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i 2905 West Benjamin Holt Drive, Stockton, California March 16, 1994 Chevron U.S.A. Products Company Page 6 3.0 ASSESSMENT OF IMPACTS 3.1 Physical and Chemical Characteristics of Gasoline and its Constituents Gasoline is the only known hazardous substance that has been detected in appreciable quantities in soil and groundwater at the site. Gasoline is a hydrocarbon blend that includes normal and branched chain alkanes, cycloalkanes, alkenes, aromatics and other additives. Included in this blend are the aromatic hydrocarbons: benzene, toluene, ethylbenzene and xylenes (BTEX), all of which have maximum contaminant levels (MCI_) set by the State Department of Health Services. The aromatic hydrocarbons are very mobile in both the dissolved and vapor phase, because of their relatively high vapor pressure and slight solubility in water. The toxicity to humans is summarized in the material safety data sheets included in Appendix A. Benzene is classified as a suspected human carcinogen. Toluene, ethylbenzene, and xylenes have no listing of human carcinogenicity. 3.2 Site Hydrogeology The site is located in the northern part of the San Joaquin Valley. The valley fill consists of late Mesozoic through Eocene dominantly marine sedimentary rocks overlain by late Tertiary and Quaternary non-marine deposits. In the Stockton area, the late Cenozoic deposits are approximately 3,000 feet thick and generally consist of alluvial gravel, sand, silt, and clay. The non-marine sedimentary units are generally fresh-water-bearing in the area of the valley east and southeast of Stockton, but brackish or saline water occurs in these sediments in the delta area west of Stockton. The saline-water/fresh-water boundary within these sedimentary units trends north to south across the western part of Stockton. The saline groundwater west of this boundary is usually overlain by a layer of fresh groundwater to a depth of approximately 50 feet SGS; therefore, shallow monitoring wells usually encounter fresh groundwater on both sides of the boundary. The site is underlain by interbedded clays, silts and silty fine sands from the surface to 20 feet BGS, fine to medium sand from 20 to 30 feet BGS, and clay to the total depth drilled (approximately 35 feet BGS). Groundwater occurs at the top of the sand layer at approximately 20 feet BGS. The shallow water-bearing zone is approximately 10-12 feet thick based on data collected during drilling (Figure 3). Based on an estimated porosity of 0.3 and a hydraulic conductivity of 1 foot/day, the velocity of groundwater in the shallow water-bearing zone is approximately 0.017 feet/day or 6 feet/year. 5073PAR.RPT ,tel=`�� GROUNDWATER �j TECHNOLOGY