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' concentrations corresponded to the maximum levels detected over the past two years in offsite downgradient well MW-7 (see Table 1) Use of maximum levels over the past two years as representative of current and future concentrations is considered conservative, given the absence of a continuous source and sporadic detection of hydrocarbons in groundwater (ET1C, 2000) Table 1 ' Tier IUser-Defined Data Parameter,'w Designation _ Comments ' Chemicals of Potential Concern All chemicals detected in soil COPCs BTEX,MTBE and/or groundwater samples Representative COPC Source Benzene <0 002 mg/kg Maximum detected concentration ' Concentrations in Soil-Onsite Toluene <0 002 mg/kg in unsaturated soil confirmation Commercial/Industrial Receptor and Ethylbenzene <0 002 mg/kg samples from the former tank Construction Worker Receptor Xylenes 0 569 mg/kg balm area MTBE <0 01 in ' Representative COPC Source Benzene 13 ppb Site maximum concentrations Concentrations in Groundwater- Toluene 22 ppb during the past two years of Onsite Commercial/Industrial Ethylbenzene 7 4 ppb sampling in onsite wells(MW-5, ' Receptor and Residential Receptor at Xylenes 38 ppb VEW-2) Offsite Agricultural/Irrigation Wells MTBE 190 pLb Representative COPC Source Benzene 1 7 ppb Site maximum concentrations Concentrations in Groundwater- Toluene <0 5 ppb during the past two years of Offsite Residential Receptor at Ethylbenzene<0 5 ppb sampling in offsite well (MW-7) Apartment Building Xylenes <0 5 ppb MTBE 13 ppb ' Distance to Nearest Offsite Water Based on July,2000 Area Well SyMIX Wells '/4 mile= 1,320 ft Survey(ETIC,2000 Source Dimensions in Groundwater Width= 100 ft Conservative estimates based on D th=5 ft groundwater quality data Target Carcinogenic Risk Level Lowest target risk level defined 1 x 10-6 by USEPA, DTSC, and ASTM Target Non-Carcinogenic Hazard. Consistent with USEPA,DTSC, Quotient 10 and ASTM guidelines Benzene Carcinogenic Slope Factor Conservative value defined by inhalation and oral 0 1 (mg/kg/day)-' DTSC(1994 MTBE Carcinogenic Slope Factor Conservative value defined by inhalation 1 6x 10-4(mg/kg/day)° California EPA OEHHA, 2000 MTBE Carcinogenic Slope Factor Conservative value defined by ' (oral) I 1 8x 10"3(m /k da "i California EPA OIHHA,2000 For evaluation of risks associated with direct exposure to groundwater at the agriculturaVirrigation wells located 14-mile south-southeast of the site, site maximum groundwater concentrations over the past two years were used to estimate potential exposure point concentrations at the wells This calculation assumed that the wells are perforated in the shallow water-bearing formation and that transport in groundwater is governed by advective and dispersive transport as depicted by the highly conservative Domenico solute transport model (ASTM, 1995 and 1998, USEPA, 1996, and Domenico, 1987) Biodegradation and other mass-loss mechanisms were conservatively ignored along the path of chemical transport form the site to the wells Hydraulic parameters used in the Domenico model correspond to conservative ASTM (1995 and 9