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GROUND HATER <br /> MONITORING AND SAMPLING REPORT <br /> for <br /> Former Mobil Service Station 04-NP4 <br /> 2375 Tracy Boulevard <br /> Tracy, California V E E rk <br /> AUG 2 4 ;"^0 <br /> ENVIRONMEN,AL HEAL FH <br /> PERfV1i i/S f, IHS <br /> INTRODUCTION <br /> This report presents the results and findings of the recent <br /> quarterly ground water monitoring and sampling activities <br /> performed by Alton Geoscience, Inc. at former Mobil Service <br /> Station 04-NP4, located at 2375 Tracy Boulevard, Tracy, <br /> California. The site location is shown in Figure 1. <br /> PROJECT BACKGROUND <br /> Preliminary assessment of the site performed by Kaprealian <br /> Engineering, Inc. (KEI ) in August 1990 following the removal <br /> of four underground storage tanks indicated detectable <br /> amounts of petroleum hydrocarbon constituents in the soil <br /> samples collected from the tank cavity. Four ground water <br /> monitoring wells (MW-1, MW-2, MW-3, and MW-4 ) were <br /> subsequently installed onsite by KEI. Results of chemical <br /> analysis indicated detectable levels of hydrocarbon <br /> constituents in the water samples from MW-3 and MW-4 only. <br /> In January 1987, four additional ground water monitoring <br /> wells (MW-5, MW-6, MW-7, and MW-8 ) were installed to define <br /> the lateral extent of the hydrocarbon contamination. All the <br /> water samples from the additional wells had detectable levels <br /> of total petroleum hydrocarbons. The extent of hydrocarbon <br /> contamination in ground water, however, could not be defined <br /> at that time. <br /> Periodic ground water monitoring and sampling, conducted from <br /> April 1987 to October 1988, indicated the continued presence <br /> of dissolved hydrocarbon constituents in the shallow ground <br /> water beneath the site. Free product had been observed <br /> continuously in MW-3, while dissolved hydrocarbon <br /> constituents had been detected in five of the existing wells. <br /> In April 1990, two more offsite monitoring wells were <br /> installed to obtain additional data related to ground water <br /> contamination in the immediate vicinity of the site. <br /> Periodic monitoring and sampling has been conducted in 1990 <br /> in addition to the ongoing subsurface investigation and <br /> remedial planning study. <br />