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July 30, 1992 Former Unocal Station 2859, Stockton, California Working To Restore Nature A RESNA began periodic monitoring of the ground water in June 1988 Dissolved hydrocarbons were detected in water samples collected from the five wells, the highest concentrations of hydrocarbons were detected in well MW-2. The direction of ground-water flow was to the northeast from June 1988 to May 1990 (before the wells were dry) In February 1992,we observed drilling of five soil borings and installation of one downgradient, offsite monitoring well (MW-7), one onsite monitoring well (MW-6), and three onsite vapor- extraction wells (VW-1, VW-2, and VW-3) PRESENT GROUND-WATER SAMPLING i A technician from RESNA was onsite May 21, 1992, to measure depths to ground water and collect ground-water samples from wells MW-4, MW-5, MW-6, and MW-7 for subjective and laboratory analyses, wells MW-1, MW-2, and MW-3 were dry Our procedures used are summarized in an attachment to this report The technician noted no subjective evidence of hydrocarbons in the four wells, except for an obvious product odor in samples from wells MW-5 and MW-6 (Table 1) The purge water was stored onsite in labeled 17-E steel drums pending the results of . laboratory analyses At Unocal's request, drummed water was removed and disposed of by i Armour Petroleum Service and Equipment Corporation of Fairfield, California, on June 19, 1992 Evaluation of Ground-Water Gradient The ground-water gradient on May 21, 1992, was 0 003, with a flow direction to the northeast (Plate 3) The ground-water flow direction and gradient are consistent with those evaluated on March 4, 1992, and before May 1990 The depth to water in each well is shown on Table 2 The ground-water surface dropped an average of 0 7 feet since the March 1992 monitoring event Survey information is discussed in an attachment to this i report Laboratory Analyses and Results The water samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg) using modified Environmental Protection Agency(EPA) Method 8015 and benzene,toluene, « ethylbenzene, and total xylene isomers (BTEX) using EPA Method 602. Laboratory analyses were performed at RESNA Environmental Laboratories in Rancho Cordova, California (Hazardous Waste Testing Laboratory No E773). The water sample collected from well MW-7 was also analyzed for fecal coliform bacteria at FGL Environmental 0528bbou 38012-9 2