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Additional Site Assessment and Second Quarter 1999 Monitoring and Sampling Report 8 <br /> 7-Eleven Inc,Store#19976, 1399 N Main St,Manteca,California _ July 15,1999 <br /> 1 <br /> crossgradient(east) of the former tank pit area based on non-detectable to low BTEX,TPH-G and MTBE <br />' concentrations in the groundwater samples collected from monitoring wells MW-3,MW-4 and MW-4A <br /> Low to high hydrocarbon and oxygenate concentrations were detected in crossgradient well,MW-1 which <br /> is located near the former UST pit and pump island (source area) Dissolved hydrocarbon and <br /> oxygenate concentrations remain and have historically been low to non-detectable in the deep source <br /> area well, MW-1A In downgradient well MW-5 (shallow)dissolved hydrocarbon and MTBE <br /> concentrations are low to moderate In MW-5A(deep) benzene and MTBE were detected at low <br /> concentrations, possibly due to limited mixing with shallow zone aquifer water during well installation <br /> These low concentrations are expected to decrease below detection limits during subsequent sampling <br />' events Copies of the analytical reports and chain-of-custody forms are included in Appendix D <br />' 50 CLOSURE ABSTRACT <br /> Soil impacted with low hydrocarbon concentrations appears restncted to a limited area between the <br /> former tank pit and underlying the former fuel dispensers Based on the reported concentrations of <br /> BTEX, TPH-G, and MTBE in the sod samples collected during the installation of the four groundwater <br /> monitoring wells and one soil boring between May 11 and May 17, 1999,the installation of three <br /> groundwater monitoring wells on September 2 and 3, 1998, and the GeoprobeTm soil borings drilled on <br />' April 29, 1998,the lateral and vertical limits of MTBE, STEX and TPH-G have been delineated <br /> The lateral and vertical extent of dissolved hydrocarbons has been adequately delineated downgradient <br /> of the source area based on the low to moderate dissolved hydrocarbon and MTBE concentrations in <br /> the groundwater samples collected from monitoring wells MW-5 and MW-5A The lateral extent of <br />' dissolved hydrocarbons has been delineated crossgradient and upgradient of the former tank pit area <br /> based on low to non-detectable BTEX , TPH-G and MTBE concentrations in the groundwater samples <br />' collected from monitoring wells MW-3, MW-4 and MW-4A The highest dissolved BTEX, MTBE, and <br /> TPH-G concentrations from the groundwater samples collected June 4, 1999,were reported in <br /> monitoring well MW-2, located southwest(slightly upgradient) of the former fuel dispenser island <br /> ' Groundwater monitoring wells MWAA(source area,deep zone), MW-3 (upgradient), MW--4 <br /> (crossgradient), MW-4A(crossgradient, deep zone) and MW-5A(downgradient, deep zone) contained <br /> ' low to non-detectable concentrations of benzene, MTBE and TPH-G <br />