; thus, <br /> leaching of contaminants downward within the subsurface to groundwater is the potential pathway. <br /> ' 3.3.3 Based on the limited information available and site conditions at the two former 20,000-gallon <br /> USTs at the AASF, the potential for the exposure of receptors (e.g., workers at the AASF)to any <br /> ' petroleum constituents from the locations of the two former 20,000-gallon USTs and the three former <br /> 5,000-gallon USTs through inhalation, ingestion, or dermal contact is possible; therefore, there is a <br /> potential health risk at the site.Potential petroleum releases at the former UST location occurred <br /> primarily below grade.Most of the site is paved; therefore, the transport of near-surface petroleum <br /> constituents through outdoor air or surface water runoff is minimal. The transport of petroleum <br /> constituents through outdoor air, surface water runoff, and infiltration to the subsurface is possible <br /> ' beyond the locations of the two former 20,000-gallon USTs and the three former 5,000-gallon USTs. <br /> The presence of groundwater impacts originating from possible releases from the two former 20,000- <br /> gallon USTs and the three former 5,000-gallon USTs may have occurred. <br />' K:\Wprocess\00251\CA ARNG\Stockton AASF S]\FWAL\SI Rp[.doc 5 <br /> hne 2008 <br />