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2.4.2 California Environmental Protection Agency, Regional Water Quality Control Board (RWQCB), Central Valley Region. Files maintained by the RWQCB on Cox & Cox pertain primarily to the RWQCB's listing of all underground storage tank sites within the region (20). The RWQCB provided technical oversight for tank removal and soil investigation operations on site and provided guidelines for the installation and sampling of groundwater monitoring wells on site in 1992 (3, 21). A copy of the report from groundwater sampling conducted by consultants in 1992 was submitted to the RWQCB (3). 2.4.3 San Joaquin County,Public Health Services,Environmental Health Division (EHD). In February 1990, the EHD notified owners of the need for further site assessment due to evidence of soil and groundwater contamination (22). The site owners responded with the submittal of a site investigation report completed in April 1992 (3). The EHD received the site investigation report, and in October 1992 required the site owners to submit a work plan for the installation of additional groundwater monitoring wells on site. 'Quarterly groundwater monitoring reports were required to be submitted as well. (23) The site owners advised the EHD in November 1992 that despite the EHD's requirements, additional site work could not be carried out because of a lack of funds (24). 3.0 HAZARD RANKING SYSTEM FACTORS 3.1 Sources of Contamination The potential sources of hazardous substances on site are: • One 55-gallon drum of used oil filters • One 55-gallon drum of waste oil • Contaminated soil in the region of the former underground storage tank A pile of contaminated soil deposited in the northeast corner of the residential trailer park. 3.2 Groundwater Pathway 3.2.1 Hy&vgeological Setting. The Cox & Cox site is located in Tracy, Calif., in the western portion of the San Joaquin Valley approximately 10 miles east of the Diablo Range. The San Joaquin Valley is a north-northwestward trending structural trough between the Sierra Nevada on the east and the Coast Ranges, which include the Diablo Range, on the west. The Sierran block underlies the valley sediments. In the Tracy area, several thousand feet of continental upper Miocene and Pliocene sediments rest on the marine upper Cretaceous Panoche Formation. The continental upper Miocene Neroly Formation has a lower member consisting of andesitic sands and cobble to pebble conglomerates, and an upper member consisting of siltstones and soft claystones. Overlying the Neroly Formation is the continental Pliocene-Pleistocene Tulare Formation, which consists of silty and clayey fine- to medium-grained sandstone. Recent alluvium covers much of the Tracy area. (25) PA Cox& Cox-BB•9193 6 Printed on 50%recyded paver.