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' Quarterly Monitoring Report July 12, 1996 Chapin Brothers Automobile Repair Facility Stockton, California Results of analytical testing indicate that gasoline is present in the two onsite wells nearest the former tanks, MW 1 and MW4, at concentrations of 66 and 190 parts per million (ppm) respectively The volatile constituents BTEX were also detected in those wells at ' relatively high proportions All of the volatile constituents were detected at very low concentrations in MW2, located north of the site Gasoline was detected in MW3 and MW5, located on properties west of the site, at concentrations of 3 1 ppm and 0 56 ppm, respectively, with relatively low volatile constituent concentrations Analytical results for each constituent tested are listed in Table 2 Analytical reports and completed chain-of-custody forms are presented in Appendix A 4.0 GROUNDWATER GRADIENT The groundwater gradient beneath the site was calculated from surveyed wellhead elevations and groundwater surface elevation measurements collected from the monitoring wells Well casing elevations were measured to the nearest 0 01 ft by Underwood and Rosenblum of San Jose, California, a licensed surveyor Installation of the vapor extraction system required modification to the wellhead at MW4 Consequently, the elevation of that well casing was resurveyed on June 27, 1996 by Lush Geosciences personnel These data were used to calculate the elevation of the groundwater surface in each well (Table 3) and to evaluate the slope of the groundwater surface across the site Groundwater elevations in the wells on June 26, 1996, were between 10 30 and 10 79 ft below mean sea level, the highest elevation recorded at this site to date by almost 1 ft The measurements indicated that the groundwater surface sloped on the average to the east southeast (N80°W) at approximately 0 0027 ft/ft(0 27 ft/100 ft) at that time, the surface had a pronounced warp which indicates a more northerly flow direction to the west This calculated gradient indicates that the groundwater surface was relatively flat-lying, with a general northwesterly flow direction in the site area, the surface is relatively complicated, however, and flow is more northerly offsite to the west Figure 2 illustrates the configuration of the surface of uppermost groundwater at the site, the direction of flow, and gradient 5.0 CONCLUSIONS AND RECOMMENDATIONS Results of analytical testing indicate that gasoline is present in the two onsite wells nearest the former tanks, MW1 and MW4, at concentrations of 66 and 190 ppm, respectively The volatile constituents BTEX were also detected in those wells at relatively high proportions I 530-001 _ 4 _ LUSH GEOSCiENCES