Levels in Exposed Organisms
For chemicals that bioaccumulate, body burdens can be used to determine exposure. However, if the exposed organisms are mobile relative to the biological half-life of the parent substances or the metabolites, care must be taken when interpreting exposure, since it may not have occurred where the animal was captured. Factors such as natural variability, migratory habits and environmental concentrations that approach analytical detection limits make the determination of exposure difficult. For example, body burdens of a number of organochlorines were measured in the study on the Grande Prairie ecosystem prior to and as chlorine dioxide substitution was increased (Swanson et al., 1993a). Hydrophobic organochlorines such as 2,3,7,8-TCDD/TCDF were found in sediments, in benthic organisms and in fish. Levels appeared to be related to feeding habit with higher concentrations in whitefish (which feed on benthic filter feeders). After implementation of 70% and then 100% chlorine dioxide substitution, levels of 2,3,7,8-TCDD/TCDF in effluents and sediments declined rapidly but levels in fish persisted somewhat longer (Figure 6). Levels of EOCl in fish declined after the change to 100% chlorine dioxide, suggesting that the compounds measured as EOX were more rapidly excreted than chlorinated dioxins and furans. These data support the predictions that 100% substitution with chlorine dioxide results in significant reductions in the production as well as the uptake by fish of organochlorines from pulp mill effluents.