Preliminary Contaminant Assessment of the Thomson, Forebay, and Fond Du Lac Reservoirs

The Thomson, Forbay, and Fond du Lac Reservoirs were created along the Lower St. Louis River in the early 1900s for the purpose of hydroelectric generation. Since their construction, several feet of soft sediment have accumulated behind the dams. In 1974, the U.S. Corps of Engineers estimated that 4.6 million cubic yards of fibrous organic sediments and sludges, approximately 5-10 feet thick, had accrued behind these dams (USCOE, 1974). Prior to 1979, the primary effluent dischargers to the St. Louis River, upstream of the reservoirs, were the City of Cloquet, the USG Corporation (formerly Conwed), and Potlatch's Northwest Paper Division facility. The wastewater from these discharges were combined and routed to the newly constructed Western Lake Superior Sanitary District (WLSSD) in 1979. Nonpoint sources of contamination from landfills, runoff, and atmospheric sources may have contributed to contamination in the reservoirs. Landfills operated by Conwed and Potlatch exist along the St. Louis River, just upstream of the Thomson, Forbay, and Fond du Lac Reservoirs. The contribution from these landfills to downstream contaminant levels is not known; however, some of the solid waste (e.g., sludge ash, sludges) may be contaminated with mercury (Hg), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (hereafter referred to as TCDD or dioxin), and/or polychlorinated biphenyls (PCBs).