Controls on Methylation of Groundwater Hg(II) in Hyporheic Zones of Wetlands

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7/1/2007 - 6/30/2009

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  • Martin Shafer, UW-Madison
  • Christopher Babiarz, UW-Madison
  • David Armstron, UW-Madison
  • Eric Roden, UW-Madison

Our proposal addresses the groundwater-related problem of methyl mercury (MeHg) formation in hyporheic zones. Groundwater transport of inorganic mercury, Hg(II), to hyporheic zones leads to bacterial transformation to MeHg and subsequent accumulation of the highly toxic MeHg in aquatic foodwebs. The Objective is to determine the main factors controlling the bioavailability of inorganic Hg(II) for production of MeHg in wetland hyporheic zones. Formation of MeHg, mainly by sulfate-reducing bacteria (SRB), is expected to depend on the activity of SRB and the concentration and speciation of Hg(II). Our Project Plan is to use a combination of field measurements and laboratory experiments to resolve the main factors controlling production of MeHg in hyporheic zones. In Field research at hyporheic sites in the Allequash Creek wetland, concurrent measurements of Hg(II) methylation rates, sediment pore-water chemistry, and microbial activity will be used to test the hypothesis that rates of Hg(II) methylation are dependent on both concentration and speciation of Hg(II) and microbial activity in hyporheic zones. In Laboratory Microcosm Experiments, sediments from hyporheic zones will be incubated in microcosms under different conditions to probe specific drivers of bioavailability. Response measures will include methylation potential and other indicators of Hg(II) bioavailability. Drivers to be varied are total concentrations of Hg(II), sulfide, NOM, pH, dominant electron acceptor for microbial respiration (Fe(III) vs. sulfate), and bacterial activity. Methodology will include total Hg and MeHg by CVAFS; methylation potential by production of MeHg from isotope-enriched Hg(II); “potential bioavailability” by measurements of by Hg(II) uptake by sensor bacteria and by reducibility by Sn(II); Hg(II) speciation by modeling and experimental measurements; and microbial activity by 14C-acetate respiration.

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