Characterizing Presence of Metals and PCBs (Polychlorinated Biphenyls) and Evaluating Potential for Bioaccumulation in a Regulated Reservoir on the Pend Oreille River, Washington, USA

摘要

Boundary Reservoir formed by Boundary Dam near the Canadian Border is located in a scenic valley of northeast Washington State This reservoir is operated by Seattle City Light as a load-following facility for hydropower production. Historic and current mining has been an active industry in the region since the turn of the nineteenth century. Daily pool level fluctuation was suspected of entraining contaminants from tailings and erosional materials from margins of the reservoir. Surface water, sediment, and pore water were sampled for metals (As, Hg, Pb, Zn, Cd) and PCBs at 14 locations. Sampling program development was guided by using a hydraulic-sediment model for predicting potential settling zones and pathway models for describing contaminant dynamics between media. The models (toxicological and pathway models) represent a two-step characterization that evaluated the potential for bioaccumulation in aquatic biota. Comparison of sampling results against components of the models influenced by water level fluctuation determined if Project operations were responsible for presence and concentration of contaminants. Surface water samples from the fluctuation zone (7 locations) detected trace dissolved concentrations of As (2 locations) and Zn (7 locations). The concentration of contaminants in the water fluctuation zone did not change at depth from beginning to end of a falling hydrograph in any portion of the reservoir; therefore, no entrainment of contaminated sediments were detected. Dissolved Pb concentrations from water samples collected near the bottom were 3.46 - 5.46 μg/L at two sites of 14 locations. Similarly, dissolved Pb concentration (3.98 μg/L) in pore water exceeded chronic criteria for surface water at one site, while a Zn concentration of 279 μg/L exceeded acute criteria at the same site. Sediment samples had low concentrations of Zn except at one location, but did not exceed freshwater sediment contamination guidelines. Subsequent and more intensive longitudinal sampling showed that sediment thresholds were not exceeded for zinc. However, lead concentrations remained confined to pore water at one of the sites and not bioavailable. Exceedance of sediment thresholds for contaminants did not occur and did not establish a diffusion gradient transferring contaminants into pore water and then to surface water. PCBs were not detected in any of the sediment samples. Water chemical factors described in toxicological models do not promote transfer of contaminants between media.