SEDIMENT MERCURY ABOVE AND BELOW THE HEAD-OF-TIDE MILL POND DAM IN DURHAM N.H

Contaminant mercury (Hg) when mobilized from dam removal, is transported to the downstream watershed which negatively impacts water quality. The Mill Pond Dam, a Head-of-Tide dam in Durham NH, was slated to be removed by public vote in March 2022. Previous studies of Hg in this area have found varying sediment Hg concentrations, and further study is important to preventing the transport of contaminants into the Great Bay Estuary downstream of the dam. Sediment samples from 12 sites in the watershed above and below the Mill Pond Dam in Durham, NH were analyzed for total mercury. Samples were collected over 3 sampling campaigns during Summer 2023. Seven sites above the dam contained total Hg concentrations above the freshwater interim sediment quality guidelines (ISQG) established by the Canadian Council of Ministers of the Environment. Two of these sites had samples with concentrations above the probable effect level (PEL). All five sites in the estuary environment below the dam contained sediments with total Hg concentrations above the marine ISQG, and two of these sites had samples which were above the marine sediment PEL. These sites were heterogeneous over the sampling period, suggesting that Hg, though high in places, is highly variable. There is not a simple relationship with organic matter and mercury concentration. This project aims to further understand the extent of contamination in the watershed, and to be an aid to sediment mitigation decisions made by the town of Durham as preparations are made for the dam’s removal.