FOREST SOIL ACCUMULATION AND SUSPENDED SEDIMENT TRANSPORT OF HG IN THE CONNECTICUT, MERRIMACK, AND THAMES RIVER WATERSHEDS

Mercury (Hg) is a toxic metal of global importance due to its long-term transport in the atmosphere. The sequestration of Hg in forest soils is a key sink to prevent its re-volatilization to the atmosphere or transport to wetlands or marine ecosystems, where it can be methylated and bioaccumulate in aquatic food webs. Here, we studied three watersheds draining southern and central New England: the Connecticut River watershed, Merrimack River watershed, and Thames River Watershed. Forest soils in upland and riparian positions were sampled in summers of 2019, 2020, and 2021. Upland forest soils had lower Hg concentrations than riparian soils in all three watersheds. Decreasing Hg concentrations with depth suggest upland forest soils are sequestering atmospheric deposition. In the Merrimack River Watershed, riparian forest soils had the highest Hg concentrations, which increased with depth, suggesting riparian soils are sequestering historical Hg pollution. Further, river water samples and suspended sediments from April 2020 to Nov 2020 were collected at least monthly. Dissolved Hg concentrations were below the limit of quantification (< 0.05 µg/L). Suspended sediment Hg concentrations, collected via sediment traps, were significantly higher in the Merrimack River than Connecticut and Thames Rivers. When normalized per square kilometer of watershed area, the Merrimack River had comparable Hg exports via suspended sediments as the much larger Connecticut River. These results show that upland forests are important for modern Hg sequestration while riparian forests are key for historical Hg pollution. Further, the higher Hg concentrations in the suspended sediments of the Merrimack River have increased the total Hg export to approach rates of the Connecticut River, which is nearly three times larger. However, adequately assessing transport rates from upland forest soils to surface waters and dissolved Hg concentrations are needed to quantify the rate of Hg transport.