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CALIFORNIA GEOPHYSICAL GROUP, INC, ;O Box 578341, Modesto, CA 95357 Tel 209 527-1247 November 16, 1998 Fox (209) 527-4810 Public Health Services ' San Joaquin County P O Box 388 Stockton, CA 95201 ' RE Quarterly Report ort October 1998 Darpetro, Inc USA Service Station 749 E Charter Way Stockton, CA ' Site No 1060 This site was sampled on November 4 and 5, 1998 Sherwood Labs 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 showing the approximate groundwater gradient on November 4, 1998 Sampling protocols are included FINDINGS The groundwater fell approximately 1 foot since the last quarterly monitoring event The ' gradient bearing is to the east-northeast The gradient is 0 0022 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 except well BM-4 was above California Drinking Water Standards for benzene and other hydrocarbon chemicals The levels in all the wells except BM-1 and BM-5 have remained relatively low It is noted that the contamination ' from sites west (upgradient of this site) is entering this site Well BM-3 has MtBE for the first time About 3 years ago in January 1996 well BM-9 had some MtBE The distance from BM-9 to BM-3 is about 85 feet and the time span was 1000 days This equates to a velocity of 0 085 feet per day Based upon the Hydraulic Conductivity of the site, 15 07 Ft/day, and the average hydraulic gradient of 0 0024 and a porosity of 0 4, the velocity calculates to be 0 09 feet per day These velocities are fairly close and may indicate the velocity of the water over the site Other factors are not the same such as the amount of BTEX chemicals, however, they may have been degraded over the years 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