Tier 1 Preliminary Evaluation of Pollutant Sources to the Impounded Reaches of Five Dams in the Neuse River Basin, North Carolina.
Tom Augspurger Sara E. Ward
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Impoundments are well recognized stressors to many species of riverine fishes, amphibians, mussels, crayfishes, and insects (Richter et al. 1997). Neves et al. (1997) and Watters (2000) reviewed effects of impoundments on freshwater mollusks, noting flow changes, population fragmentation, water quality problems and sediment issues. Dams also alter normal nutrient dynamics of riverine systems (Freeman et al 2003) and can degrade water quality within the impounded reach as well as downstream (Arnwine et al. 2006). Removal of dams has re-established important anadromous fish runs in North Carolina and has restored habitat and improved water quality for a variety of species. One issue among the many to consider in evaluating the risks and benefits of dam removal is the chemical nature of the sediments accumulated behind the dam. Impounded reaches behind some dams accumulate silt and detritus through time, and both organic and inorganic contaminants have a strong affinity for the silt and organic fraction of sediments (Anderson et al. 1987; Rodgers et al. 1987). In addition to their potential in-place effects, contaminated sediments can impair surface waters and associated aquatic life upon mobilization and transfer of water-soluble pollutants to the water column.
The degree of the concern is a function of site-specific pollutant loading based on age of the dam, dominant landuses, pollutants discharged into the watershed, and extent of watershed development. While there is no sediment evaluation protocol specific to dams, the issue is very similar to evaluating sediments proposed for dredging and disposal. The U.S. Environmental Protection Agency and U.S. Army Corps of Engineers (USEPA/USACE 1998) have a guidance manual on disposal of dredged sediments which recommends a phased approach to sediment evaluation. The phases start with a ‘tier 1’ assessment of the potential for sediment contamination to be an issue warranting any further consideration.
U.S. Fish and Wildlife Service
http://www.fws.gov/raleigh/pdfs/FWS_NeuseDams_FinalTier1Rpt.pdf, 4 MB
U.S. Fish and Wildlife Service - Raliegh, North Carolina, Ecological Services Field Office