A systematic uncertainty analysis of an evaluative fate and exposure model

Multimedia fate and exposure models are widely used to regulate the release of toxic chemicals, to set cleanup standards for contaminated sites, and t o evaluate emissions in life-cycle assessment. CalTOX, one of these models, is used to calculate the potential dose, an outcome that is combined with the toxicity of the chemical to determine the Human Toxicity Potential (HTP ), used to aggregate and compare emissions. The comprehensive assessment of the uncertainty in the potential dose calculation in this article serves t o provide the information necessary to evaluate the reliability of decision s based on the HTP. A framework for uncertainty analysis in multimedia risk assessment is proposed and evaluated with four types of uncertainty. Param eter uncertainty is assessed through Monte Carlo analysis. The variability in landscape parameters is assessed through a comparison of potential dose calculations for different regions in the United States. Decision rule unce rtainty is explored through a comparison of the HTP values under open and c losed system boundaries. Model uncertainty is evaluated through two case st udies, one using alternative formulations for calculating the plant concent ration and the other testing the steady state assumption for wet deposition . This investigation shows that steady state conditions for the removal of chemicals from the atmosphere are not appropriate and result in an underest imate of the potential dose for 25% of the 336 chemicals evaluated.