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QUARTERLY GROUNDWATER MONITORING REPORT f v99 East Yosemite Avenue Manteca California Aegis began a program of groundwater monitoring in February 1991 Monthly monitoring of groundwater levels and quarterly sampling of groundwater has continued since then On June 20, 1991, Aegis installed monitoring wells MW-1 B, MW-4, and MW-5 on the site Wells MW-4 and MW-5 were completed with 4-inch-diameter casing to a depth of 35 feet 1 below grade Well MW-1 B was completed with 6-inch-diameter casing by 'over-coring" the former MW-1A 4 0 GROUNDWATER MONITORING 41 Groundwater On July 27, 1992, Aegis personnel collected measurements of the depths to groundwater in wells MW-1 B, MW-2, MW-3, MW-4, and MW-5 Since April 9, 1992, groundwater levels have fallen in all wells (Figure 3) an average of approximately 0 49-feet (Table 1) On the ' basis of the July 27 measurements, groundwater is estimated to flow to the west- northwest (Figure 4) at an average gradient of approximately 0 014 ft/ft For April 9, 1992, the flow direction was estimated to be to the west Previous groundwater levels are summarized in Table 1 All groundwater elevation measurements were conducted according to the Aegis standard operating procedures (SOP) included as Appendix A Figure 4 is a potentiometnc surface map of the shallow water-bearing zone on July 27, 1992 4 2 Water Sampling and Analysis On July 27, 1992, Aegis personnel collected groundwater samples from four of the five I groundwater monitoring wells at the project site The samples were collected according to the Aegis SOP included in Appendix A, and delivered under chain-of-custody to NET Pacific, Inc , of Santa Rosa, California, a state-certified laboratory, for chemical analysis ' Well MW1-B was not sampled due to the presence of floating liquid hydrocarbons The samples were analyzed for concentrations of • total petroleum hydrocarbons (TPH), as gasoline, by GC/FID Method 5030, and, ' benzene, toluene, ethylbenzene, and total xylenes by EPA Method 8020 Groundwater analytical results obtained to date, including data reported by previous consultants, are summarized in Table 2 Concentrations of TPH, as gasoline, and benzene reported for samples collected on July 27, 1992, are also shown on Figure 5 Copies of the laboratory analytical reports and chain-of-custody form are included in Appendix B 90 0986 am 2