Dioxins and furans are organic chemicals that are of environmental interest because of their acute toxicity to some animal species. Recently, they have been implicated as potential endocrine disrupters--chemicals that can interfere with the normal function of hormones, and thereby affect reproductive success. A review of health risks associated with dioxins and furans has been published in Environmental Science and Technology (1995, v. 29, p. 24A-35A).
Dioxins and furans are not single chemicals, but rather families of related compounds that differ in the number and position of their chlorine atoms. Each different compound is called a congener. Congeners that have the same number of chlorine atoms form a congener class.
Toxicity varies among congeners. The most toxic congener is 2,3,7,8-TCDD (tetrachlorodibenzo-p-dioxin), which has four chlorine atoms located at the 2,3,7, and 8 positions. Toxicity decreases as chlorine atoms are added or removed. For example, OCDD (octachlorodibenzo-p-dioxin), the congener with eight chlorine atoms, is considered a thousand times less toxic than 2,3,7,8-TCDD. However, regardless of the number of chlorine atoms, the most toxic congeners are those that have chlorine atoms in at least the 2,3,7, and 8 positions. Because toxicity varies among congeners, concentrations of dioxins and furans can be expressed as equivalent concentrations of 2,3,7,8-TCDD in order to provide an indication of potential total toxicity.
Toxicity of dioxins and furans also varies widely among animal species. The USEPA has estimated that 2,3,7,8-TCDD equivalent concentrations below 60 pg/g (picograms per gram) in sediment are of low risk to fish exposed to the sediment (U.S. Environmental Protection Agency, 1993). Similarly, concentrations below 50 pg/g in fish tissue are estimated to be of low risk to predator fish.
Dioxins and furans are not deliberately manufactured. Rather, small quantities of these compounds are inadvertently produced as by-products of a variety of industrial processes. Two important sources are waste incineration, especially of plastics, and effluent discharge from pulp and paper mills that use chlorine bleaching. In addition, dioxins and furans are trace contaminants in many chlorinated chemicals, such as some wood preservatives and pesticides. Chlorination of sewage effluent also may produce dioxins and furans.
Dioxins and furans contain from one to eight chlorine atoms located in the positions indicated by the numbers 1-4 and 6-9. There are 75 different dioxin congeners and 135 different furan congeners. Dioxins and furans containing four or more chlorine atoms were analyzed in this study.
A survey of dioxin and furan occurrence in the Willamette Basin during 1992-95 found a wide range of concentrations in bed sediment, with the highest concentrations being about 1,000 times greater than the lowest concentrations (Bonn, 1998; in press). Samples from industrial areas generally contained the highest concentrations. Concentrations of 2,3,7,8-TCDD equivalent were found at levels greater than the USEPA threshold for risk to fish at two sites (A-3 Channel in Eugene and Middle Fourth Lake near Albany), both of which are downstream from industrial areas.
Throughout the basin, total dioxin and furan concentrations were 100-1,000 times more than 2,3,7,8-TCDD equivalent concentrations because the most abundant congeners also were the least toxic. OCDD was the dominant congener found in bed sediment. It accounted for about 80 percent of total concentrations. The sum of concentrations for the TCDD congener class was usually about 100 times lower than the OCDD concentration at the same site, and 2,3,7,8-TCDD was rarely detected.
Dioxins were detected in every bed sediment sample, including those from forested reference basins, such as Mack Creek in the Cascade Range east of Eugene and Fir Creek in the Bull Run Watershed east of Portland. Concentrations below 100 pg/g occurred in sediment from Mack Creek and from Cottage Grove Reservoir, which receives drainage from a large forested basin. Concentrations at other forested sites were somewhat higher (100-300 pg/g) and were similar to those at agricultural sites.
Atmospheric deposition is the most likely source of dioxins and furans in bed sediment at forested and agricultural sites. |
Atmospheric deposition contains low concentrations of dioxins and furans from wood burning and waste incineration. This is the most likely source of these compounds in bed sediment at forested and agricultural sites. Low concentrations at Mack Creek and Cottage Grove Reservoir probably result because these sites are farther from atmospheric sources than other sampling sites. In general, dioxin and furan concentrations at forested and agricultural sites in the Willamette Basin are similar to or less than those at reference sites located elsewhere in the United States where atmospheric deposition is the presumed source.
In addition to atmospheric deposition, some sites may be influenced by specific local sources. For example, a bed sediment sample obtained beneath a wooden bridge at one forested reference site showed unusually high concentrations of furans. Other samples obtained from the same general area, but upstream from the bridge, did not. The high furan concentrations may have been associated with wood preservatives used on the bridge.
In the main stem Willamette River, dioxin and furan concentrations in bed sediment increased as the predominant adjacent land use changed from agricultural to industrial. Sediment from RM (river mile) 55 near Newberg exhibited essentially background concentrations. In contrast, higher concentrations at RMs 13 and 5 may reflect chemical use and production in the Portland Harbor area. Interestingly, sediment from Corvallis (RM 137) contained considerably higher 2,3,7,8-TCDD equivalent concentrations than that from Newberg, even though total concentrations were not very different. This is because the Corvallis site exhibited a larger proportion of 2,3,7,8-TCDD than the other sites. The Corvallis site is downstream from a pulp and paper mill that has used elemental chlorine bleaching--a process known to produce trace amounts of 2,3,7,8-TCDD. The measured decrease in concentration of 2,3,7,8-TCDD equivalent in bed sediment at Corvallis between 1992 and 1995 may be related to the mill's reduction of elemental chlorine use in 1993.
Dioxin and furan concentrations in bed sediment from the main stem Willamette River increased downstream from Newberg through the Portland area. Because the less toxic congeners predominated, 2,3,7,8-TCDD equivalent concentrations were 100-1,000 times less than total dioxin and furan concentrations.[ Note different ordinate scales; samples collected in 1992 unless otherwise indicated.]
Dioxin and furan concentrations in whole fish generally were about 10 times less than in bed sediment from the same area. However, when expressed as 2,3,7,8-TCDD equivalent, concentrations in fish exceeded those in sediment at more than one-half the sites because fish contained a larger proportion of the most toxic compounds (2,3,7,8-chlorinated congeners and TCDD) than did sediment.
No fish analyzed in this study contained 2,3,7,8-TCDD equivalent concentrations that exceeded the USEPA threshold for risk to predator fish. |
Dioxin and furan concentrations in fish tissue were much lower than in bed sediment obtained from the same area. Fish tissue, however, generally contained greater proportions of the most toxic congeners (especially 2,3,7,8-TCDD) than sediment. Consequently, 2,3,7,8-TCDD equivalent concentrations usually were higher in fish than in bed sediment at a given site.
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Bed sediment samples (
) were obtained from 22 sites; composite samples of 5-20 whole fish (
) were obtained from 8 sites (p. 25). Industrial, urban, agricultural and forested reference areas were represented. Three reaches of the main stem Willamette River were sampled both for bed sediment and fish tissue.
All samples were analyzed for tetra- through octa-chlorinated congener class totals of dioxins and furans. In addition, concentrations of all congeners with chlorine atoms in at least the 2,3,7, and 8 positions were determined. Results are expressed as picograms per gram dry weight. Detailed analyses of the data are presented by Bonn (1998; in press).
