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When cedar ash is mixed with water and allowed to remain for a time, it creates a substance known as caustic pot ash. This ash is corrosive and high in pH. (7) The waste stream from a solid waste container containing wet fly ash was sampled and found to have a pH of 12.5 (19). The size of the solid waste container is estimated to be 20 cubic yards (8). In 1990, five underground storage tanks were located on site. The underground tanks were used for gasoline storage. These tanks have been excavated and removed from the site. Soil samples collected beneath these tanks indicated the presence of benzene, toluene, xylene, and ethyl benzene (20). However, under Section 101 (14) and (33) of CERCLA, petroleum is excluded from the definition of CERCLA hazardous substances, pollutants, and contaminants. The underground fuel storage tanks and associated contaminated soils were, therefore, not considered as hazardous substance sources for this PA evaluation. 3.2 Groundwater Pathway 3.2.1 Hydrogeological Setting. The Cal Cedar site is located in Stockton, within the Great Valley geologic province in northern San Joaquin County. To the east are the metamorphic and igneous rocks of the Sierra Nevada geologic province. The Stockton area is underlain by continental rocks and deposits from the Miocene to Holocene ages. Heterogeneous mixtures of poorly sorted clay, silt, sand, and gravel, as well as some beds of claystone, siltstone, sandstone, and conglomerate, are present beneath the area to a depth of about 2,000 feet. Informal units of significance to fresh water supplies in the Great Valley include, from ground surface downward, the Younger Alluvium, Older Alluvium, Turlock Lake Formation, Laguna Formation, Mehrten Formation, and the Valley Springs Formation. Continental and marine rocks and deposits occur below a depth of about 2,000 feet. These deposits are predominantly sandstone, with some conglomerate and shale, marine rocks, and deposits of clay, shale, siltstone, and sandstone. (31) The younger sedimentary units of the alluvial deposits, Turlock Lake, and Laguna formations are the most widely used for groundwater in the Great Valley. The Mehrten Formation has been less developed because of generally higher concentrations of iron and manganese. The cause of this is the high concentrations of these elements in the minerals present in the volcanic-source rocks that dominate the Mehrten Formation. The older Valley Springs Formation is relatively undeveloped because of its fine-grained nature and its depth of occurrence. However, some production is made from this formation, especially along the eastern boundary of the Great Valley. (31) In the Stockton area, the Turlock Lake and Laguna formations are the main water-producing zones. The Younger Alluvium unit occurs along the Calaveras River Valley as a thin complex of terrace and floodplain deposits. The Younger Alluvium's importance is most likely a route of groundwater from the river and stream channels to the deeper and older formations. (31) Most of the groundwater in the Younger Alluvium, and the Turlock Lake and Laguna formations for the Stockton area can be found within semi-confined aquifers because of the large number of clay lenses and clay layers within this geologic formation. (32) The depth to groundwater on site is 12 feet to 25 feet below ground surface (bgs) (6). The direction of regional groundwater flow is to the east and southeast (33, 34). 3.2.2 Groundwater Targets. The nearest municipal drinlcing water well is 1.25 miles east of the site. This well is owned by the California Water Service, and is part of a blended system that serves 139,619 people. The California Water Service has 49 wells, 36 of which are within 4 miles PA California Cedar Products-W (F) • 5/93 8 Printed on 50% recycled paper. c9