The chemical form in which mercury exists in water is its speciation. Mercury species are distinguishable from one another, stoichiometrically, and with respect to their bioavailability. In addition, mercury can exist in different phases, for example, as gaseous species, as solid phases, or in adsorbed states. The concept of chemical speciation is central to the equilibrium, kinetic and biogeochemical aspects of mercury in the Everglades ecosystem.

We have used the computer ionic speciation model WHAM (Windermere Humic Aqueous Model) to characterize the mercury-organic species present in Everglades surface water. WHAM focuses on humic and fulvic acid-metal interactions. These calculations indicate that the major mercury species in solution changes from uncharged chloro- and hydroxy-complexes to DOC-bound complexes in the presence of high DOC concentrations (that is, greater than about 10 milligrams per liter). Inorganic speciation calculated using WHAM has been compared to speciation determined with PHREEQC (pH-redox-equilibrium-equations) with good agreement. WHAM has also been satisfactorily tested using laboratory measurements of calcium ion binding to a soil fulvic acid. The presence of sulfide and sulfur-containing ligands shifts mercury speciation to mercury-sulfur and mercury-organosulfur complexes. Mercury sulfide solid phases appear to be supersaturated in some Everglades' surface and pore waters. Analysis of DOC and sulfide competition for mercury binding is in progress as well as measurement of Everglades fulvic acid mercury interaction, in cooperation with the Department of Civil Engineering, University of Colorado at Boulder.

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