To elucidate the fate of mercury (Hg) and methylmercury (MMHg) in estuaries we have been studying the biogeochemical cycling of Hg in the Chesapeake Bay and its tributaries. Measurements of total Hg and mercury speciation (MMHg and dissolved gaseous Hg (DGHg)) have been made in the surface waters of the Chesapeake Bay system, including sampling transects into Baltimore Harbor, a region where sediment Hg concentrations can exceed 5 nmol g-1 and sediment MMHg concentrations approach 50 pmol g-1. Surface water Hg concentrations are typical of an impacted urban estuary that has a high suspended sediment load (SPM; typically > 5 mg l-1), and for the upper Bay, where total Hg is generally 20 pM or less, particulate Hg is the dominant fraction. Total Hg concentrations tend to decrease down-estuary, and this is also true for the sub-estuaries (Potomac and Patuxent Rivers). This decrease coincides with a decrease in SPM, but urban inputs are also higher up-estuary. Measurement of total Hg and MMHg in the anoxic bottom waters that develop seasonally in the harbor demonstrate the increased concentration of Hg, and especially MMHg, under low oxygen conditions, and the most likely source is the release of these species from sediments. Both intensive studies and long-term measurements show that the concentration of DGHg is relatively low in this system (typically < 0.2 pM) and DGHg is a small fraction of the total Hg. Based on the information available, a preliminary mass balance budget is constructed for the Chesapeake Bay. Direct atmospheric deposition contributes a substantial portion of the Hg to the Bay while sedimentary removal is the principal sink. Overall, 29% of the riverine and atmospheric inputs to the Bay is exported to the coastal ocean. Thus, the Bay is an efficient trap for Hg. The Bay is, however, a net source of MMHg suggesting that significant methylation of Hg occurs within the estuary. On a watershed scale, only about 5% of the total atmospheric deposition to the watershed is exported to the coastal ocean.
- Chesapeake Bay