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funderground fuel storage tanks, a used oil storage tank,-and dispenser islands detected .:_ petroleum hydrocarbons at concentrations of up to 120 parts per million (ppm) total s petroleum hydrocarbons as gasoline (TPH-G) and 16 ppm benzene. Groundwater samples collected from Monitoring Wells MW-3 and MW-4 contained concentrations of up to 10,000 parts per billion (ppb) TPH-G and 6.7 ppb benzene (Resna, 1993). 1.4 Potential Offsite Sources'of Contamination Review of regulatory agency files revealed the following two properties neighboring the BP Oil service station that may effect or be effected by the subsurface conditions at the BP Oil site: • An Arco service station is located approximately 100 feet north of the BP Oil site. In 1987, three wells were installed after removal of a used oil tank. Total petroleum hydrocarbons were detected at concentrations of up to 29 parts per million (ppm) in two soil samples; and total lead-were detected at concentrations of up to 12 parts per r billion in groundwater samples collected at the site. The groundwater gradient direction at the Arco service station has varied from southwest to southeast (Brown s and Caldwell, 1992). • The J. R. Simplot Company (formerly, Occidental Chemical Agricultural Products, Inc.) operates a fertilizer manufacturing and formulation plant approximately 1 mile east- southeast of the subject site. The groundwater remediation system, which currently includes more than 20 groundwater'monitoring, recovery, and reinjection wells, was designed to address groundwater containing nitrate (NOA radium 226, total uranium, s and chloride in aquifers at depths of over 200 feet below grade. Historical contamination detected in groundwater collected from domestic water supply wells in _ the immediate area of the BP Oil service station may have been associated with contamination from the fertilizer plant (California Regional Water Quality Control - Board, Central Valley Region, 1983). 1.5 Regional Geology and Hydrogeology The site is situated in the Central Valley, which comprises the lowlands between the Sierra " ? Nevada to the east and the coastal range to the west. As a structural trough, the Central 1 Valley received sediment from the adjacent mountains under both marine and continental conditions. Continual uplift during the Pleistocene resulted in formation of the Sierra Mountains and accelerated deposition of silt, sand, and gravel in alluvial fans along the valley margins. The topography of the site vicinity slopes gently to the west toward the San Joaquin River, which is 1.5 miles to the west.of the site (Page, Ben M., 1966). E:\0\10-151\SSI.RPr/020494 2 I G I�1 I ti