title: Influence of porewater sulfide on methylmercury production and partitioning in sulfate-impacted lake sediments.
abstract: In low-sulfate and sulfate-limited freshwater sediments, sulfate loading increases the production of methylmercury (MeHg), a potent and bioaccumulative neurotoxin. Sulfate loading to anoxic sediments leads to sulfide production that can inhibit mercury methylation, but this has not been commonly observed in freshwater lakes and wetlands. In this study, sediments were collected from sulfate-impacted, neutral pH, surface water bodies located downstream from ongoing and historic mining activities to examine how chronic sulfate loading produces porewater sulfide, and influences MeHg production and transport. Sediments were collected over two years, during several seasons from lakes with a wide range of overlying water sulfate concentration. Samples were characterized for in-situ solid phase and porewater MeHg, Hg methylation potentials via incubations with enriched stable Hg isotopes, and sulfur, carbon, and iron content and speciation. Porewater sulfide reflected historic sulfur loading and was strongly related to the extractable iron content of sediment. Overall, methylation potentials were consistent with the accumulation of MeHg on the solid phase, but both methylation potentials and MeHg were significantly lower at chronically sulfate-impacted sites with a low solid-phase Fe:S ratio. At these heavily sulfate-impacted sites that also contained elevated porewater sulfide, both MeHg production and partitioning are influenced: Hg methylation potentials and sediment MeHg concentrations are lower, but occasionally porewater MeHg concentrations in sediment are elevated, particularly in the spring. The dual role of sulfide as a ligand for inorganic mercury (decreasing bioavailability) and methylmercury (increasing partitioning into porewater) means that elucidating the role of iron and sulfur loads as they define porewater sulfide is key to understanding sulfate's influence on MeHg production and partitioning in sulfate-impacted freshwater sediment.
authors.affiliation: Water Resources Science Program, University of Minnesota, United States.
authors.forename: Logan T
authors.initials: LT
authors.lastname: Bailey
authors.affiliation: Department of Physical and Environmental Sciences, University of Toronto Scarborough, Canada.
authors.forename: Carl P J
authors.initials: CPJ
authors.lastname: Mitchell
authors.affiliation: St. Croix Watershed Research Station, Science Museum of Minnesota, United States.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Engstrom
authors.affiliation: Minnesota Department of Natural Resources, Division of Lands and Minerals, United States.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Berndt
authors.affiliation: St. Croix Watershed Research Station, Science Museum of Minnesota, United States.
authors.forename: Jill K
authors.initials: JK
authors.lastname: Coleman Wasik
authors.affiliation: Water Resources Science Program, University of Minnesota, United States; Department of Civil Engineering, University of Minnesota Duluth, United States. Electronic address: nwjohnso@d.umn.edu.
authors.forename: Nathan W
authors.initials: NW
authors.lastname: Johnson
keywords: Bioavailability; Iron loading; Methylmercury; Sulfate loading; Sulfide