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CALIFORNIA GEOPHYSICAL GROUP, INC. i O Box 578341, Modesto, CA 95357 Tel (209) 527-1247 May 18, 2000 Fax. (209) 527-4810 1 Public Health Services ' San Joaquin County 304 East Weber Ave 3rd Floor Stockton, CA 95202 ' RE Quarterly P P Report April 2000 Darpetro, Inc USA Service Station 749 E Charter Way Stockton, CA ' Site No 1060 This site was sampled on April 29, 2000 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, 3A and 313 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 time of operation Sampling protocols are included FINDINGS The groundwater rose approximately 1 7 feet since the last quarterly monitoring event The gradient bearing is to the east-southeast The gradient is 0 0021 Ft/ft The attached hydraulic gradient plot shows the bearing of the gradient This quarter we measured all the wells and checked the oxygen levels in the water, however, we only sampled wells BM-1, BM-2, BM-3, BM-4, BM-5, BM-7, BM-8, BM-9 and BM-12 as approved by the EHD See Figure 1 iThe 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 BMA, BM-4, BM-5, BM-7, BM-8, and 1BM-9 was 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 wells BM-2 and BM-8 was possibly caused by the mounding of the groundwater near the vapor sparging well between BM-5 and BMA The TPH level in wells 1BM-1, BM-4, BM-5 and BM-8 rose since last quarter We believe this is a result of the air sparging system not working We have located the problem and it will be corrected this quarter i% MtBE was detected in BM-2, BM-3, and MW-4 TBA tertiary Butanol was found in wells BM-1, BM-3, BM-8 and BM-9, this product can be a byproduct of MtBE when it is 1