Mercury in the Chesapeake Bay

To elucidate the fate of mercury (Hg) and methylmercury (MMHg) in estuaries we have been studying the biogeochemical cycling of Hg in the Chesapeake B ay and iri. tributaries. Measurements of total HD and mercury speciation (M MHg and dissolved gaseous Hg (DGHg)) have been made in the surface waters o f the Chesapeake Bay system, including sampling transects into Baltimore Ha rbor, a region where sediment Hg concentrations can exceed 5 nmol g(-1) and sediment MMHg concentrations approach 50 pmol g(-1). Surface water Hg conc entrations are typical of an impacted urban estuary that has a high suspend ed sediment load (SPM; typically > 5 mg l(-1)), and for the upper Bay, wher e total Hg is generally 20 pM or less, particulate Hg is the dominant fract ion, Total Hg concentrations tend to decrease down-estuary, and this is als o true for the sub-estuaries (Potomac and Patuxent Rivers). This decrease c oincides with a decrease in SPM, but urban inputs are also higher up-estuar y. Measurement of total Hg and MMHg in the anoxic bottom waters that develo p seasonally in the harbor demonstrate the increased concentration of Hg, a nd 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 l ow in this system (typically < 0.2 phl) and DGHg is a small fraction of the total Hg. Based on the information available, a preliminary mass balance b udget is constructed for the Chesapeake Bay, Direct atmospheric deposition contributes a substantial portion of the Hg to the Bay while sedimentary re moval is the principal sink. Overall, 29% of the riverine and atmospheric i nputs to the Bay is exported to the coastal ocean. Thus, the Bay is an effi cient trap for Hg. The Bay is, however, a net source of MMHg suggesting tha t significant methylation of Hg occurs within the estuary. On a watershed s cale, only about 5% of the total atmospheric deposition to the watershed is exported to the coastal ocean. (C) 1999 Elsevier Science B.V. All rights r eserved.