Concentrations of dioxin-like compounds, primarily polychlorinated dibenzofurans (PCDFs), in soils and sediments of the Tittabawassee River (TR) and associated floodplains downstream of Midland, Michigan (USA) were greater than upstream sites and prompted a site-specific risk assessment of great blue herons (GBH). Dietary exposure of GBH to PCDFs and polychlorinated dibenzo-p-dioxins (PCDDs) was evaluated based on site-specific concentrations of residues in prey items. Concentrations of ∑PCDD/DFs and 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents (TEQ(WHO-Avian)) in prey items collected from the TR were consistently greater than those collected from associated reference areas (RAs) and further downstream in the Saginaw River (SR). The average daily dose (ADD(pot)) of ∑PCDD/DFs to GBH was 45- to 54-fold greater along the TR and 12-fold greater along the SR when compared to the RA. ∑PCDD/DFs were normalized to TEQ(WHO-Avian), and fold differences in the ADD(pot) increased, being 150- to 190-fold greater along the TR and 36-fold greater along the SR than they were in the RA. Greater fold changes in the ADD(pot) based on TEQ(WHO-Avian) between the RA and the TR and SR was due to prey items from the latter reaches having a greater relative toxic potency of ∑PCDD/DFs, primarily from greater amounts of 2,3,7,8-tetrachlorodibenzofuran but also 2,3,4,7,8-pentachlorodibenzofuran. Potential for adverse population-level effects from site-specific contaminant exposures were evaluated via comparison to selected toxicity reference values. The prediction of minimal to no risk of adverse population-level effects resultant from the assessment of site-specific dietary exposure of GBH to ∑PCDD/DFs along the TR and SR is consistent with site-specific assessments of tissue-based exposures as well as population condition.
