Exposure Assessment Using Analytical and Numerical Models: Case Study

Exposure assessment studies attempt to link environmental pollution with increased health risk to humans. Most occurrences of past exposure have sparse databases from which to estimate health risks. Thus, analytical and numerical models can be used to help quantify estimates of past exposures. In this paper, we present a case study describing a situation in which tetrachloroethylene (PCE), a volatile organic chemical, has been detected in ground-water supplies in the Osborn Connecticut Correctional Institution (OCCI) area, including the Rye Hill Circle neighborhood, in Somers, Connecticut. Contaminant concentrations based on measured ground-water samples on the OCCI property were found to range from 2,553 parts per billion (ppb) in glacial till to 1,860 ppb in the underlying bedrock aquifer. In the residential wells tapping the same bedrock aquifer, PCE concentrations ranged from 545 ppb (well RHC-082) to below detection limits (<1 ppb). Analysis of the site included applying simplified analytical and numerical models for flow and transport in one and two dimensions and a parameter uncertainty analysis for vertical migration of contaminants. Estimates based on results of modeling analyses indicate that residential wells in the Rye Hill Circle area have probably been contaminated since their installation in 1978 through 1981. Thus, citizens have probably been exposed to PCE-contaminated water for 16 years from 1978 through 1993, when carbon-activated filters were installed on each well. However, no acute or long-term adverse health effects, including cancer, are likely to occur as a result of these exposures. The important lesson that should be derived from this study is that the use of simplified oneand two-dimensional fate and transport models in an appropriately simplified hydrogeologic setting can yield meaningful and useful results.