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CALIFORNIA GEOPHYSICAL GROUP, INC. P O Box 578341, Modesto, CA 95357 Tel (209) 527-1247 August 13, 1999 Fax. (209) 527-4810 I Public Health Services ' San Joaquin County 3 304 East Weber Ave 3 floor Stockton, CA 95202 Ej 1t - RE Quarterly Report July 1999 Darpetro, Inc USA Service Station ' 749 E Charter Way Stockton, CA Site No 1060 r This site was sampled on July 27 & 28, 1999 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 Table 3 lists the analyses of the groundwater Also attached is a hydraulic gradient plot showing the approximate groundwater gradient on April 27, 1999 Sampling protocols are included FINDINGS The groundwater fell approximately 0 7 feet since the last quarterly monitoring event The gradient bearing is to the northeast The gradient is 00019 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 IThe groundwater in all the wells except BM-1, BM-4, BM-5, BM-6, BIVI-7, BM-8 and BM-9 was above California Drinking Water Standards for benzene The benzene levels are high in wells BM-1, BM-7 and BM-9 The last two wells are probably related to the contamination entering from the Arco Station to the west The benzene level in well BM- 11 is non-detectable for the first time since September 1994 The TPH level in wells, BM-1, BM-3, BM-5, BM-6, BM-7 and BIVI-9 rose this quarter The contamination in BM-3, BM-6, BM-7 and BM-9 is probably from water entering from the s ARCO station Wells BM-1 and BM-5 are located in the old tank pit area Well BM-9 is �I in the path of contamination from the ARCO station and BM-7 is directly dowmgradient of well BM-9 Wells BM-3 and BM-6 are located in the path of previous gradients and the contamination could have come from the ARCO station Well BM-12 had no contamination This indicates that there is no contamination extending out as far as BM-12 The fact that well BM-11 had a slight amount of xylene in