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CALIFQRNIA GEOPHYSICAL GROUP, INC. , , ,' " ' r1�r � ;Y O Box 578341, Modesto, CA 95357 Tel (209) 527-1247 August 9, 1998 - =1] ,8 Fax (209) 527-4810 Public Health Services 'r San Joaquin County P 0 Box 388 Stockton, CA 95201 RE Quarterly Report July 1998 ' Darpetro, Inc USA Service Station 749 E Charter Way Stockton, CA Site No 1060 This site was sampled on July 28, 29 and 30, 1998 Sherwood Labs, Corp of Hilmar, California analyzed the samples, their report is 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 showng the approximate groundwater gradient on July 28, 1998 Sampling protocols are included FINDINGS The groundwater has risen approximately 6 4 feet since the last quarterly monitoring event The gradient bearing is to the northeast The gradient is 0 0023 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 all the wells was above California Drinking Water Standards for benzene and other hydrocarbon chemicals There has been a decrease of contamination in all wells except well MW-9 This decrease is probably due to the vapor extraction system, which has removed about 39,560 pounds of hydrocarbon to date Dissolved oxygen levels were recorded this quarter at this site The levels were measured with a calibrated YSI model 51 B meter by placing the sensor 5 feet below the surface of the water table after the bailing of the wells The measured levels are shown on Table 3A The readings of dissolved oxygen are between 1 5 at well BM-2 and 8 6 at well BM-7 It appears that oxygen placed in the aquifer is still present even though the oxygen release agent has been removed I We also tested the groundwater for oxygenates as requested by the EHD We found Tertiary Butyl Alcohol (TBA) and MtBE in the groundwater in some of the wells The TBA was found in 7 of the wells and the MtBE was found in 2 of the wells The MtBE is in wells BM-1 and BM-3 In January it was also found in BM-5 We believe this was caused by 1