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All <br /> Date July 26,2005 <br /> Quarter 2"d Quarter 2005 <br /> ARCO QUARTERLY GROUNDWATER MONITORING REPORT <br /> Facility No 6020 <br /> Address 1711 E Yosemite Ave , Manteca, CA <br /> ARCO Environmental Engineer Paul Supple <br /> Consulting Co/Contact Person URSNernon Elarth <br /> Consultant Project No 38466530 OA143 <br /> Primary Agency/Regulatory ID No San Joaquin County Environmental Health Department <br /> Assembly Bill 681 Status Rropedy owned by Atlantic Richfield Company <br /> WORK PERFORMED THIS QUARTER (Second 2005): <br /> 1 Performed quarterly groundwater monitoring and sampling <br /> 2 Prepared and subnutted quarterly groundwater monitoring report <br /> WORK PROPOSED FOR NEXT QUARTER (Third 2005): <br /> 1 Perform quarterly groundwater monitoring and sampling <br /> 2 Prepare and submit quarterly groundwater monitoring report <br /> QUARTERLY RESULTS SUMMARY (Second 2005): <br /> Current Phase of Project Monitoring <br /> Frequency of Groundwater Sampling MW-5 through MW-12, and MW-13D sampled quarterly <br /> Frequency of Groundwater Monitoring Quarterly <br /> Is Free Product (FP) Present Onsite No T <br /> Current Remediation Techniques Natural attenuation <br /> Approximate Depth to Groundwater _20 20 to 22 79 feet below ground surface (bgs) _ <br /> Groundwater Flow and Gradient North to Northwest @ 0 002-0 003 feet/foot (ft/ft) <br /> ANALYTICAL DATA REVIEW: <br /> Based on the review of the analytical and hardcopy data (chain-of-custody records,blanks,dilutions, holding times, <br /> LCS/LCSD results,MS/MSD results,and surrogate recoveries), the data are acceptable as reported by the laboratory <br /> with the following limitation <br /> • The reported results for methyl tert-butyl ether (MtBE) in samples MW-6 and MW-13D should be considered <br /> estimated due to a high relative percent difference reported between the associated LCS and LCSD <br /> DISCUSSION <br /> MtBE was the only analyte detected above laboratory reporting limits in groundwater samples collected from site <br /> monitoring wells MW-6 and MW-13D this quarter MtBE concentrations in MW-6 and MW-13D were generally <br /> consistent with the previous quarter No other analytes were detected above laboratory reporting limits in <br /> groundwater samples collected from site monitoring wells this quarter <br /> Groundwater elevations increased relative to the previous quarter The inferred direction of groundwater flow across <br /> . the site generally ranged from the north to northwest at an average hydraulic gradient that ranged from 0 002 to <br /> 0 003 ft/ft <br /> K%Wprocesm\15 RP ARC0W6W0\Quantcr y0005\2 QTR 05\6020 GWMR 2Q05 doe Page 1 <br />