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ground wafter to deeper water-bearing units do not represent realistic exposure pathways for these sites • The presence of shallow ground water precludes the construction of buildings with basements or other subsurface structures Thus, the ' migration of VOCs from ground water into confined spaces does not represent a realistic exposure pathway ' 5.2.4 Summary of Realistic Exposure Pathways In summary, the following exposure pathways were selected for evaluation in the HRA Medium Scenario Exposure Route Soil Construction Incidental ingestion ' Dermal contact Inhalation of vapors(VOCs only) ' Ground water Commercial/Industrial Inhalation of vapors(VOCs only) Ground water Construction Inhalation of vapors(VOCs only) 53 DEVELOPMENT OF SITE-SPECIFIC SCREENING LEVELS Based on the realistic exposure pathways identified in the preceding section, three sets of risk-based screening levels were developed, as ' follows • Risk-based screening levels for soil, based on a construction scenario, ' • Risk-based screening levels for ground water, based on a commercial/ industrial scenario, and ' • Risk-based screening levels for ground water,based on a construction scenario ' Table 9 presents the risk-based screening levels for soil These calculated levels were developed in accordance with guidance issued by USEPA ' (1996), USEPA Region 9 (1999), and ASTM (1996), and Incorporate current toxicity information published by Cal-EPA and USEPA Exposure assumptions are based on default values recommended by USEPA in all ' cases, except with regard to exposure frequency and exposure duration Default values are not available for these parameters under a construction scenario, and so site-specific assumptions were made regarding exposure frequency and exposure duration In light of the limited area of impact, ERM 21 HEINZ USA 3374 60-3/27/00