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4 QL-r"Groundwater MonkodM Report 14800 West Frontage Road Manteca Callfornia i BACKGROUND Aegis began a program of monthly monitoring and quarterly sampling of the wells in January 1991 Work performed at the site by previous consultants included investigations by Earth Treatment Systems, Inc (ETS), of Merced, California, Delta Environmental Consultants, Inc (Delta), of Rancho Cordova, California, and Nations Groundwater Associates of Sacramento, California In August 1988, two groundwater monitoring wells MW-1 and MW-2, were installed by ETS In April and May 1989, Delta installed two additional monitoring wells MW-3 and MW-4 In November 1989, Delta conducted soil sampling during the removal of three 10,000-gallon underground fuel storage tanks Delta installed four more monitoring wells MW-5 through MW-8 in November and December 1989 The previous consultants' groundwater data, along with Aegis' data, is included in Table 1 In November 1991, Aegis supervised drilling and completion of groundwater monitoring well MW-9 (Figure 2) GROUNDWATER MONITORING On September 23, 1992, Aegis personnel collected measurements of depths to groundwater in wells MW-1 through MW-9 Groundwater levels have fallen in all wells (Figure 3) an average of approximately 2 83 feet, ranging from 2 35 to 4 39 feet (Table 1) Figure 4 is a potentiometnc surface map of the shallow water-bearing zone on September 23, 1992 The data indicate a reversal in flow direction during the period reported On the basis of the September 23, 1992, measurements, groundwater flow direction is estimated to be to the west with a gradient of approximately 0 007 ft/ft The previous quarterly groundwater measurements indicated a gradient of 0 008 ft/ft to the northeast Groundwater measurements are summarized in Table 1 All groundwater elevation measurements were made according to the Aegis standard operating procedures (SOP) included in Attachment 1 90-097C QMR 2