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i
<br /> r� Interim Remedial Action Test Report,Former Unocal#0187,Stockton,CA
<br /> August 15,2001
<br /> concentrations of TPHg and benzene were detected in wells NP-1-22 and NP-2-60, respectively MtBE
<br /> was detected only in well NP-1-22 at a concentration of 6 70 ppb Quarterly monitoring and sampling of
<br /> the wells is performed by GR Historical groundwater chemical analytical data are summarized in the
<br /> most recent GR report(Second Quarter 2001 Groundwater Monitoring and Sampling Report,dated June
<br /> 25, 2001) Depth to water in the wells ranged from 22 98 to 27 22 feet below top of casing (btoc), and
<br /> overall groundwater flow was to the northeast at gradients ranging from 0 001 to 0 03,which is consistent
<br /> with recent historical data Since August 1994, groundwater beneath the site has risen steadily from a
<br /> depth of approximately 48 feet btoc to its present level
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<br /> GR personnel performed a five-day vapor extraction(VE)pilot test at the site from November 27 through
<br /> December 1, 2000 The VE system operated for a total of 40 hours, and soil vapors were extracted from
<br /> wells UV-1, UV-2, and UV-3 The purpose of the VE pilot test was to evaluate the feasibility of the VE
<br /> technology and collect needed site specific data for the design of a VE system if deemed necessary The
<br /> conclusion of the VE pilot test was that VE technology is not appropriate for this site
<br /> SITE CONCEPTUAL MODEL
<br /> The lack of a significant difference in groundwater elevation between wells screened in Zone A (screened
<br /> from 20 to 35 feet bgs), Zone B (screened from 65 to 75 feet bgS), and Zone C(screened from 110 to 120
<br /> feet bgs) suggests these zones are all part of the same hydrogeolouic unit, and there is hydrostatic
<br /> communication between the uppermost and the deepest depths explored Previous investigations have not
<br /> encountered any fine-grained units that appear to act as a confining layer for groundwater, and all of the
<br /> fine-grained units encountered below first-encountered water are saturated, suggesting that none of these
<br /> units would act as a complete barrier to the downward migration of hydrocarbons
<br /> Based on groundwater analytical results from the most recent groundwater monitoring and sampling
<br /> event on May 14 through 16, 2001, the lateral extent of the dissolved hydrocaibon plume is delineated in
<br /> Zone A and Zone B wells, but not in Zone C wells Well locations are shown on Figure 3 In Zone A,
<br /> impact is limited to the vicinity of wells U-1, U-2, NP-1-22, and NP-2-22 Dissolved hydrocarbon
<br /> concentrations in Zone A drop significantly from the source area near the southeast corner of the site, to
<br /> downgradient well NP-2-22 In Zone B wells, dissolved hydrocarbons were detected in wells NP-1-65,
<br /> NP-2-65, and U-13 Again, there is a drop in concentrations from the source area to downgradient well
<br /> U-13
<br /> The dissolved hydrocarbon plume in Zone C is delineated to the west(upgradient), north, and southeast,
<br /> but not downgradient (east to northeast) of the Unocal site Dissolved hydrocarbons were detected in
<br /> wells U-10, NP-2-116, U-12, U-19, and U-21, with a trace concentration of benzene detected in well
<br /> U-17 (0 853 ppb) TPHg and benzene concentrations detected in Zone C wells range from 175 to 534
<br /> ppb, and 0 853 to 59 8 ppb, respectively, with concentrations diminishing toward the perimeter of the
<br /> plume A conceptual model of the dissolved hydrocarbon plume is shown on Cross Section A-A'
<br /> (Figure 4)
<br /> Reconnaissance observations in the vicinity of the Unocal site indicate there are several sites that have
<br /> had USTs USTs have been removed from the parking area on the northwest comer of East Miner
<br /> Avenue and American Street (Chet's Auto Service and Repair, 545 East Miner Avenue), the parking area
<br /> on the southwest corner of East Miner Avenue and California Street (a former service station, 448 East
<br /> Miner Avenue), and from a site at 601 East Miner Avenue, located southeast of the corner of Lindsay
<br /> Avenue and American Street,which was the Former Vintage Car Wash(Vintage) The building currently
<br /> 24004209-1 5
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