ARCHIVED REPORTS_XR0005603 CASE 1

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URS Mr Mike Infurna Page 2 • San Joaquin County Environmental Health Department ARCO 6020 February 16, 2006 reported at maximum concentrations of 1,430 and 20 8 mlli rams per kilogram(mg/kg), respectively, in post-excavation soil samples Lead was detected in only two of the soil samples collected 21 mg/kg in sample location D-5 at 2 5 feet bgs and 3 4 mg/kg in sample locat n D-8 at 2 5 feet bgs Three new e 38A gasoline USTs were installed at the site in A4rrA19G� In January 2001, six groundwater monitoring wells (MW-5 through MW-10) were installed at the site to assess the extent of petroleum hydrocarbon impacts MtBE was reported in three soil samples collected during the well installation at a maximum concentration of 0 657 mg/kg (MW-6 at 25 feet bgs) Benzene was reported in one soil sample at a concentration of 0 00978 mg/kg (MW-9 at 30 feet bgs) All other analytes were reported as below laboratory detection limits in the boring soil samples Depth to groundwater was approximately 20 feet bgs, therefore, the soil sample results above may be attributable to smearing of groundwater and not reflect true soil contamination Soil analytical results from this investigation are included in Attachment B I In July 2003, two onsite wells (MW-11 and MW-12) were installed to monitor groundwater down gradient from the USTs, dispenser islands, and MW-6 Monitoring well MW-13D was installed to assess the vertical extent of contamination near MW-6 No analytes were reported at or above the detection limit in the soil samples collected with the exception of one detection of MtBE in sample location MW-13D at 25 feet at a concentration of 0 016 mg/kg (Attachment C) Groundwater was first encountered in this boring at 23 5 feet bgs, therefore, this result may be due to groundwater smear and not be reflective of soil contamination Quarterly groundwater monitoring activity utilizing groundwater monitoring wells MW-5 through MW- 10 has been conducted at the site since February of 2001 Monitoring wells MW-11, MW-12, and MW- 13D were added to the quarterly sampling cycle in July 2003 MtBE and tert-amyl methyl ether(TAME) have been reported at maximum concentrations of 4,200 and 7 1 µg/L in well MW-6 (June 2001) MtBE and other oxygenates have never been reported above laboratory detection limits in wells MW-5 and MW-7 through MW-10 The January 2006 sampling data show that all constituents are below detection limits with the exception of one detection of MtBE identified in MW-13D (0 70µg[L) Site Geology and Hydrogeology Soils encountered during drilling indicate the presence of interbedded gravelly sand, sand, silty sand, ' sandy silt, and silt from ground surface to approximately 45 feet In MW-13D, clay and clayey sand was encountered from 46 to 48 feet bgs, silty sand and sandy silt was encountered from 48 to 55 feet bgs, and clay was encountered between 56 and 60 feet bgs Soil boring logs and a geologic cross-section of the site are included to Attachment D i K 1WprccessV5 SP ARC01060201Closure Rpts\Text 2_16_06 doc