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Chevron U S A , Inc February 20, 1992 02320 2338 Page 3 0 within the Chevron property and vertically bound a a depth of approximately 35 feet The latest round of quarterly sampling indicates that no detectable concentrations of BTEX and TPH-G remain in the groundwater 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 sod 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 (MCL) set by the State Department of Health Services The aromatic hydrocarbons are very mobile in both the dissolved and vapor phase, because of their low vapor pressure and solubility in water [Material Safety Data Sheets (MSDS), Appendix C] The toxicity to humans is summarized in the MSDS contained in Appendix C Benzene is classified as a suspected human carcinogen Toluene, ethylbenzene and xylenes have no listing of human carcinogenicity 3 2 Site Hydrogeology Two hydrogeologic divisions are recognized beneath this site and consist of consolidated to semi-consolidated units containing saline connate water, and the overlying unconsolidated fresh water- bearing units The consolidated to semi-consolidated units are composed of the Mehrten and the Valley Spring Formations The groundwater within these two formations is saline and is hydraulically separated from the overlying fresh water bearing units by an impermeable bed within the upper section of the Mehrten Formation The unconsolidated units consist of alluvial sediments of the Victor Formation, Arroyo Seco Gravel, and the Laguna Formation Total thickness of these sedimentary units is approximately 1,000 feet Fresh groundwater is present in these units under confined and unconfined conditions 3 3 Proximity and Quality of Nearby Water Supplies Local surface water bodies located within one mile of the site include Fourteen mile Slough and associated drainage canals to the east and Mokelumne Aqueduct, and the Calaveras River to the south Local domestic, municipal and irrigation wells were located in a well search conducted by Groundwater Technology in 1986 (Figure 6, Appendix A) Groundwater pumped for drinking water use is generally withdrawn from depths of greater than 100 feet 233SCAP RPT GROUNDWATER TECHNOLOGY