INFLUENCE OF SOIL HALF-LIFE ON RISK ASSESSMENT OF CARCINOGENS

Risk estimates for contaminants in soil are currently calculated assum ing that concentrations remain unchanged over time. In reality, biolog ical and physicochemical processes can substantially diminish contamin ant concentrations in soil. For exposure periods typically evaluated i n USEPA risk assessments, failure to consider the decline in contamina nt levels from environmental transport and degradation can result in a significant overestimation of the average daily dose of toxicant. Thi s overestimation may be up to 2- to 3-fold for compounds with long hal f-lives (15-20 years) in soil and as much as 40-fold for compounds wit h short half-lives (0.5 years). Overestimation of dosages affects esti mation of cancer risks because of the assumption that the probability of cancer increases directly with the cumulative dose of carcinogen. T hus, assuming static contaminant concentrations in soil adds unacknowl edged conservatism to cancer risk estimates and target concentration l imits. Furthermore, as significant time may elapse before future-use s cenarios could possibly occur, soil half-life can affect the estimatio n of noncarcinogenic health hazards as well. Therefore, an increase in target concentration limits for some compounds could be allowed and c orresponding remediation costs reduced by considering how soil half-li fe changes the dosage calculation. Specific examples of the influence of soil degradation rates on estimates of cancer risk are presented an d the degree of added conservatism imparted to risk assessments throug h assumption of static site contaminant levels is discussed. Consideri ng the potential importance of this parameter for risk assessment and risk management decisions, soil degradation of contaminants under site -specific conditions should be performed whenever possible and incorpo rated into the risk assessment exercise. When the soil degradation rat e cannot be measured or reliably predicted, an estimate of the degree of conservatism should be made to provide risk managers with an apprec iation of the degree of uncertainty in the calculation of risk. (C) 19 95 Academic Press, Inc.