Article

Mercury Cycling in Stream Ecosystems. 1. Water Column Chemistry and Transport

U.S. Geological Survey, 2280 Woodale Drive, Mounds View, Minnesota 55112, USA.
Environmental Science and Technology (Impact Factor: 5.48). 05/2009; 43(8):2720-5. DOI: 10.1021/es802694n
Source: PubMed

ABSTRACT This paper is freely available via Open Access: http://pubs.acs.org/doi/abs/10.1021/es802694n

We studied total mercury (THg) and methylmercury (MeHg) in eight streams, located in Oregon, Wisconsin, and Florida, that span large ranges in climate, landscape characteristics, atmospheric Hg deposition, and water chemistry. While atmospheric deposition was the source of Hg at each site, basin characteristics appeared to mediate this source by providing controls on methylation and fluvial THg and MeHg transport. Instantaneous concentrations of filtered total mercury (FTHg) and filtered methylmercury (FMeHg) exhibited strong positive correlations with both dissolved organic carbon (DOC) concentrations and streamflow for most streams, whereas mean FTHg and FMeHg concentrations were correlated with wetland density of the basins. For all streams combined, whole water concentrations (sum of filtered and particulate forms) of THg and MeHg correlated strongly with DOC and suspended sediment concentrations in the water column.

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    • "deposition, mobilization from watersheds and groundwater discharges (Johannesson and Neumann, 2013). Several authors suggested that mobilized Hg leaves the watershed as Hg(II) complexed to dissolved organic carbon (DOC) compounds through overland flow and, to a lesser extent, groundwater flow (Krabbenhoft et al., 1995; Babiarz et al., 2003; Brigham et al., 2009; Bradley et al., 2012; Lamborg et al., 2013). Although several investigations have studied Hg cycling in continental aquifers (Dooley, 1992; Krabbenhoft and Babiarz, 1992; Krabbenhoft et al., 1995; Bollen et al., 2008; Bagnato et al., 2009; Bradley et al., 2012; Johannesson and Neumann, 2013; Lamborg et al., 2013; Guédron et al., 2014) our knowledge related to Hg reactivity, mobility and speciation in groundwater remains limited. "
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    ABSTRACT: Mercury (Hg) mobility and speciation in subsurface aquifers is directly linked to its surrounding geochemical and microbial environment. The role of bacteria on Hg speciation (i.e., methylation, demethylation and reduction) is well documented, however little data is available on their impact on Hg mobility. The aim of this study was to test if (i) Hg mobility is due to either direct iron oxide reduction by iron reducing bacteria (IRB) or indirect iron reduction by sulfide produced by sulfate reducing bacteria (SRB), and (ii) to investigate its subsequent fate and speciation. Experiments were carried out in an original column setup combining geochemical and microbiological approaches that mimic an aquifer including an interface of iron-rich and iron depleted zones. Two identical glass columns containing iron oxides spiked with Hg(II) were submitted to (i) direct iron reduction by IRB and (ii) to indirect iron reduction by sulfides produced by SRB. Results show that in both columns Hg was leached and methylated during the height of bacterial activity. In the column where IRB are dominant, Hg methylation and leaching from the column was directly correlated to bacterial iron reduction (i.e., Fe(II) release). In opposition, when SRB are dominant, produced sulfide induced indirect iron oxide reduction and rapid adsorption of leached Hg (or produced methylmercury) on neoformed iron sulfides (e.g., Mackinawite) or its precipitation as HgS. At the end of the SRB column experiment, when iron-oxide reduction was complete, filtered Hg and Fe concentrations increased at the outlet suggesting a leaching of Hg bound to FeS colloids that may be a dominant mechanism of Hg transport in aquifer environments. These experimental results highlight different biogeochemical mechanisms that can occur in stratified sub-surface aquifers where bacterial activities play a major role on Hg mobility and changes in speciation. Copyright © 2015 Elsevier B.V. All rights reserved.
    Journal of Contaminant Hydrology 08/2015; 180:56-68. DOI:10.1016/j.jconhyd.2015.08.001 · 2.70 Impact Factor
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    • "The U.S. Geological Survey (USGS) has conducted extensive studies of national and regional fish-tissue contaminant data by combining results of several separate targeted studies conducted over time, analyzing the data and characterizing contaminant levels on the basis of region and watershed type, including propensity for methylation and fish characteristics (Schmitt, 2002; Scudder et al., 2009; Chalmers et al., 2011). These studies have led to identification of patterns and mechanisms of Hg accumulation in fish including the propensity for methylation to occur in watersheds with abundant wetlands (Brigham et al., 2009; Scudder et al., 2009). USGS continues to emphasize locally-focused studies which are national in their distribution (Brigham et al., 2014; Feaster et al., 2014; Chalmers et al., 2014) and explore causative mechanisms for the observed variations in Hg concentrations in a variety of media and settings. "
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    • "Interactions between Hg and organic matter influence the transport and bioavailability of Hg in riverine and estuarine environments (Laurier et al., 2003; Choe et al., 2003; Conaway et al., 2003; Han et al., 2007; Lee et al., 2011). Dissolved Hg distribution across watersheds is explained by dissolved organic carbon (DOC) distribution in a number of river systems (Peckenham et al., 2003; Schuster et al., 2008; Brigham et al., 2009). In estuarine systems, complexation of Hg with dissolved organic matter , coupled with colloidal coagulation, is reported to influence estuarine mixing behavior (Stordal et al., 1996; Choe et al., 2003; Lee et al., 2011) and bioavailability of Hg (Pan and Wang, 2004; Zhong and Wang, 2009). "
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