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WI w eJS 1I auart*dy Groundwater Morrtonnp Report 14800 West Frontage Road Manteca Callforrua BACKGROUND 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, MW-1 and MW-2 were installed by ETS In April and May 1989, Delta installed 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 MW-5 through MW-8 in November and December 1989 Aegis began a program of monthly monitoring and quarterly sampling of the wells in January 1991 In November 1991 , Aegis supervised the drilling and completion of MW-9 (Figure 2) The previous consultants' groundwater data, along with Aegis' data, are included to Table 1 GROUNDWATER MONITORING On July 8, 1993, Aegis personnel collected measurements of depths to groundwater from the nine wells Since the last monitoring event in April 1993, groundwater levels rose in all wells (Figure 3) an average of approximately 3 34 feet, ranging from 2 90 to 4 13 feet (Table 1 ) Figure 4 is a potentiometric surface map of the shallow water- bearing zone on July 8, 1993 On the basis of the current measurements, the groundwater flow direction ,s estimated to be generally to the northeast, at a gradient of approximately 0 0008 ft/ft During the April 1993 sampling event, groundwater flow was estimated to be toward the southeast 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 WATER SAMPLING AND ANALYSIS On July 8, 1993, Aegis personnel collected groundwater samples from the nine wells The samples were collected according to the Aegis SOP included in Attachment 1, and submitted under chain-of-custody to Pace Inc , of Novato, California, a state- certified analytical laboratory The samples were analyzed for concentrations of • total petroleum hydrocarbons (TPH), as gasoline, by modified EPA Method 8015, • benzene, toluene, ethylbenzene, and total xylenes by EPA Method 8020, • 90 097F QMR 2