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Soil Boring Report–Former Unocal Station No 0123,Tracy,California <br /> March 10 2000 <br /> given the inability to distinguish between hydrocarbon contributions from the former Shell Station and <br /> any downgradient migration of hydrocarbons from the subject site <br /> J <br /> The most recent quarterly groundwater monitoring data show that in all monitoring wells except U-4, <br /> TPHg and BTEX are non-detectable, and TPHd and MtBE are present up to 71 and 4 4 ppb, respectively <br /> Hydrocarbon concentrations in monitoring well U-4 have been decreasing over time Given the recent <br /> detection of MtBE, which was not used in gasoline at the Unocal station when it closed in the 1960's, <br /> suggests hydrocarbon contributions from a recent release at an offsite source In a review of the <br /> California State Water Quality Control Board's leaking underground storage tank database, one potential <br /> upgradient source exists—M&M Automotive at 60 East 10`h Street, located approximately 50 feet <br /> southeast of the subject site Given that MtBE was not detected in boring B-5, MtBE concentrations in <br /> on-site wells are less than the secondary MCL of 5 ppb as stated in the California Code of Regulations <br /> (CCR) for drinking water standards, no water wells are located within 1,000 feet downgradient of the site <br /> (Pacific Environmental Group, Inc report #910-017 IA, Sod and Groundwater Investigation, dated <br /> December 2, 1998), and elevated hydrocarbon concentration in boring B-5 suggesting a more proximal <br /> source(such as the former Shell Service Station),GR recommends that no further investigation by Unocal <br /> is warranted and the subject site be closed <br /> s - <br /> M J <br />. 24007303-1 5 <br />