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• Inhalation of volatiles from soil to indoor air (onsite commercial/industrial receptor), • Inhalation of volatiles from groundwater to outdoor air (onsite commercial/industrial receptor), ' • Inhalation of volatiles from groundwater to indoor air (onsite commercial/industrial receptor), and • Ingestion, dermal contact, and inhalation of volatiles and particulates from soil (onsite construction worker) ' 5.3.2 Offsite Receptors As previously indicated, water supply wells downgradient of the site are limited to two agricultural/irrigation wells located approximately 14 miles south-southeast of the site Although the perforated depths for these wells are unknown, they are most likely screened in water-bearing zones deeper than that impacted beneath the site Nevertheless, as a conservative measure, potential offsite migration of chemicals from the site to these wells was evaluated in the RSCA analysis Specifically, this exposure pathway was assumed complete based on a highly conservative assumption of direct (i e , 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) 5 4 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 1 evaluation involves ' comparison of the site-specific chemical source concentrations to highly conservative, generic risk-based screening levels (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 noncarcinogenic hazards Tier I input data are included in Appendix H, with user-defined data summarized in Table 1 of Appendix H As indicated in Table 1 of Appendix H, consistent with ASTM (1995) guidelines, this RSCA 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 (c g , BTEX), the risk associated with exposure to hydrocarbon F 5pm�ects%701371NlwlerlWplClosure I 1 OO10137Lrtext JOC 15