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QUARTERLY GROUNDWATER MONROPoNG R610R7 10 620 West Charter Way Stockton California In January and February 1990, a manifold piping system was installed to wells MW-2, IMW-3, MW-4, MW-5, and MW-7 for future remediation The piping was installed in conjunction with station reconstruction The planned remedial activities were reported to Shell by Delta in "Interim Remedial Action Plan and Proposed Schedule," dated April 13, 1990 On May 20 and 21, 1990, a source test was conducted to evaluate the remedial Ieffectiveness of groundwater pumping and soil vapor extraction at the site The results of these activities and the continued floating hydrocarbon recovery were reported to Shell Iby Delta in a "Status Report," dated October 8, 1990 In January 1992, Emcon was retained by Shell to conduct groundwater monitoring at the site In April 1992, Aegis was assigned the project 4 0 GROUNDWATER MONITORING 41 Groundwater On April 30, 1992, Emcon personnel collected measurements of the depths to groundwater in wells MW-1, MW-3, MW-5, MW-6, MW-8, MW-9, and MW-10 Since ` January 28, 1992, groundwater levels have risen in the seven wells (Figure 3) The average rise was 1 25 feet (Table 1) On the basis of the April 30, 1992, measurements, groundwater is estimated to flow to the north with an easterly component (Figure 4) at an average gradient of approximately 0 04 ft/ft Figure 4 is a potentiometric surface map of the shallow water-bearing zone on April 30, 1992 Previous groundwater levels are summarized in Table 1 All groundwater elevations were made from the referenced wellhead elevations, measured to the nearest 0 01 foot, and conducted according to the Emcon sampling procedures (SP) included in Appendix A 4 2 Water Sampling and Analysis On April 30, 1992, Emcon personnel collected groundwater samples from wells MW-1, MW-3, MW-5, MW-6, MW-8, MW-9, and MW-10 The samples were collected according to the Emcon SP included in Appendix A, and delivered under chain-of-custody to Sequoia Analytical of Sacramento, California, a state-certified analytical laboratory The samples were analyzed for concentrations of a) total petroleum hydrocarbons, as gasoline, by EPA Method 8015, and b) benzene, toluene, ethylbenzene, and total xylenes ' by EPA Method 8020 The analytical results are summarized in Table 2 The analytical reports and chain-of-custody form are included in Appendix B Concentrations of ' benzene reported in Table 2 are also shown on Figure 5 f� ' 91 246A QM 2