Estimating exposure point concentrations for surface soils for use in deterministic and probabilistic risk assessments

Abstract

When estimating human or ecological risks from exposures to surface soils at terrestrial properties regulated as hazardous waste sites by federal or state agencies, risk assessors must estimate exposure point concentrations (EPCs) for compounds in the surface soils. In this manuscript, we demonstrate non‐parametric methods to estimate the EPC for a single compound in surface soils for use in deterministic and/or probabilistic human or ecological risk assessments. Since regulatory agencies instruct risk assessors to consider scenarios involving long‐term (chronic or lifetime) average exposures in most human risk assessments (USEPA, 1992, Exposure; Lioy, 1990; USEPA, 1989, HHEM), it is essential to distinguish (i) the number of soil samples taken in the field program, Ns, for which the field geologist has laboratory measurements from (ii) the number of exposure events, Ne (>>Ns) over which the exposure is properly averaged. By taking spatial information into account, we demonstrate new methods for computing the upper 95th‐percentile of the uncertainty in the mean concentration that overcome the limitations in the method currently recommended by the US Environmental Protection Agency for deterministic human risk assessments. We also extend the new methods to developing EPCs for multiple compounds and to developing second‐order distributions for use in probabilistic risk assessments.