partition coefficient, Kow. The range of uncertainty in the predicted concentrations also varied among the chemicals. In-lake removal processes (sedimentation, volatilization, and degradation) were important for all chemicals.
CASE STUDY 3B: Ecological Risk Assessment of TCDD and TCDF
M. Zeeman, U.S. Environmental Protection Agency
This paper is based on a full-scale ecological risk assessment of chlorinated dioxin and furan emissions from paper and pulp mills that use the chlorine bleaching processes (Schweer and Jennings, 1990). Although the risk assessment addressed potential risks to terrestrial and aquatic wildlife exposed to TCDD and 2,3,7,8-tetrachlorodibenzofuran (TCDF) via a number of environmental pathways, the case study was limited to exposure of terrestrial wildlife to TCDD resulting from land disposal of paper and pulp sludges. This route of exposure was identified as one of the most hazardous in the multiroute risk assessment.
The specific exposure pathway considered was uptake of TCDD by soil organisms (earthworms and insects) from soil to which pulp sludge has been applied, and the consumption of soil organisms by birds and other small animals. Transfer factors were estimated both by modeling and from data collected in a field study in Wisconsin, in which an average soil TCDD concentration of 11 ppt led to concentrations of up to 140 ppt in a composite of six robin eggs. The models used three alternative sets of assumptions: low estimate, best estimate, and high estimate. The best estimates of tissue concentrations derived from the model were often similar to those observed in the field study: the low and high estimates were lower and higher, respectively, by a factor of roughly 10.
Risk estimates for terrestrial wildlife were derived by comparing exposure estimates (usually converted to daily intake rates) with benchmark toxicity values. The values used as benchmarks were either lowest-observed-adverse-effect levels (LOAELs) or no-observed-adverse-effect levels (NOAELs) for reproductive toxicity in birds and mammals