Mercury in Temperate Forest Soils and Suspended Sediments in the Connecticut River, Merrimack River, and Thames River Watersheds, USA

Quantifying the sequestration in forest soils and watershed transport of Hg is key to reducing exposure of aquatic food chains to the toxic pollutant. We explored if forest soils, proximity in human developments, or watershed characteristics could shed light on sediment driven Hg transport in the Connecticut River, Merrimack River, and Thames River watersheds, in the northeastern USA. We sampled 48 upland forest soils and 32 riparian forest soils and collected bottled and trapped suspended sediments from April 2020 to November 2020 across the three watersheds. Forest soil Hg concentrations were greater in riparian forest soils (median = 153 ng g−1) than in upland forest soils (median = 71 ng g−1) across all three watersheds and developed sites near urbanization had higher Hg concentrations in the Merrimack River watershed (median = 407 ± 119 ng g−1). The Connecticut River had the highest suspended sediment Hg export rate but when normalized by total area of the watershed, the Merrimack River had 0.19 kg km−2 month−1 while the Connecticut River had 0.13 kg km−2 month−1 and 0.04 km−2 for the Thames River. Our findings suggest that riparian forest soils sequester while suspended sediments transport historical Hg pollution within the Merrimack River Watershed.