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Qsrterty Gra Khm der Modmrrg Lkn r Report 5491"1"'Street,Banta,California BACKGROUND Aegis was assigned as consultant, and resumed groundwater monitoring at the site on February 1, 1991. Work performed at the site prior to February 1991 included investigations by Earth Treatment Systems, Inc., of Manteca, California, Delta Environmental Consultants, Inc., of Rancho Cordova, California,and Nations Groundwater Associates of Sacramento, California. GROUNDWATER MONITORING Groundwater On July 27, 1992, Aegis personnel collected measurements of the depths to groundwater in wells MW-1 through MW-6. Since April 1992, groundwater levels have risen in all wells (Table 1, Figure 3) an average of approximately 3.16 feet; ranging from 3.03 to 3.38 feet. Based on the depth-to-groundwater measurements, the groundwater flow direction is estimated to be to the east (Figure 4) at an average gradient of approximately 0.003 ft/ft. Figure 4 is a potentiometric surface map of the shallow water-bearing zone. Previous groundwater levels are summarized in Table 1. All depth-to-groundwater measurements were made from the referenced wellhead elevations, measured to the nearest 0.01 foot, and conducted according to the Aegis standard operating procedures (SOP) included in Attachment 1. Water Sampling and Analysis On July 27, 1992, Aegis personnel collected groundwater samples from monitoring wells MW-1 through MW-6. The samples were collected according to the Aegis SOP included in Attachment 1, and delivered under chain-of-custody to NET Pacific, Inc., of Santa Rosa, California, a state-certified analytical laboratory. The samples were analyzed for concentrations of: a) total petroleum hydrocarbons (TPH), as gasoline, by GC-FID/EPA Methods 5030/8015; b) benzene, toluene, ethylbenzene, and total xylenes by EPA Method 8020; and, c) TPH, as diesel, by GC-FID/EPA Methods 3510/8015. The analytical results are summarized in Table 2. The analytical reports and chain-of-custody form are included in Attachment 2. Concentrations of TPH, as gasoline and diesel, and benzene reported in Table 2 are also shown on Figure 5. 90-099C.OMR -2.