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Atlantic Richfield Company 13 <br /> ARCO 4932 lb E Hairding Way,Stockton,CA June 7 2002 <br /> CONCLUSIONS AND RECOMMENDATIONS - <br /> ' Based on the data collected during this assessment, petroleum hydrocarbons have <br /> impacted both soil and groundwater beneath and downgradlent of the site The absence <br /> of TPHg, benzene, and MtBE and other fuel oxygenates in deep soil borings SB-1 and <br /> SB-2 advanced in July 2001, and In the 80-foot soil sample collected from the boring <br /> for well DMW-1, indicates that petroleum hydrocarbons have been vertically defined In <br /> soil beneath the site Concentrations of benzene and MtBE in groundwater samples <br /> from deep well DMW-1, and from on-site shallow extraction well EW-1 and off-site <br /> shallow groundwater monitoring wells MW-5 through MW-7 reported during the April <br /> 11, 2002 groundwater monitoring event indicates that the vertical and lateral extent of <br /> dissolved petroleum hydrocarbons and MtBE have not been defined beneath and <br /> downgradlent of the site As proposed in past work plan submittals, the installation of <br /> ' two shallow off-site monitoring wells, two deep off-site monitoring wells, and one off- <br /> site extraction well are scheduled to be completed between June 10 and 14, 2002 <br /> Based on the measurable drawdown of water levels in the pumping well and observation <br /> wells at pumping rates ranging between 4 and 6 gpm, and the calculated capture zone <br /> radii of 31 feet and 48 feet In the downgradlent and cross-gradient directions, <br /> ' respectively, GWE appears to be a capable remedial alternative for removing petroleum <br /> hydrocarbons In soil and groundwater beneath the site, however additional groundwater <br /> {' extraction wells are needed to capture the dissolved petroleum hydrocarbon plume <br /> downgradlent of the site As proposed in SECOR's March 11, 2002 work plan <br /> addendum, SECOR has scheduled the installation of a downgradlent off-site extraction <br /> well, which will capture and prevent the further downgradlent migration of the <br /> dissolved petroleum hydrocarbon/MtBE plume Following the completion of the <br /> scheduled off-site assessment scheduled for June 10 through 14, SECOR will submit a <br /> report including the results of the off-site assessment, and a work plan for the <br /> installation of additional GWE wells and a GWEISVE treatment system by July 26, <br /> 2002 <br /> 1 <br /> 1 <br /> 1 <br /> ' 11ARC0149321Reports'Asmt&AgfrTstRpt doc <br />