Sediment Contaminants at Dillsboro Reservoir: Report on Site Assessment and Sediment Analyses. : Final Report
Tom Augspurger Mark Cantrell John Wishon
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Potential removal of Dillsboro Dam, located on the Tuckasegee River near Sylva (Jackson County), North Carolina, has caused some concern for the potential mobilization of sediment-associated contaminants accumulated behind the dam in Dillsboro Reservoir. We used the framework of the U.S. Environmental Protection Agency / U.S. Army Corps of Engineers technical guidance manual on disposal of dredged material in inland waters to evaluate this issue. A tier 1 review of existing information indicated no major pollutant sources or contaminant concerns upstream of the dam. The review also indicated that sediments within the reservoir might have low potential to accumulate contaminants from a physical standpoint, being comprised primarily of sand and gravel. Finally, it appears that watershed land uses upstream and downstream of the reservoir are similar and that any mobilized sediments from behind the dam may merely subject downstream areas to the same sources of contaminants to which they have been historically exposed. While no major concerns were noted in the review of existing information, new data were collected to support management decisions (tier 2). Six sediment samples from within the reservoir and four samples downstream were collected in June 2003 and analyzed for elemental contaminants. None of the sediment samples from within the reservoir or downstream exceeded probable effects concentration screening values, indicating no obvious concern. Over 80 percent of the sediment sample results were also less than threshold effects screening values, indicating they are unlikely of toxicological significance. Slightly less than 20 percent of the sample results fell between the screening values and they were further evaluated by comparing their magnitude to the geometric mean of the screening values. Only two values (both for nickel) exceeded the geometric mean of the screening values. The highest nickel concentration (41.5 ug/g dry weight) was from a downstream sediment sample near Dillsboro gage, and the average of the four downstream samples (27.4 ug/g dry weight) exceeded the average concentration of the six samples collected within the reservoir (21.8 ug/g dry weight). Accordingly, the nickel concentrations behind the dam should not be a concern relative to movement downstream where concentrations are slightly higher. Our review of existing data and an on-site assessment (tier 1) and results of sediment chemistry (tier 2) indicated no significant sediment contamination. This assessment is limited to the toxicological properties of the sediments evaluated. It does not address the potential physical impacts of sediment mobilization.
U.S. Fish and Wildlife Service
http://www.fws.gov/raleigh/pdfs/DillsboroReportFinal.pdf, 4 MB
U.S. Fish and Wildlife Service - Raliegh, North Carolina, Ecological Services Field Office
U.S. Fish and Wildlife Service - Asheville, North Carolina, Ecological Services Field Office