CYCLING OF MERCURY ACROSS THE SEDIMENT-WATER INTERFACE IN SEEPAGE LAKES

The magnitude and direction of Hg fluxes across the sediment-water interface were estimated by groundwater, dry bulk sediment, sediment pore water, sediment trap, and water-column analyses in two northern Wisconsin seepage lakes. Little Rock Lake (Treatment Basin) received no groundwater discharge during the study period (1988-1990), and Pallette Lake received continuous groundwater discharge. In Little Rock Lake, settling of particulate matter accounted for the major Hg delivery mechanism to the sediment-water interface. Upward diffusion of Hg from sediment pore waters below 2-4-cm sediment depth teas apparently a minor source during summer stratification. Time-series comparisons suggested that the observed buildup of Hg in the hypolimnion of little Rock Lake was attributable to dissolution and diffusion of Hg from recently fallen particulate matter close to the sediment-water interface. Groundwater inflow represented an important source of new Hg, and groundwater outflow accounted for significant removal of Hg from Pallette Lake. Equilibrium speciation calculations revealed that association of Hg with organic matter may control solubility in well-oxygenated waters, whereas in anoxic environments sulfur (polysulfide and bisulfide) complexation governs dissolved total Hg levels.