Use of California Human Health Screening Levels (CHHSLs) in ...

3.2 Tier 2 Human Health Risk Assessments3.2.1 PurposeThe Tier 1 CHHSLs were developed with default or generic assumptions that arenot specific to any particular site condition. If site soil concentrations exceedCHHSLs, site-specific exposure assumptions may be used in the standard riskmodels described in Appendix 1 or the PEA guidance to estimate risk and/ordevelop site-specific CHHSLs. Using alternative exposure assumptions in thesestandard risk models could reduce the time and cost incurred by both theregulated business and the overseeing responsible party in finalizing the riskassessment. Modifications to the default assumptions must be described andjustified in the text of the report, presented with the revised set of screening orcleanup levels, and agreed to beforehand with the regulatory agency.3.2.2 Examples of Site-Specific AdjustmentsPotential site-specific modifications include:• Use of alternative target risk levels, and/or alternative exposureassumptions;• Elimination of direct-exposure concerns through imposition ofinstitutional controls;• Inclusion of potential exposure of construction and trench workers tocontaminated soil not likely to be exposed at the ground surface in thefuture (e.g., capped soils or soils isolated at depth);• Consideration of method reporting limits or natural background orambient concentrations of a chemical in place of the CHHSL.After incorporating site-specific parameter values into the Tier 1 direct-exposuremodels, alternative human-health-based screening levels can be calculated and recomparedto site data.3.3 Tier 3 (Baseline) Human Health Risk Assessments3.3.1 PurposeIn a site-specific baseline human health risk assessment, alternative models andassumptions are used and fully justified to develop a detailed, comprehensiveJanuary 20053-2CHHSLS