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11 <br /> "sir y� <br /> t yV` <br /> c <br /> A S S O C I A T E S I N C <br /> average decrease in the water table elevation of approximately 198 feet since July 2003 The water level <br /> data were used to develop the groundwater elevation contour map (Figure 3) Assuming that horizontal <br /> isotropic conditions prevail, groundwater in the uppermost aquifer beneath the site flowed generally towards <br /> the east with an apparent slight depression in the vicinity of the former UST area The average hydraulic <br /> gradient on October 23, 2003 was calculated to be 0 0009 ft/ft or approximately 4 7 ft/rmle A summary <br /> of groundwater monitoring data is presented in Table 1 <br /> CONCLUSIONS <br /> Concentrations of petroleum hydrocarbon constituents decreased in the groundwater sample collected from <br /> MW-2 during the fourth quarter 2003 below levels detected during recent quarterly monitoring events Low <br /> concentrations of toluene, ethylbenzene, total xylenes, and TPHg were detected in the groundwater sample <br /> collected from MW-3 Concentrations of petroleum hydrocarbons remained at nondetectable levels in <br /> groundwater samples collected from MW-1, MW-4, MW-5, and MW-6 The direction of groundwater flow <br /> and magnitude of the hydraulic gradient are generally consistent with previous quarters <br /> RECOMMENDATIONS <br /> Based on the following, ATC recommends the site be considered for closure <br /> • Petroleum hydrocarbon constituents have never been detected in samples collected from MW-4, <br /> MW-5, and MW-6 (downgradlent monitoring well) <br /> o Benzene was not detected in any of the groundwater samples collected in October 2003 Benzene <br /> has never been detected in samples collected from MW-1 and has not been detected in samples <br /> collected from MW-2 since September 1994 <br /> • MTBE has not been detected in samples collected from MW-1 and MW-2 since February 2001, <br /> and July 2001, respectively The concentrations of MTBE detected in samples collected from these <br /> wells have not exceeded the Maximum Contaminant Level for this constituent <br /> • Laboratory analytical results of groundwater samples collected from MW-3 indicate a decreasing <br /> trend in petroleum hydrocarbon concentrations MTBE has not been detected in samples collected <br /> from this well since April 1998 <br /> • It is ATC's understanding that the nearby municipal supply well has been taken out of service and <br /> therefore reduces the potential for impacted groundwater from the site to impact the local drinking <br /> water supply <br /> Please contact our office at(209) 579-2221 if you have any questions or comments <br /> Respectfully submitted, <br /> vL�sr4e �$ <br /> ATC Associates Inc s <br /> Gt taf <br /> O <br /> Todd C Hafner Jeanne Homsey, P E <br /> Protect Geologist CA Registered Civil Engineer <br /> cc Gerald Thtemann <br /> Jun Barton, RWQCB <br /> s lenvuronmental1626051quartly14QR-2003 dx 3 <br />