Mercury sources and cycling in the Connecticut River and Long Island Sound

Abstract

Long Island Sound (LIS) is one of the “Great Waters” of the United States, and one of the world's most productive and utilized water bodies. It is a classic “urban sea” with about 16 million people living and working near its shores. Commercial, industrial, and recreational uses of LIS abound, as do current and historic pollution from heavy metals, nutrients, and organic chemicals. Mercury cycling in such anthropogenically impacted near shore and fisheries-rich systems is poorly understood. Here, we are presenting a well-constrained assessment of the status and impact from contamination on the behavior and fate of Hg in LIS. Mercury (Hg), as with many substances in the Sound, is affected by localized discharges (e.g., rivers, water pollution control facilities [WPCFs]), and direct and indirect (i.e., watershed leaching) inputs from airborne transport and deposition of Hg. Agreement in total Hg (14–21 pM) and monomethylmercury (MMHg; 0.5–0.9 pM) levels (1995–1997) among the major rivers (Connecticut [CTR], Housatonic, and Thames) entering LIS suggests that deposition and subsequent leaching of atmospherically derived Hg from the respective watersheds accounts for much of the river fluxes. Although CT WPCF effluents contain varying amounts of HgT and MMHg, effluent from major CT WPCFs discharging to the CTR (Hartford and Mattabassett-Cromwell) contain similar amounts of total Hg (60–92 pM) and MMHg (1.5–2.1 pM), which are elevated compared to river concentrations. However, only a minor portion (2% of HgT flux) of Hg in the CTR is derived from CT WPCFs. Similarity in the atmospheric Hg flux (wet+dry deposition; 40±10 nmol HgT m −2 year −1) among sampling sites (Groton, Madison, Milford, CT; Rye, NY) points to the importance of long-range transport and deposition of Hg to this coastal region. The principal sources of total Hg (1160 mol year −1) to LIS are rivers entering from CT (∼680 mol year −1; 59% of total inputs), the East River (∼290 mol year −1; 25%; 110 mol year −1 from New York City WPCFs), WPCFs (∼60 mol year −1; 5%), and direct atmospheric deposition (∼130 mol year −1; 11%). Although direct atmospheric Hg deposition to LIS is small, modest leaching (25–30% watershed delivery; 410–490 mol Hg year −1) of the Sound-wide atmospheric deposition normalized to its watershed could account for 60–75% of river Hg inputs. Principal external sources of MMHg to LIS (28 mol year −1) include rivers entering from CT (∼15 mol year −1), the East River (∼6.0 mol year −1), and direct atmospheric deposition (∼3.5 mol year −1); MMHg concentrations are typically only 2% of HgT. In situ sedimentary production is predicted to be the major source of MMHg in LIS, with an estimated sediment–water flux of 55±20 mol MMHg year −1 (70% of total inputs).