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REMEDIAL ACTION PLAN 1� USA STATION#65 - LODI NOVEMBER, 1995 Following completion of MW-6 and MW-7, Applied Geosystems prepared a Problem Assessment Report (Problem Assessment Report (PAR), report no. 87050, dated December 5, 1989). The PAR concluded that hydrocarbon impacted water had migrated beyond the property boundaries, and it was their opinion that the groundwater in the upper aquifer, above 42-feet was not penetrating the deeper aquifers. It was also their opinion that further delineation of the impacted soil and water was not required at that time. On February 8, 1993,MW-8 was completed to a depth of approximately 60-feet bgs. by Western Geo-Engineers (WEGE). The well is down gradient from the suspected source and approximately 8-feet southeast of the tank field. The well was completed as part of WEGE "Workplan Proposal" of January 4, 1993. On March 14, 1995,WEGE completed a one-day vapor extraction treatability test. The test was performed on monitoring wells MW-4, S-1, S-2, and MW-8. Test results are summarized in WEGE 'Results of Pilot Test for Vapor Extraction", dated April 17, 1995. ` 4.0 GEOLOGIC AND HYDROGEOLOGIC SETTING 4.1 Geology and Regional Hydrogeologyl The site is located within the south-central portion of the Sacramento Valley. The valley is a �. broad northwest-trending structural trough bounded by the Sierra Nevada to the east, the Coast range to the west, the Cascades to the north and the Transverse Range to the south. The site is underlain by the Pleistocene Modesto Formation. Deposits of the Modesto Formation were laid down as a series of coalescing alluvial fans and consist in large part of gravel, sand and silt. According to March and Allwardt(1977), numerous channels are present within the Modesto Formation that create rapid lateral changes in texture. The site is in the Western San Joaquin County Groundwater Basin (California Department of T Water Resources, 1980). The regional groundwater gradient in the Eastern San Joaquin '— Groundwater Basin is to the southwest (California Department of Water Resources, 1980). The Mokelumne River lies approximately one mile north of the site. The South Main Canal on the eastern edge of the property is used to transport water down seasonally from the Mokelumne River for irrigation. Water in the canal flows to the south and since it is unlined it should act as a source of groundwater recharge in the area. ' Applied Geosystems "Problem Assessment Report", dated December 5, 1989 3