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CALIFORNIA GEOPHYSICAL GROUP, INC. �r O Box 578341, Modesto, CA 95357 Tel- (209) 527-1247 August 9, 2002 Fax. (209) 527-4810 V Public Health Services fit?. f HEMMED n/J San Joaquin County GEM V ED ' 304 East Weber Ave 3rd Floor Stockton, CA 95202 AUG 19 ZOOZ ! RE Quarterly Report July 2002 ENVIRONMENT HEALTH PERMIT/SERVICES Darpetro, Inc USA Service Station 749 E Charter Way Stockton, CA Site No 1060 ' This site was sampled on July 24, 2002 Sherwood Labs of Hilmar, California analyzed the samples, their reports are attached ' Table 1 lists the history of the water table depths and elevations at this site since January 26, 1990 Table 2 lists the hydraulic gradients and bearings and Tables 3 and 3B lists the analyses of the groundwater Table 5 lists the readings at the Vapor Extractor during operation, Table 6 lists the laboratory tests of the input and exhaust gasses and Table 7 lists the times of operation Sampling protocols are included '• FINDINGS The groundwater fell approximately 1 14 feet since the last quarterly monitoring event ' The gradient bearing is N 75 E The gradient is 0 0013 Ft/ft The attached hydraulic gradient plot shows the bearing of the gradient See Figure 1 The wells were purged with a decontaminated pump and the measured field parameters stabilized at the end of the purging Water samples were taken with a decontaminated bailer See Protocols Water samples were placed in appropriate glass containers with Teflon-lined seals The groundwater in wells BM-1, BM-5, BM-6, BM-7, BM-8, BM-9 and BM-13 were above California Drinking Water Standards for benzene Wells BM-9 and BM-7 are related to the ' contamination entering from the Arco Station to the west There is very little to no benzene in wells BM-3 and BM-4, which are between the Arco Station and the old tank pit at this site The benzene in well BM-13 is possibly caused by the mounding of the groundwater ' near the vapor sparging well between BM-5 and BMA See Figure 2 TPH levels have the same pattern as the benzene levels Well BM-5 and BM-7, 65000 ' and 10100 ug/L respectively have the highest levels of TPH BM-1 shows a decrease probably due to the air sparging The sparging point is within the old tank pit See Figure 1 for the AS well ' MtBE was detected in wells BM-1, BM-2, BM-3, BM-5, BM-6, BM-7, BM-8, BM-9 and BM- 13 TBA (tertiary Butanol) was found in wells BM-2, BM-3, BM-8 and BM-9 The MtBE 1 Darpetro