HR: 15:10h
AN: B13F-06    [Abstracts]
TI: The Influences of Dissolved Organic Matter on Mercury Biogeochemistry in Mesocosm Experiments in the Florida Everglades
AU: * Aiken, G R
EM: graiken@usgs.gov
AF: US Geological Survey, 3215 Marine Street, Boulder, CO 80303,
AU: Gilmour, C A
EM: gilmour@si.edu
AF: Smithsonian Environmental Research Center, PO Box 28 647 Contees Wharf Road, Edgewater, MD 21037,
AU: Krabbenhoft, D P
EM: dpkrabbe@usgs.gov
AF: US Geological Survey, 8505 Research Way, Middleton, WI 53562-3581,
AU: Orem, W
EM: borem@usgs.gov
AF: US Geological Survey, 12201 Sunrise Valley Drive, Reston, VA 20192,
AB: Interactions of mercury (Hg) with dissolved organic matter (DOM) play important roles in controlling reactivity, bioavailability and transport of Hg in aquatic systems. Laboratory experiments using a variety of organic matter isolates from surface waters in the Florida Everglades indicate that DOM binds Hg very strongly and is the dominant ligand for Hg in the absence of sulfide. These experiments have also shown that the presence of DOM influences the geochemical behavior of cinnabar (HgS) through the stabilization of nanocolloidal HgS resulting in relatively high Hg concentrations under supersaturated conditions with respect to HgS, a common condition in waters containing measurable sulfide concentrations. In this paper, the results of in-situ mesocosm experiments designed to directly measure the effects of DOM -Hg interactions on Hg biogeochemistry will be described. In these experiments, mesocosms (wetland enclosures), located in the central Everglades region of Water Conservation Area 3A (WCA 3A15), were amended with isotopically enriched Hg (²⁰⁰Hg, ²⁰²Hg), sulfate (SO₄⁼) and the hydrophobic organic acid (HPOA) fraction of DOM from a site (F1) in the eutrophic northern Everglades. The use of stable isotope spikes in these studies allowed us to examine the delivery of Hg to surface soils (which are the predominant zones of methylation); partitioning of Hg and MeHg among phases (which impacts bioavailability); net MeHg production; loss of Hg and MeHg through photodemethylation, reduction and volatization; and bioaccumulation. The F1 HPOA isolate, obtained using XAD resins, was more aromatic, had a greater specific ultra-violet absorbance and had previously been shown to be more reactive with Hg than the DOM present at the 3A15 site. The F1 HPOA isolate formed strong DOM-Hg complexes (K_DOM') = 10^23.2 L kg^-1 at pH = 7.0 and I = 0.1) and effectively inhibited the precipitation of HgS in laboratory experiments. Select mesocosms were amended with either F1-HPOA or SO₄⁼ resulting in a range of concentrations for each constituent. For the DOM amended mesocosms, DOC concentrations increased from 50-100% and the overall SUVA increased from 2.9 to 3.7 L mg C^-1 m^-1 relative to control mesocosms, indicating that both the concentration and overall reactivity of the DOM in the amended mesocosms had been altered substantially. In these mesocosms, the concentrations of both ambient and isotopically enriched dissolved Hg increased significantly compared to controls. Greater concentrations of both dissolved ambient and labeled methylmercury were also observed in the DOM amended mesocosms indicating that the added DOM increased Hg bioavailabilty of both Hg pools for methylation. In addition, DOM shielded Hg and MeHg from photodemethylation and volatilization, however, it inhibited subsequent MeHg bioaccumulation. Overall, the addition of DOM resulted in increased concentrations of labeled methylmercury comparable to those measured in mesocosms amended with SO₄⁼ suggesting that DOM is an important constituent influencing the methylation of Hg. This effect is likely due to increased concentrations of dissolved Hg in the DOM amended mesocosms.
DE: 0409 Bioavailability: chemical speciation and complexation
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0448 Geomicrobiology
DE: 0489 Trace element cycling (4875)
DE: 0496 Water quality
SC: Biogeosciences [B]
MN: 2007 Fall Meeting