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1 chemicals from the site to these wells was evaluated in the RBCA analysis Specifically, this exposure ' pathway was assume complete based on a highly conservative assumption of direct(i a ingestion) residential exposure to groundwater at these wells ' Chemicals at residual levels in groundwater may migrate offsite beneath the existing residential apartments, located 100 feet southeast of the site As a result, indirect exposure of residents to volatile emissions from groundwater was evaluated Offsite exposure to commercial and light industrial receptors was not ' quantified, as the need for corrective action would be driven by the higher risk to onsite commercial receptors and/or offsite residential receptors To summarize,the following offsite receptors were evaluated ' Inhalation of volatiles from groundwater to outdoor air(offsite residential receptor), • Inhalation of volatiles from groundwater to indoor air(offsite residential receptor), and • Ingestion of groundwater at nearest offsite water supply well(offsite residential receptor) 3.2 Tier I RBCA Analysis ' The initial attempt at evaluating health risks associated with potential exposure to hydrocarbons and MTBE at the site was based on a Tier I evaluation The Tier I evaluation involves comparison of the site- specific chemical source concentrations to highly conservative, generic RBSLs based on simplified ' equations and generalized site conditions characterized by default input data Default data are supplemented by user-defined data including chemicals of potential concern(COPCs), source concentrations for each COPC, target carcinogenic risk levels, and target noncarcmogenic hazards Tier I input data are included as Appendix II, with user-defined data summarized in Table 1 As indicated in Table 1, consistent with ASTM (l 995)guidelines, this RBCA analysis focused on ' evaluation of potential risks due to individual chemical constituents such as BTEX and MTBE, identified as COPCs The risk associated with hydrocarbon mixtures (i e TPH-g and TPH-d)was not quantified, however, by evaluating the risk associated with the most toxic and mobile hydrocarbons(e g BTEX), the ' risk associated with exposure to hydrocarbon mixtures is mdirectly evaluated In fact, the conservative approach for quantifying risks associated 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 1551 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 a 1991 and 1992) in tank basin product Iine 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 conformation samples(post-remediation) from unsaturated soils beneath the former tank basm 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) For offsite residential exposure to groundwater at the apartment building located 100 feet southeast of the site, COPC source