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Quarterly Monitoring, Second Quarter 1990 October 31, 1990 Former Unocal Station No. 2859, Stockton, California AGS 38012-4 analyses were performed at Applied Analytical Environmental Laboratories in Fremont. California (Hazardous Waste Testing Laboratory No. 1211). The results of these and pret'.jus analyses are summarized in Table 2. Copies of the laboratory analysis reports are included with this report. Results of laboratory analyses indicate that concentrations of TPHg and BTEX are highest in samples from monitoring well MW-2, downgradient of a former dispenser island, concentrations of TPHg continue to increase in this well. Concentrations of BTEX in monitoring wells MW-.1, MW-3, MW-4, and MW-S ranged from 9.0 parts per billion (ppb) to below detection levels for the tests used;however, concentrations of TPHg ranged from 30 ppb to 760 ppb. lYvaluation of_Ground-Water_Gradient On May 30, 1990, the geologist measured the depth to ground water in monitoring wells MW-1 through MW-5. The depth to the groundwater surface was measured from the top of the well casing to the nearest 0.01 fact with a Solinst water-level indicator. Depth-to- ground-water data were combined with the elevation of the top of the well casing,previously surveyed by Moldenhauer Engineering Company (a licensed land surveyor from Davis, California)on September 21, 1989,to compil?a Potentiometric Map(Plate P-3). Data used to compile Plate P-3 are shown in Table 3. We evaluated the direction of groundwater flow to be to the northeast and the average gradient to be 0.0023. These data are generally consistent with the flow direction and gradient previously evaluated. Conclusions and Recommendations ,Dissolved gasoline hydrocarbons were detected in groundwater samples from the five wells; the highest concentrations were in samples from well N[W-2. We recommend continuing the quarterly monitoring of the groundwater in the wells while remedial alternatives are evaluated. We have scheduled the next sampling for the week of August 20, 1990 3