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1 Page 4 January 30, 1992 Job No 400511-SA Structurally, the Great Valley is an asymmetrical synclinal trough bordered on the east by the Sierra Nevada and on the west by the Coast Range Basement rocks underlying the Great Valley sediments ' probably include Mesozoic granitic intrusives and pre-Jurassic ametamorphics The Valley began to form in the late Jurassic with sediments derived from the rising Nevadan Mountains to the east Deposition continued-through-the Cretaceous mamly--represented by shallow-water marine sediments ---Extremely ---' thick marine sections accumulated during the Miocene Continental and marine deposits of the Great Valley were deformed by periodic folding and faulting through mid-Cenozoic time By Pliocene time, , most of the Valley's seas were drained Brackish water and fresh water lakes replaced marine waters and the Great Valley assumed its present form (Norris and Webb, 1990) to is located within the San Joaquin Valle Groundwater Basin This basin includes roughly the The site q Y l�Y southern two-thirds of the Great Valley of California, south of Sacramento The maximum thickness of fresh water-bearing deposits in the basin is 4,400 feet Total Dissolved Solid (TDS) content of the water varies from 64 milligrams per liter (mg/1) to more than 10,000 mg/l The character of the water on the east side of the Valley is predominantly sodium-calcium bicarbonate, water on the west side principally contains sodium sulfate Development of groundwater resources has been intensive for irrigation,domestic,industrial, municipal,and stock use The Morrison Slough is located one-mile south of the site and drains surface water from agricultural areas east of Stockton into the San Joaquin River Much of the Valley is in overdraft condition,which has caused declining water levels and excessive land subsidence in parts of the`basm A major water quality problem is the rising saline connate waters in the Sacramento - San Joaquin Delta from Stockton to Tracy (Department of Water Resources, 1975) 1 4 0 FIELD INVESTIGATION The groundwater monitoring wells and the vapor extraction wells were installed and completed as proposed in DERS' work plan dated July 12, 1991, with a few changes based on field conditions and laboratory analyses The deviations from the work plan were summarized in a letter to Conoco dated f October 28, 1991 Appendix A contains a description of field procedures Well locations for-the vapor extraction system were chosen to intersect hydrocarbon-contaminated soil in the vadose zone ' Groundwater monitoring well locations were chosen to redefine the petroleum hydrocarbon concentrations in the groundwater, and to determine the groundwater gradient at the site The screen intervals of the groundwater monitoring wells were situated to accommodate expected seasonal variations in groundwater elevation Four monitoring wells were installed on-site Two monitoring wells were installed to an approximate depth of 93 feet with 20 feet of screen, and two monitoring wells were installed to an approximate depth 4005111 Rev 1 Du Pont Environmental Remediation Services Pnnted on Recycled Paper