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J CALIFORNIA GEOPHYSICAL GROUP, INC. t�O Box 578341, Modesto, CA 95357 Tel (209) 527-1247 April 23, 2002 Fax (209) 527-4810 Public Health Services San Joaquin County 304 East Weber Ave 3rd Floor WAY Stockton, CA 95202 $ Z00 '1 F ENVIROWENT HEALTH s ' l RiE Quarterly Report April 2002 TWIT/SERVICES Darpetro, Inc USA Service Station 749 E Charter Way Stockton, CA Site No 1060 ,T i This site was sampled on April 8, 2002 Sherwood Labs of Hilmar, California analyzed the samples, their reports are attached r` 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, 3A and 3B lists the analyses of the groundwater Table 5 lists the readings at the Vapor Extractor durlog operation and Table 6 lists the laboratory tests of the input and exhaust gasses and Tabje 4� K11 7, lists the times of operation Sampling protocols are included „ fFINDINGS The groundwater rose approximately 0 44 feet since the last quarterly monitoring event The gradient bearing is N 75 E The gradient is 0 0017 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 7 +, 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-9, BM-11 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 BM-1 See Figure 2 b TPH levels have the same pattern benzene levels Well BM-5 along with BM-1 have ' contained the highest levels of TPH in the past 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-1,13M-3, BM-5, BM-6, BM-7, BM-9 and BM-13 TBA (tertiary Butanol) was found in wells BM-1 and BM-3 The MtBE plume appears to be � 1 t