USING 7BE AND 210PB CONCENTRATIONS IN RIVER SEDIMENT SAMPLES TO UNDERSTAND THE FATE OF PARTICLE-BOUND MERCURY CONTAMINATION IN A REGULATED RIVER—ANDROSCOGGIN RIVER, NORTHERN NEW HAMPSHIRE

Sediment-sorbed mercury contamination from past chlor-alkali production waste disposal occupies, and is mobilized through, hydropower reservoir sediments on the Androscoggin River in northern New Hampshire. The amount of transport and accumulation of mercury within the reservoirs is unknown. As Superfund remediation of the upstream mercury point-source diminishes the introduction of mercury into the river, mapping where new mercury-free sediment is deposited may identify regions where incoming sediment is sequestering older contaminated sediment. Likewise, mapping where older sediment is exposed at the riverbed may highlight areas where contaminated sediment is scoured into the river, remaining a long-term threat to local ecology. This study detected concentrations of naturally occurring ⁷Be and ²¹⁰Pb radionuclides in sediment-water interface samples throughout the Gorham Dam reservoir and plotted results on a bathymetric map. Results indicate that modern sedimentation is not uniform throughout the reservoir; rather, lower energy regions within the reservoir, such as along the shorelines, contain higher concentrations of ⁷Be and ²¹⁰Pb, and are radiometrically “younger”. Samples collected from the thalweg of the reservoir have lower ⁷Be and ²¹⁰Pb concentrations, and are closer to being radiometrically “dead”. Using relative sediment age distribution as a proxy for distribution of sediment-sorbed mercury contaminants indicates that buried sediment on the flanks of the reservoir may contain a larger volume of mercury contamination beneath the river bed, while higher velocity regions within the reservoir are scoured, supplying relatively uncontaminated sediment downriver. An understanding of modern sedimentation within the reservoir will serve the ultimate goal of identifying the fate of particle-bound contaminants in regulated rivers.