Abstract
Triplicate porewater depth-profiles of pH and concentrations of total Hg (HgT), methylmercury (MeHg), Fe, Mn, sulfate, total sulfide, total zero-valent sulfur, organic C and major ions were determined at two sampling dates in a perennially oxygenated basin and a seasonally anoxic basin from Lake Tantaré, a Canadian Shield lake. The vertical distribution of HgT, MeHg, acid volatile sulfide, total S, Fe, Mn, Al and organic C were also determined in dated sediment cores from the same lake basins and from the deepest site of two other lakes, one also located in the Canadian Shield and the other in the Northeastern part of the Appalachian Mountains. Application of a one-dimensional transport-reaction equation to the dissolved HgT and MeHg profiles constrains the depth intervals (zones) where these species are produced or consumed in the sedimentary column and yields estimates of net reaction rates of HgT or MeHg in each of the zones as well as their fluxes at the sediment–water interface.Dissolved HgT and MeHg diffused from the overlying water into the sediments, except for MeHg at one of the sampling dates in the perennially oxygenated basin. About 97% and 50% of the MeHg flux to the sediments is presently deposited with settling particles in the perennially oxygenated and seasonally anoxic basins, respectively. Removal of porewater HgT and MeHg occurred at all dates and sampling sites. Comparison of the consumption zones of porewater HgT and MeHg with the profiles of ancillary parameters, coupled with thermodynamic calculations, suggest that pure Hg mineral phases do not form in the sediments, that HgT and MeHg adsorption onto authigenic Fe oxyhydroxides occurs in minor proportions, and that the association of HgT and MeHg to Fe sulfide phases or sulfidized organic matter is possible. Assuming that the net consumption of MeHg in the porewaters was essentially due to demethylation, an apparent first-order rate constant for MeHg demethylation of 0.04–0.8d−1 was estimated. Production of porewater MeHg occurred only in the perennially oxygenated basin, at sediment depths where SO4 was consumed. Assuming that the net production of porewater MeHg was essentially due to methylation, an apparent first-order rate constant for Hg methylation ranging between 0.006d−1 and 0.1d−1 was calculated. These field-derived Hg methylation and MeHg demethylation rate constant values are within the range of those derived from Hg-spiked experiments. We also show that the post-depositional redistribution of total Hg during the early stages of sediment diagenesis is minor and that the solid-phase HgT record can be used to reconstruct the evolution of the anthropogenic HgT deposition.
Highlights
The increase in atmospheric deposition of mercury as a result of industrialization has stimulated the in situ production of methylmercury (MeHg) in aquatic systems and the incorporation of this neurotoxic compound into food chains (Evers et al, 1998; Hammerschmidt and Fitzgerald, 2006b; Munthe et al, 2007)
MeHg with the profiles of ancillary parameters, coupled with thermodynamic calculations, suggest that pure Hg mineral phases do not form in the sediments, that HgT and MeHg adsorption onto authigenic Fe oxyhydroxides occurs in minor proportions, and that the association of HgT and
We provide evidence that pure Hg mineral phases do not form in the sediments and that HgT and MeHg adsorption onto authigenic Fe-ox is of minor 903 importance; the assessment of HgT and MeHg association with Fe sulfide phases or sulfidized organic matter would require additional field and laboratory measurements
Summary
The increase in atmospheric deposition of mercury as a result of industrialization has stimulated the in situ production of methylmercury (MeHg) in aquatic systems and the incorporation of this neurotoxic compound into food chains (Evers et al, 1998; Hammerschmidt and Fitzgerald, 2006b; Munthe et al, 2007). This study is in line with this general objective; it focuses on Hg dynamics in recent sediment deposits which are considered to be a key location for MeHg formation (Krabbenhoft et al, 1998; Kainz et al, 2003; Hammerschmidt et al, 2006). We submit that measuring HgT, MeHg and ancillary parameters in sediments and porewaters, combined with thermodynamic and kinetic modeling, provides an alternative to constrain in situ HgT and MeHg mobility, physico-chemical processes involving HgT and MeHg and their reaction kinetics. Porewaters are sensitive indicators of reactions that occur in the solid phase, most studies reporting porewater profiles of HgT and
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