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,, CALIFORNIA GEOPHYSICAL GROUP, INC. P O Box 578341, Modesto, CA 95357 Tel (209) 527-1247 May 7, 2001 Fax (209) 527-4810 Public Health Services =� ' San Joaquin County 304 East Weber Ave 3rd Floor Stockton, CA 95202 --- RE Quarterly Report April 2001 _ Darpetro, Inc USA Service Station 749 E Charter Way Stockton, CA ' Site No 1060 This site was sampled on April 24, 2001 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 3113 lists the ' analyses of the groundwater Table 5 lists the readings at the Vapor Extractor during operation and 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 rose approximately 0 6 feet since the last quarterly monitoring event The gradient bearing is to the east The gradient is 0 0094 Ft/ft The attached hydraulic gradient plot shows the bearing of the gradient This quarter we measured all of the wells and sampled nine wells 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 131M-1, BM-4, BM-5, BM-7, BM-9, BM-11 and BMA 3 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 The benzene in well BMA 3 was ' possibly caused by the mounding of the groundwater near the vapor sparging well between BM-5 and BM-1 There are TPH levels in wells BM-1, BM-2, BM-5, BM-7, BM-9, BM-11 and BM-13 We believe the TPH levels in well BM-1, BM-5 and BMA 3 are caused by the air sparging The sparging point is located 50 feet below the surface and 20 feet below the water table The soil at this depth is very contaminated and the sparging has pushed the lower contamination to the water surface The contamination in BM-11 is perhaps a zone of water which migrated from one of the sites The water at BM-11 would take about 18 years to flow from our site to this well