ARCHIVED REPORTS_XR0006716 CASE 1

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EXECUTIVE SUMMARY The site is currently an active Valero gasoline service station located at 1001 E Yosemite Avenue in the city of Manteca (San Joaquin County), California The property was formerly a RINO service station, which included three 10,000-gallon gasoline underground storage tanks (UST) and two dispenser islands In 1999, new USTs and dispensers were installed Petroleum hydrocarbon impacts to soil and groundwater were identified, investigated and monitored between 1999 and the present The objective of this project was to define the lateral extent of impacted groundwater to the northwest and west of the site To achieve this objective, three additional groundwater table monitoring wells (MW-7, MW-8, and MW-9) were permitted, installed, developed and surveyed Groundwater in the new and existing wells was monitored, sampled and analyzed Consistent with previous investigations, the uppermost 30 feet of soil in the new well borings consists of a massive sandy soil sequence interbedded with intervals of clay, silt and fine sand mixtures Based on previous cone penetration test (CPT) borings, the soil between 30 and 150 feet consists of a massive sequence of clay and silt soils interrupted by discontinuous intervals of sand and gravel soils The static depth to the groundwater table, as measured in all nine monitoring wells, ranged from 18 73 feet below ground surface (bgs) in MW-6 to 20 62 feet bgs in MW-4 The resulting groundwater surface appears as an irregular plane dipping to the north-northeast at a gradient of approximately 0 0071 foot/foot The soil samples collected at 15, 20 and 25 feet from well borings MW-7, MW-8, and MW-9 did not contain detectable concentrations of gasoline compounds From the nine groundwater monitoring wells, only the sample collected from MW-2 contained detectable concentrations of gasoline compounds 0 6 µg11 of ethylbenzene, 1 3 µg11 of xylenes, and 1 4 µg/l of methyl tertiary butyl ether (MTBE) Results of this investigation confirmed that the direction of groundwater flow is toward the north- northeast, impacted soil and groundwater remains defined to the vicinity of the site, and concentrations of gasoline compounds have declined to below drinking water standards Based on these conclusions, Ramage Environmental recommends that the nine monitoring wells continue to be monitored and sampled for the two remaining quarters of 2005, after which, a request for closure will be submitted if residual gasoline compounds are not detected or remain below drinking water standards I4 4