Cycling of volatile mercury in temperate lakes

The cycling of dissolved gaseous Hg (DGM) has been examined in our studies of the troposphericHg cycle, air-water exchange and their importance to the biogeochemical behavior and fate of Hg in temperate lakes. Five seepage lakes in northcentral Wisconsin, ranging in pH from 4.7 to 7.2, have been studied under a variety of limnological conditions which included the following seasonal periods: summer (peak stratification), fall (following turnover) and late winter (under ice). Analytically, DGM was determined after purging lake water with argon and collecting the volatile Hg fraction on gold coated sand. The Hg collections were analysed by pyrolysis in a two-stage Au amalgamation gas train with detection by atomic fluorescence spectroscopy (AFS). In addition, chemical speciation of the volatile fraction has been achieved by trapping on a nondestructive substrate followed by gas chromatographic separation and AFS detection. The DGM consists principally of elemental Hg (Hg^o) under all sampling conditions, with no significant contribution from volatile organic Hg species (detection limit of 3 femtomolar). Atmospheric gaseous Hg, which also consists principally of Hg^o, was measured and the air-water partitioning determined. In general, the lake waters have been supersaturated with Hg^o relative to the atmosphere. Supersaturation was greater in the summer, ranging from ca. 1.4 to 12 times (x) the saturation concentration. During the other sampling periods, Hg^o ranged from saturation to ca. 7x the equilibrium concentration. The flux of Hg from the lakes due to gas evasion is significant and is estimated at approximately 10% of the annual atmospheric input of Hg to the lakes. An apparent relationship between Hg^o and pH has been observed with lakes of lower pH having smaller Hg^o concentrations.