Student Research: Joseph E. Johnson
Exposure to soil-borne contaminants can occur through ingestion, dermal absorption and/or inhalation pathways. The uncertainty associated with estimates of exposures vie these pathways is large, particularly for the dermal route. Site cleanup costs may be very sensitive to cleanup targets. If remediation goals are driven by dermal pathway risk estimates, the appropriateness of cleanup expenditures is especially subject to question.
Screening of 235 Superfund risk assessments from the period 1989-1992 indentified of 38 sites where lifetime excess cancer risks attributable to dermal soil contac were greater than the nominal regulatory threshold of 1 x 10-4. At 19 of these sites, the dermal soil pathway presents the largest soil-based carcinogenic risk and may therefore drive soil cleanup targets. At 9 sites the dermal soil pathway presents the largest carcinogenic risk of any pathway.
Statistical analysis of risk algorithm inputs indicates that both exposure and toxicity-related factors contribute to elevated dermal soil risks. In many cases, the plausible range of imput parameters is very broad. Monte Carlo simulations utilizing soil adherence data from a University of Washington field study predict a two-to-threefold decrease in dermal soil risks compared to using default adherence factors. Although substantial, the downward adjustment suggested by these results is insufficient to eliminate the plausibility of dermal pathway dominance in risk assessment of contaminated soils. Frequency and duration of exposure estimates remain poorly quantified, and represent the greatest area of uncertainty in the current methodology.