Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) are related compounds that are of interest primarily because of their potential toxicity. They are considered carcinogens and have been implicated as hormone disrupters. An occurrence and distribution study of these compounds in the Willamette Basin, Oregon, was done by the U.S. Geological Survey in 1992- 1995. Bed sediment samples were collected at 22 sites, and fish tissue samples were collected from 8 sites. Samples were analyzed for 10 tetra- through octa- congener class totals and for 17 individual 2,3,7,8-substituted congeners. PCDD/Fs were found in bed sediment and fish tissue throughout the basin, including samples from the most remote forested sites. PCDD/F concentrations in bed sediment at most sites in agricultural and forested areas were similar to those at reference sites worldwide and are probably background concentrations due to atmospheric deposition. The highest concentrations in bed sediment were found at sites where industrial or urban inputs were likely. Potential toxicity at these sites (as measured by toxicity equivalents concentration) was high enough to be associated with increased risk to sensitive wildlife. From 30-60 percent of the toxicity equivalents concentration in bed sediment was due to hepta- and octa- congeners. The most toxic congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), was detected at only 6 of 22 sites. Compared to bed sediment from the same site, fish tissue usually had a lower total PCDD/F concentration, but contained a higher proportion of the most toxic congeners, such as 2,3,7,8-TCDD and 2,3,7,8-tetrachlorodibenzofuran. Because of these differences, toxicity equivalents concentrations in fish were higher than those in bed sediment from the same site at half of the sites where both media were analyzed.
