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E May 24, 1996 Quarterly Monitoring Report Chapin Brothers Automobile Repair Facility Stockton, California h groundwater surface in each well (Table 3) and to evaluate the slope of the groundwater surface across the site. A I� Groundwater elevations in the wells on March 20, 1996, were between 11.33 and 11.55 ft p below mean sea level. The measurements indicated that the groundwater surface sloped on the average to the east southeast (N65°W) at approximately 0.0019 ft/ft (0.19 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 northwesterly flow direction. 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, MW 1 and MW4, at concentrations of 23 and 70 ppm, respectively. The volatile constituents BTEX were also detected in those wells at relatively high proportions. Except for xylenes at 0.0009 ppm, no other petroleum constituents were detected in MW2 located north of the site. Gasoline was detected in MW3 and MW5, located on property west of the site, at concentrations of 1.7 and 2.9,jrespectively. - 6 Currently, permitting, design, an'd construction of a remediation system utilizing vapor extraction/air sparging is in progress. In the interim, we recommend continued quarterly monitoring of the groundwater as required by San Joaquin County Environmental Health Department and Central Valley Regional Water Quality Control Board guidelines , 1 �r it 530-001 - 5 - Lusx GEOSCIENCES