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WAN"EVAN- MIN- R �.r December 18, 1991 U20 WOVA=M Dormer Unocal Station 2859, Stockton, California r� the soil underlving the tank cavity (Applied GeoSvstems, April 28, 1988. Report No. 38012-1). In June 1988,we installed ground-water monitoring wells MW-1 through LAW-3 (Applied GeoSystems, July 28, 1988, Report No. 35012-2). Results indicated ,L further investigation of ground water down-eradient«as warranted. Therefore. in August 1989.two V5 ground-water wells were installed offsite to delineate dissolved hydrocac bons (Applied GeoSystems, December 12, 1990, Report No. 38012-5). In response to the detection of hydrocarbons in soil and ground water, Applied GeoSystems began periodic monitoring of the ground water in June 1988. Results of previous r monitoring events indicated that dissolved hydrocarbons were detected in water samples collected from the rive wells; the highest concentrations of hydrocarbons were detected in well MW-2. We evaluated the direction of groundwater flow to be northeast F1 0111 JUTIC 1988 to May 1990 (before the wells were dry). The average gradient was 0.002. The direction of ground water flow for May 30. 1990, was to 'he northeast as ,hown on Platte 2, GROUNDWATER SAMPLING A technician from RESNA was onsite November 6, 1991, to collect ground-water samples from wells MW-4 and MW-5 for subjective analysis using procedures summarized in an attachment to this report. Wells MW-I, NIW-2,and MW-3 were dry. The technician noted no subjective evidence of hydrocarbons in the two wells. Cumulative results of subjective analyses are presented in Table 1. The technician purged wells MW-4 and 1vIW-5 and collected water samples for laboratory analyses. A Chain of Custody Record was initiated in the field and accompanied the samples to the laboratory. A copy of that record is attached. The methods of ground-water sampling are discussed in an attachment to this repo:t. The purge water was stored onsite in properly labeled 17-E steel drums pending the results of laboratory analyses. At Unocal's request, cirummed water was removed and disposed of by Armour Petroleum Service and Equipment Corporation of Fairfield, California, an November 20, 1991. Laboratory Analyses and Results The water samples were analyzed for total petro.eum hydrocarbons as gasoline (TPHg) using modified Environmental Protection Agency(EPA)Method 8015 and benzene,toluene, ethyiberzene, and total xylene isomers (BTEX) using EPA Method 602. Laboratory analyses were performed at Applied Analytical Environmental Laboratories in Rancho Cordova, California(Hazardous Waste Testing Laboratory No. E773). Cumulative results of analyses are summarized in Table 2. Copies of the laboratory analysis reports are included with this report. u tz,sis 2 3801x•7 I