Alternative modeling approaches for contaminant fate in soils: uncertainty, variability, and reliability

The fate of contaminants in soils is an issue that is relevant to the management of both uncontrolled hazardous waste sites and to engineered facilities that accept hazardous materials. Addressing long-term potential human exposures requires the use of models. Large uncertainties are associated with predictions from these models. Some of this uncertainty derives from uncertainty about chemical properties and transformation half-lives and some derives from the variation among sites. The purpose of the paper is to consider two alternative modeling approaches and the reliability with which each approach can represent variations of contaminant behavior in the California landscape. One approach is based on the exact solution of the dispersion/advection differential equations that describe contaminant fate in the soil. The second approach is based on a mass-balance approach with less resolution in the soil layers. Incorporation of trichloroethylene (TCE) in the top ten meters of soil is used as a case study to consider how well the two models can be expected to predict cumulative inventory in this ten-meter zone over five years. Using rank correlation as a measure of the importance of parameter uncertainty, it is demonstrated that these two models represent variability of contaminant behavior in the California landscape with comparable reliability.