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• GEOLOGICAL AUDIT SERVICES, INC. THIRD QUARTER 1993 PROJECT REPORT SITE: ARCO MINI-MART ADDRESS: 4511 Pacific Ave. Stockton, CA CLIENT: Arthur Wyatt TELEPHONE: 951-1207 LEAD AGENCY: SAN JOAQUIN COUNTY PHS/EHD AGENCY CONTACT: Diane Hinson TELEPHONE: 468-3452 SUMMARY OF WORK COMPLETED During the third quarter, monitoring wells MW-1 through MW-3 (Figure 1) were resurveyed to check previous measurements and recalculate the groundwater gradient. Quarterly monitoring took place on August 9 , 1993 . The depth to standing water was measured in each well, and then disposable bailers were used to purge the wells of between 9 and 10 gallons of water prior to sampling (Table 1) . Water samples were collected in disposable bailers and transmitted to Alpha Analytical in Ukiah, California. GROUNDWATER DEPTH AND GRADIENT . Monitoring Date: August 9, 1993 Depth Measurements: See Table 2 Average Depth: 46. 1 feet Gradient: . a044 ft/ft to N 660 E Groundwater flow at this site is slightly north of east (Figure 2) . Previous reports of northwestward flow were incorrect, due to an error in the original survey data. The error was discovered during surveying in August 1993 , and corrected groundwater gradient maps for January and May 1993 are presented in Figures 3 and 4 . Both maps are consistent with the August map and with neighboring sites. LABORATORY ANALYTICAL RESULTS For the first time since monitoring began, no contamination was detected in MW-1. Gasoline, benzene, and toluene were detected in MW-2 , and gasoline, diesel , benzene, and xylene were detected in MW-3 . Concentrations are reported in Table 3 and Attachment A and illustrated in Figure 5. CONCLUSIONS AND RECOMMENDATIONS The revised survey data indicate that groundwater flow is to the east or east-northeast, and the contaminant plume may extend beyond MW-2 beneath Pacific Avenue. Although this well has been . contaminated in all three sampling events, concentrations have fluctuated. In contrast, concentrations have consistently decreased in MW-1, which may be due to its upgradient location. Concentrations have also fluctuated in MW-3 , but no reason is apparent.