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The interactive and user-friendly PC-computer-based RIVER/FISH mechanistic modeling procedure was evaluated and found to provide credible simulations of aquatic organism bioaccumulation for nonpolar hydrophobic organic chemicals (NPHOCs) using a four-trophic-level food web calculation methodology. Model evaluation included calibration to observations of tetrachlorinated dibenzodioxin and -furan (TCDD/F) bioaccumulation in largemouth bass (Micropterus salmoides) and channel catfish (Ictalurus punctatus) that were exposed in pilot-scale experimental streams receiving biologically treated pulp mill effluent and subsequent comparison of model predictions without bioaccumulation parameter adjustment (validation) to similar but independently obtained bioaccumulation observations. When calibrated with site-specific information on NPHOC loading rates and organism feeding habits, model prediction uncertainty was within U.S. Environmental Protection Agency (EPA) established standards averaging 1.1 pg/g for the observations used for model validation, which all measured < 20 pg/g. The procedure appears suitable for field-scale applications provided the site-specific feeding habits of each organism in the simulated food web can be estimated accurately, all significant chemical loadings are quantified, and the natural variability of bioaccumulation processes is considered when interpreting simulation results.

Field measurements of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) concentrations in fish tissues from riverine environments impacted by treated pulp and paper industry effluent in the United States were analyzed. Data from 39 sites in 14 states across the four major U.S. paper-making regions were assembled and analyzed to determine the annual change in lipid-normalized concentrations of TCDD in fish tissue. The results provide strong evidence of a nationwide trend of decreasing lipid-normalized TCDD concentrations in fish tissue, with 84% of the examined sites showing a decrease. While the paucity of data currently limits any conclusions regarding the statistical significance at individual sites, the overall median rate calculated indicates a 0.36 annual fractional decrease in lipid-normalized concentrations of TCDD in fish tissue (0.18 to 0.51, 95% confidence interval); the average annual fractional decrease was 0.35 (0.23 to 0.47, 95% confidence interval). Subdividing fish into benthic and nonbenthic categories resulted in rates which were not significantly different from one another for both the median and mean statistics.