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5.0 ASSESSMENT OF RISK AND BENEFICIAL USES 1 To assess whether residual hydrocarbon constituents in soil and ground ' water at each former UST site pose an unacceptable risk to human health under current and realistic future land use conditions, a health risk assessment (HRA) was conducted The HRA methodologies, ' assumptions, and results are presented below ' The approach used to develop the HRA followed methodologies outlined by the American Society for Testing and Materials (ASTM) as described in their Standard Guide for Risk-Based Corrective Action Applied at Petroleum ' Release Sites (E1739-95, ASTM, December 1996) In developing this analysis, modifications were made to ASTM's general procedures to incorporate applicable guidance issued by the California Environmental 1 Protection Agency (Cal-EPA) and the U S Environmental Protection Agency (USEPA) ' Consistent with the ASTM methodology, development of site-specific, risk-based screening levels for the former tank sites entailed three major steps, as follows o Identification of constituents of potential concern in each medium (i e , soil and ground water) o Identification of realistic exposure pathways associated with each medium Development of risk-based screening levels for each medium, using appropriate exposure assumptions and current toxicity information ' from Cal-EPA and USEPA Each of these steps is described in the following subsections ' 51 IDENTIFICATION OF CONSTITUENTS OF POTENTIAL CONCERN Despite the remedial activities conducted at each former tank site, some ' residual constituents remain in subsurface soils and ground water Specific constituents found in post-remediation samples include BTEX compounds All of these constituents are commonly associated with ' weathered gasoline, and all were considered constituents of potential concern for the purposes of the HRA ' ERM 18 HEINZ USA 3374 60-3/27/00