Analysis of Uncertainty and Variability in Exposure to Characterize Risk: Case Study Involving Trichloroethylene Groundwater Contamination at Beale Air Force Base in California

Abstract

Quantitative assessments of potential human-health consequences from contaminants in environmental media routinely involve conservative deterministic, screening-level calculations of exposure and risk. Because these calculations generally are based on multiple upper-bound point estimates of input parameters, particularly for exposure attributes, they can yield results for decision makers that actually overstate the need for costly remediation. Alternatively, quantifying uncertainty and variability in exposure can provide a more informative and quantitative characterization of health risk. To illustrate, uncertainty and variability in exposure were analyzed for a hypothetical population at a specific site in California where there is trichloroethylene (TCE) contaminated ground water and a potential for its residential use. When uncertainty and variability in exposure were addressed jointly, the 95th-percentile upper-bound value of individual excess lifetime cancer risk was a factor approaching 10 lower than the most conservative deterministic estimate. Also, the probability of more than zero additional cases of cancer can be estimated, and in this case study it is less than 0.5 for a prospective residential population of up to 26,900 individuals present for any 7.6-y interval of a 70-y time period. Clearly, this probabilistic approach can provide reasonable and equitable risk-acceptability criteria for contaminated sites.