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Work Plan for the
<br /> Former Two 20,000-Gallon USTs Location at Stockton AASF
<br /> California Army National Guard
<br /> 3.2.3 Groundwater.Groundwater samples were collected from three borings in first groundwater
<br /> around the vicinity of the three former 5,000-gallon and two former 20,000-gallon USTs(shown on
<br /> Figure 3-1).Groundwater samples from boreholes B-1,B-2,and B-28 were analyzed for TPHG,
<br /> TPHD,TPHJ,TPHO,BTEX and oxygenates,and lead by EPA Methods SW8015M,SW8260,and
<br /> SW601OB,respectively.These analytical results indicated that no contaminant concentrations were
<br /> detected at levels greater than reporting limits.Table 3-1 lists the groundwater sampling results for
<br /> the samples collected from B-1,B-2,and B-28.Figure 3-2 shows the detected concentrations in
<br /> groundwater for each constituent at each sample depth.
<br /> 3.2.4 Groundwater sampling was conducted from 10 February 1993 to 8 March 2004 at three MW
<br /> locations(MW-1,MW-2, and MW-3)with in the vicinity of the three former 5,000-gallon and two
<br /> former 20,000-gallon USTs.Table 3-2 lists the groundwater sampling results for the samples
<br /> collected from MW-1,MW-2,and MW-3.Figure 3-3 shows the detected concentrations in ground-
<br /> water for each constituent at each MW.Groundwater samples from MW-1,MW-2,and MW-3 were
<br /> analyzed for TPHG,TPHD,TPHJ,TPHG,BTEX and MTBE by EPA Methods SW8015M and
<br /> SW8260,respectively.Analytical results indicated that TPHG and benzene concentrations are
<br /> present in groundwater west of the previous UST location at levels greater than the taste and odor
<br /> threshold of 100 micrograms per liter(gg/L)and maximum contaminant level(MCL)of 1 gg/L,
<br /> respectively(RWQCB, 2003). MW-1 groundwater sampling events reported all analytes less than
<br /> their respective reporting limits,except for the samples collected on 23 March 2000.These ground-
<br /> water samples had benzene,toluene,and xylene concentrations detected at 1.1 gg/L, 1.0 gg/L,and
<br /> 1.7 gg/L,respectively. The detected benzene concentration of 1.1 gg/L exceeded its respective MCL
<br /> of 1.0 gg/L and the detected toluene and xylene concentrations were less than MCLS.
<br /> 3.2.4 MW-2 groundwater sampling events had several detections of TPHG,benzene,ethylbenzene,
<br /> xylenes,and MTBE at concentrations greater than their respective reporting limits in numerous
<br /> sampling events from 1993 to 2004.The maximum detected concentrations of TPHG,benzene,
<br /> ethylbenzene,xylene,and MTBE in groundwater for MW-2 were 149 gg/L for TPHG, 14 gg/L for
<br /> benzene, 0.7 gg/L for ethylbenzene, 1.8 gg/L for xylenes,and 1.49 gg/L for MTBE.The detected
<br /> maximum TPHG and benzene contaminant concentrations of 149 gg/L and 14 gg/L within MW-2
<br /> exceeded their respective taste and odor thresholds and MCL,respectively. TPHG was detected
<br /> greater than the taste and odor threshold in MW-2 during a second sampling event(8 March 2004)at
<br /> a concentration of 110 gg/L.
<br /> 3.2.6 MW-3 groundwater sampling events reported all analytes less than their respective reporting
<br /> limits,except for the samples collected on 31 December 1996.For this event,groundwater samples
<br /> had benzene and xylene concentrations detected less then MCLS at 0.5 gg/L and 0.5 gg/L,
<br /> respectively.
<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 former two 20,000-gallon USTs area. Surface water runoff at the site flows toward
<br /> the north.The potential for surface water infiltration is limited because approximately 90%of the
<br /> site is paved. Therefore,any petroleum constituents present in the subsurface soils could possibly
<br /> continue to migrate to groundwater.
<br /> KAWprocess\00251\CA ARNG\Stockton AASF WP\FINALAWP text.doc 5 June 2007
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