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F: s1 15, 1991, April il Sampling of Ground-Water Monitoring Wells AGS Aprr85102-1 Beacon Station No. 492, Manteca, California , The geologist then collected ground-water samples from each monitoring well for laboratory analysis. Water purged from the wells was placed in 55-gallon drums approved for this use by the Department of Transportation and stored temporarily at the site pending removal by Ultramar. The ground-water samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg)using Environmental Protection Agency(EPA) Methods 5030 and 502 and benzene, toluene,ethylbenzene,and total xylene isomers (BTEX) using EPA Methods 5030 and 602. The samples were analyzed at Mobile Chem Labs Inc. in Turlock, California (Hazardous Waste Testing Laboratory No. 1223). The results of these and previous analyses are summarized in Table 3. The concentration of TPHg in the sample from well MW-3 was below detection limits for the analytical methods used. A concentration of 56 parts per billion(ppb)TPHg was detected in the sample from well MW-2, and a concentration of 64 ppb TPHg was detected in sample collected from well MW-1. Benzene was detected at concentrations of 2.4 ppb in wells MW-1 and MW-2; benzene was not detected in well MW-3. Rinsate blank samples were collected from the sampling bailer immediately before sampling each well. Two of these rinsate blanks were analyzed(MW-1 and MW-2)to eliminate false positive results that may arise from sampling error. Hydrocarbon concentrations in both rinsate blanks were below detection limits for the analytical methods used. Those results are summarized in Table 3. Concentrations of dissolved benzene and other hydrocarbons continue to fluctuate in the samples from wells MW-1 and MW-2 over the last four sampling events. According to Ultramar, there have been no known releases of product at the site; the increased hydrocarbon concentrations mz.; be due to an offsite source. 2 APP/fed a 49OSYSter"s