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' a <br /> 1 URS <br /> Mr Mike Infurna Page 2 <br /> San Joaquin County Environmental Health Department <br /> ARCO 6020 <br /> September 29, 2003 <br /> In late January of 2001, SECOR International, Inc (SECOR) installed six groundwater monitoring wells <br /> (MW-5 through MW-10) at the site to assess the extent of petroleum hydrocarbon impacts MtBE was <br /> reported in three soil samples collected during the well installation at a maximum concentration of 0 657 <br /> mg/kg(MW-6 at 25 feet bgs) Benzene was reported in one soil sample at a concentration of 0 00978 mg/kg <br /> (MW-9 at 30 feet bgs) All other analytes were reported as below laboratory detection limits in the boring soil <br /> samples Depth to groundwater was approximately 20 feet bgs Soil analytical results from previous <br /> investigations are included in Attachment B <br /> Quarterly groundwater monitoring activity(utilizing groundwater monitoring wells MW-5 through MW-10) <br /> has been conducted at the site since February of 2001 MtBE and tert-amyl methyl ether(TAME)have been <br /> reported at maximum concentrations of 4,200 and 7 1 jcg/L in well MW-6 (June 2001) MtBE and other <br /> oxygenates have never been reported above laboratory detection limits in wells MW-5 and MW-7 through <br /> MW-10 Groundwater flow direction at the site has been inferred to range from northeast to northwest <br /> (Table 1) <br /> Local Groundwater Use <br /> In August 2003, URS conducted a well records file review at the Department of Water Resources (DWR) <br /> The information requested was as follows general well information,well construction information,and well <br /> locations within a radius of 2,000 feet from the site No existing water supply wells were identified within the <br /> search area According to the City of Manteca Water Department,there are no municipal wells located within <br /> the search area <br /> ASSESSMENT ACTIVITIES AND FINDINGS <br /> Three onsite wells (MW-11, MW-12, and MW13D) were installed to monitor groundwater further down <br /> gradient from the USTs, dispenser islands and MW-6 The following paragraphs present a discussion of <br /> drilling and monitoring well installation, the subsurface conditions encountered, monitoring well <br /> development,sampling,and surveying,and the analytical data for groundwater samples collected from all site <br /> monitoring wells <br /> Drilling and Monitoring Well Installation <br /> On July 15,2003,Woodward Drilling of Rio Vista,California,installed three 2-mch groundwater monitoring <br /> wells at the site Prior to drilling,utility clearance was obtained from Underground Service Alert(USA)and <br /> the locations were cleared to a depth of 5-feet bgs with a water wand device <br /> The borings were drilled using a hollow stem auger(HSA)drill rig with 8-mch outside-diameter(OD)augers <br /> to accommodate the installation of 2-inch diameter groundwater monitoring wells The total depth of MW-I 1 <br /> • <br /> K 1WprocessU5 BP ARCO\0602012003 Assessment16020 Assessrnent&SCM 2003 doc <br />