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Site Investigation Report for the
<br /> Former UST Locations at Stockton AASF
<br /> California Army National Guard '
<br /> 4.0 CONCLUSIONS ,
<br /> 4.1 Based on information obtained during the preliminary assessment,previous investigation efforts
<br /> by Versar,Inc,and the URS 2008 site investigation, soil and groundwater adjacent to the two former '
<br /> 20,000-gallon USTs at the AASF has been impacted by releases of petroleum and associated
<br /> constituents to the environment from either the three former 5,000-gallon or two former 20,000-
<br /> gallon USTs.VOCs have also been detected;however,no potential source areas for the VOCs have
<br /> been identified. ,
<br /> 4.2 The results of the soil investigation conducted in February 2004(Versar hic.,2004) determined
<br /> that soil south and southeast of both the three 5,000-gallon and two 20,000-gallon former USTs had '
<br /> no detectable petroleum constituents(TPH-G,TPH-D,TPH-J,TPH-O,BTEX,TAME,TBA,DIPE,
<br /> ETBE,MTBE,or lead) at concentrations greater than their respective reporting limits.The soil
<br /> sampling event determined that the soil adjacent to the southeast and south corner of the two
<br /> 20,000-gallon former USTs partially defined the lateral extent of the previously discovered
<br /> petroleum contamination resulting from leakage from the two USTs.The near-surface soil samples
<br /> (0 to 5 feet bgs)that contained TPH-G concentrations greater than Tri-Regional Board guidelines ,
<br /> were considered the result of minor fuel spills and did not impact soil at 10 feet bgs or deeper. The
<br /> limited area of TPH-G impacts was considered to pose no significant threat to human health or the
<br /> environment because the vertical migration of heavy hydrocarbons is typically limited to the soluble
<br /> fraction and is not considered a concern in shallow soils at concentrations less than 1,000 mg/kg
<br /> (Versar,Inc., 2004).
<br /> 4.3 Based on the URS 2008 site investigation, the lateral extent of groundwater(downgradient and
<br /> cross-gradient)impacts within the area of the two former 20,000-gallon USTs and the three former
<br /> 5,000-gallon USTs has been determined.Previous groundwater sampling by Versar,Inc., determined '
<br /> that the groundwater beneath the vicinity of the two former 20,000-gallon USTs contains TPH-G and
<br /> benzene concentrations at levels that exceed taste and odor thresholds and primary MCLS,
<br /> respectively.The URS investigation found similar contaminants in groundwater at the borings
<br /> located adjacent to the former USTs (B4 and 139). Gasoline was detected in boring B4,and benzene, ,
<br /> CC14,and cis-1,2-DCE were detected in boring B9 at concentrations above primary MCLS,in
<br /> addition to diesel detected at 11,000 µg/L.The lateral extent of TPH-D/TPH-G and BTEX
<br /> contamination in groundwater has been defined in all directions from the former UST source areas. '
<br /> 5.0 RECOMMENDATIONS
<br /> Based on the information obtained for the AASF, the following is recommended: '
<br /> Additional groundwater samples upgradient of URS borings B4 and B9 should be collected to '
<br /> confirm the southern lateral extent of groundwater contamination.Boring B 13,located southeast
<br /> of the former 20,000-gallon USTs, contained TCE above the MCL.This boring is upgradient of
<br /> the former USTs, and no other groundwater samples contained TCE;therefore,the upgradient
<br /> groundwater conditions are a data gap.
<br /> Sample the three existing groundwater monitoring wells because these wells have only been '
<br /> sampled twice in the past seven years and have not been sampled since March 2004. If the wells
<br /> contain high turbidity,redevelop them by surging and bailing for a minimum of 48 hours before
<br /> sampling. '
<br /> K:\Wprocess\00251\CA ARNG\Stocktan AASF Sl\FiNAUSI Rpt.doc 6
<br /> June 2008 I
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