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1 mixtures is indirectly evaluated In fact, the conservative approach for quantifying risks associated with hydrocarbon mixtures makes use of indicator and/or surrogates (such as BTEX) for TPH-g and TPH-d (MDEP 1994), hence, results of this RBCA analysis may be used to ' formulate conclusions with respect to chemical constituents and hydrocarbon mixtures Selection of COPC source concentrations involved a detailed review of historical soil and ' groundwater quality data Based on removal via SVE operation of an estimated 1,551 pounds of hydrocarbons (EA 1998) from soils, the results of confirmation soil samples, and the potential ' for natural reduction of hydrocarbon levels detected historically (i e , 1991 and 1992) in tank basin product line samples, historical COPC levels in pre-remediation tank basin samples are not considered representative of the current or future magnitude of hydrocarbons present in ' unsaturated soils at the site Accordingly, soil COPCs and source concentrations correspond to maximum concentrations of chemicals detected in confirmation samples (post-remediation) from unsaturated soils beneath the former tank basin area Post-remediation soil samples from ' beneath the water table were not used to represent soil source concentrations, as the detected chemicals will not be subject to volatilization from beneath the water table Moreover, volatilization of chemicals from the saturated zone is already accounted for in evaluation of volatilization from groundwater Representation of groundwater COPC concentrations for onsite exposure pathways involved use ' of site maximum levels detected in all onsite wells over the past two years (see Table 1 of Appendix H) For offsite residential exposure to groundwater at the apartment building located 100 feet southeast of the site, COPC source concentrations corresponded to the maximum levels detected over the past two years in offsite downgradient well MW7 (see Table 1 of Appendix H) 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 (ETIC 2000) For evaluation of risks associated with direct exposure to groundwater at the ' agneultural/imgation wells located 14 miles 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 1995, and Domenico 1987) Biodegradation and other mass- loss mechanisms were conservatively ignored along the path of chemical transport from the site to the wells Hydraulic parameters used in the Domenico model correspond to conservative ASTM (1995 and 1998) default values (see Appendix H), while plume dimensions correspond to highly conservative estimates based on the extent of hydrocarbon detection across the site Selected target nsk/hazard levels represent the most conservative values outlined by USEPA (1989, 1991), ASTM (1995 and 1998), and DTSC (1999) risk assessment guidelines Use of a target risk level of 1 x 10-6 is particularly conservative with respect to standard risk assessment practice for evaluation of exposure under commercial/industrial land use F ip,gects1701371Mmtei\%VplClosw e l IM0137mert doc 16