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Supplemental Investigation Report,Problem Assessment Report and Work Plan Cleanu and Abatement Order No 5-00-707 EAI Pro ect No 1232 3.0 GEOLOGY AND HYDROGEOLOGY 3.1 Regional The Wards and Unocal sites are located near the center of the Great Central Valley of California The Central Valley is a large northwestward-trending asymmetrical trough, which was formed by filling of a large structural trough or downward in the bedrock formation The trough is situated between the Sierra Nevada Mountains on the east and the Coast Range Mountains on the west The sediments which filled the through originated as erosional debris from the adjacent mountains and foothills The bedrock complex is composed of metamorphosed marine sediments The overlying sediments exhibit a fairly complete stratigraphic section of Cretaceous, Tertiary, and Quanternary deposits which have a maximum thickness of approximately 9,000 feet beneath the Stockton area The recent sediments were ' apparently deposited by flooding of the major rivers, i e , San Joaquin The sites are located in the San Joaquin River Basin The Basin covers 15,880 square miles ' and includes the entire area drained by the San Joaquin River It includes all watersheds tributary to the San Joaquin River and the Delta south of the Sacramento River and south of the American River watershed The principal streams in the Basin are the San Joaquin River and its larger tributaries the Cosumnes, Mokelumne, Calaveras, Stanislaus, Tuolumne, Merced, Chowchilla, and Fresno Rivers The sites are located between the Calaveras River to the south and Bear Creek located approximately four miles to the north Further, the sites are located in the Lower Mokelumne Hydrologic Area of the North Valley Floor Hydrologic Unit of the San Joaquin Hydrologic Basin 3.2 Site Specific Sediments beneath the site are fluvial deposits associated with the San Joaquin River and its tributaries, particularly the Calaveras River which is located approximately one mile south of the site Based on available logs for borings and wells associated with the Wards and Unocal sites, and the RIW, sediments consist of anastomosing silts and clays with minor sand to a depth of approximately 50 feet bgs Below 50 feet, sand generally predominates with minor slit and clay (see Appendix A) The direction of ground water flow beneath the two sites has been monitored since 1993 Data from these monitoring events indicate that the natural ground water flow is easterly, during times when the RIW is not in operation However, when the RIW is in operation, the direction of ground water flow changes and ground water from both sites flows towards the RIW (see Section.A ? 1) In March 1993, ground water beneath the two sites was encountered at approximately 55 feet bgs In June 2000, ground water was encountered beneath the sites at approximately 33 feet bgs, i e , the water table has risen approximately 22 feet since March 1993 ss 1131P Iaw_wr -9- ENVIRONMENTAL AUDIT, INC 0