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URS <br /> Mr Mike Infurna Page 3 <br /> San Joaquin County Environmental Health Department <br /> ARCO 6020 <br /> February 16, 2006 <br /> Based on quarterly water-level measurements from October 2002 through January 2006, the inferred <br /> groundwater flow direction is generally to the north at a gradient of 0 001 to 0 016 feet/foot <br /> Groundwater gradients and flow directions are summarized in Table 3 <br /> REMEDIAL ACTION SUMMARY <br /> Impacted soils were over-excavated from beneath the USTs, dispensers, and piping during facility <br /> upgrade activities conducted in April 1999 Soil samples collected after over-excavation was completed <br /> indicated that petroleum hydrocarbon impacts remained in these areas No additional soil or <br /> groundwater remediation has been conducted at the site <br /> CONTAMINANT MASS REMAINING AND DISTRIBUTION <br /> IMass calculations for TPHg and MtBE in soil and groundwater are presented in Attachment E and <br /> summarized below <br /> • Mass Remaining in Soil <br /> Assuming an area of impacted soil of approximately 900 square feet remains beneath the dispenser <br /> islands and UST complex, and that the interval of soil contamination extends between 6 feet bgs and the <br /> water table, which is at approximately 21 feet, the volume of impacted soil is approximately 13,500 <br /> cubic feet Using average historical soil analytical data collected during the UST upgrade (Attachment <br /> A), the average TPHg and MtBE concentrations were assumed to be 220 parts per million (ppm) and 2 S <br /> ppm, respectively Based on these assumptions, the estimated masses of TPHg and MtBE remaining in <br /> soil are 72 7 pounds and 0 57 pounds, respectively <br /> Mass Remaining in Groundwater <br /> The area of impacted groundwater is assumed to be approximately 64 square feet and the MtBE plume <br /> is assumed to be an average thickness of 10 feet Assuming a porosity of 0 32, the volume of impacted <br /> groundwater is 192 cubic feet Assuming an average MtBE concentration of 0 6 gg/L and multiplying <br /> by the two unit conversion factors yields an estimated 0 003 pounds of MtBE dissolved in groundwater <br /> The groundwater mass calculations performed in September 2003 indicated 0 74 pounds of MtBE on <br /> site, therefore, the mass has been reduced by over 99% to 0 737 pounds <br /> ns4�is <br /> 7� a �77 � <br /> K\WprocessV5 BP ARC010602010Eosure Rpts\Text 2_16_06 doc <br />