Application of a Tiered Human Health Risk Analysis Approach to TPH-Contaminated Arctic Sites. S. Erdal and S. H. Youngren, EA Engineering, Science, and Technology, Inc.
A technical approach was developed for deriving risk-based cleanup goals for total petroleum hydrocarbons (TPH) in soil and groundwater at the Naval Arctic Research Laboratory (NARL) in Barrow, Alaska. The approach is consistent with the three-tiered approach of the risk-based corrective action (RBCA) framework developed by American Society for Testing and Materials (ASTM). However, we incorporated the surrogate chemical/fraction method developed by the TPH Criteria Working Group (TPHCWG) into the tiered RBCA decision-making framework. The TPHCWG grouped petroleum hydrocarbons into fractions by using chemical-specific leaching and volatilization factors. The fractions for aliphatic and aromatic chemical classes, which are based on relative carbon number index, were delineated by order-of-magnitude differences in the fate and transport properties of the individual compounds. The TPHCWG evaluated existing toxicological data for compounds within defined fate-and-transport fractions to select a surrogate chemical that could be used to represent the toxicity of each fraction for evaluation of health effects other than cancer. In this approach, Tier 1 involves development of risk-based chemical-specific and fraction-specific screening levels (RBSLs) for direct and indirect exposure pathways using conservative exposure parameters and contaminant fate and transport models. For carcinogenic chemicals (i.e., benzene and carcinogenic PAHs) chemical-specific RBSLs are developed. For noncarcinogenic chemicals, fraction-specific RBSLs are derived for three relative carbon number index ranges of C5-C8, C9-C16, and C17+ within aliphatic and aromatic chemical classes. However, the noncarcinogenic components of BTEX are evaluated on a chemical-specific basis. Tier 1 relies on using the diesel range organics (DRO), gasoline range organics (GRO), and BTEX data collected from the site in 1994/1995. If onsite concentrations of BTEX, carcinogenic PAHs, and aliphatic and aromatic carbon number fractions exceed Tier 1 chemical-specific and fraction-specific RBSLs, or remediation to the RBSLs is not practical or cost-effective, then site-specific target levels (SSTLs) will be developed in Tier 2. Chemical-specific and fraction-specific SSTLs are derived by using the same methodology outlined in Tier 1, except site-specific data and population-specific exposure parameters are used in the exposure and risk calculations. The presence of a shallow aquifer, permafrost, and saline-intruded surface waters are considered in the fate and transport modeling. If necessary, additional sampling to further characterize the site, to refine fate-transport modeling, and perform probabilistic analyses of exposures and risks will be performed to calculate Tier 3 chemical-specific site-specific target levels.