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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"The speciation of Hg(II) recently deposited in organic-rich soils is dominated by complexation with thiol groups in soil organic matter (SOM) (Skyllberg et al., 2006; Nagy et al., 2011). Similarly, complexation of Hg(II) with thiol groups in dissolved organic matter (DOM) (Haitzer et al., 2002) in soil pore waters is thought to be responsible for the co-mobilization of mercury with DOM from riparian soils to streams (Mierle and Ingram, 1991; Shanley et al., 2008; Brigham et al., 2009; Dittman et al., 2009, 2010). In particular, the hydrophobic organic acid fraction of DOM, compared to other DOM fractions, contains greater thiol group content (Haitzer et al., 2003) and has been observed to correlate positively with the filter-passing total mercury (Hg T ) concentration in streams (Shanley et al., 2008; Dittman et al., 2009, 2010). "
[Show abstract][Hide abstract] ABSTRACT: Riparian soils are an important environment in the transport of mercury in rivers and wetlands, but the biogeochemical factors controlling mercury dynamics under transient redox conditions in these soils are not well understood. Mercury release and transformations in the Oa and underlying A horizons of a contaminated riparian soil were characterized in microcosms and an intact soil core under saturation conditions. Pore water dynamics of total mercury (HgT), methylmercury (MeHg), and dissolved gaseous mercury (Hg0(aq)) along with selected anions, major elements, and trace metals were characterized across redox transitions during 36 d of flooding in microcosms. Next, HgT dynamics were characterized over successive flooding (17 d), drying (28 d), and flooding (36 d) periods in the intact core. The observed mercury dynamics exhibit depth and temporal variability. At the onset of flooding in microcosms (1–3 d), mercury in the Oa horizon soil, present as a combination of ionic mercury (Hg(II)) bound to thiol groups in the soil organic matter (SOM) and nanoparticulate metacinnabar (β-HgS), was mobilized with organic matter of high molecular weight. Subsequently, under anoxic conditions, pore water HgT declined coincident with sulfate (3–11 d) and the proportion of nanoparticulate β-HgS in the Oa horizon soil increased slightly. Redox oscillations in the intact Oa horizon soil exhausted the mobile mercury pool associated with organic matter. In contrast, mercury in the A horizon soil, present predominantly as nanoparticulate β-HgS, was mobilized primarily as Hg0(aq) under strongly reducing conditions (5–18 d). The concentration of Hg0(aq) under dark reducing conditions correlated positively with byproducts of dissimilatory metal reduction (∑(Fe,Mn)). Mercury dynamics in intact A horizon soil were consistent over two periods of flooding, indicating that nanoparticulate β-HgS was an accessible pool of mobile mercury over recurrent reducing conditions. The concentration of MeHg increased with flooding time in both the Oa and A horizon pore waters. Temporal changes in pore water constituents (iron, manganese, sulfate, inorganic carbon, headspace methane) all implicate microbial control of redox transitions. The mobilization of mercury in multiple forms, including HgT associated with organic matter, MeHg, and Hg0(aq), to pore waters during periodic soil flooding may contribute to mercury releases to adjacent surface waters and the recycling of the legacy mercury to the atmosphere.
Full-text · Article · Dec 2015 · Geochimica et Cosmochimica Acta
"The predominant source of MeHg to many streams is production in wetlands and subsequent transfer during times of high hydrological connectivity (Brigham et al. 2009). While it is well known that beaver-induced channel alterations change the way materials flow through streams (Naiman et al. 1986, 1988), only recently has the enhancement of landscape hydrological connectivity by beaver damming been considered in the Hg literature. "
[Show abstract][Hide abstract] ABSTRACT: Beavers (Castor spp.) are ecosystem engineers and important modifiers of freshwater ecosystems. They create impoundments that flood the surrounding landscape and modify the flow of materials through streams, thus potentially increasing nutrients, productivity and the availability of toxic methyl mercury (MeHg) to downstream food webs. Here we quantify food web-available MeHg in water, periphyton, and invertebrates collected from 15 streams up- and down-stream from beaver impoundments in the Rocky Mountain foothills of Western Canada. While nutrients, algal biomass, and total invertebrate standing stock were not significantly elevated below ponds, MeHg concentrations (average increase of 1.73) and percent of total Hg that was MeHg (average increase of 1.33) showed a trend of higher values in all compartments downstream and the difference was significant in predatory invertebrates. This suggests that beaver impoundments can increase the availability and subsequent uptake of MeHg by basal food web organisms even if their immediate influence on nutrients and resources is limited. As beaver populations continue to rebound, more research is needed to fully characterise the effects of beavers on nutrient and contaminant cycling under different biogeochemical conditions.
"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. "
[Show abstract][Hide abstract] ABSTRACT: The U.S. EPA conducted a national statistical survey of fish fillet tissue with a sample size of 541 sites on boatable rivers =>5th order in 2008-2009. This is the first such study of mercury (Hg) in fish tissue from river sites focused on potential impacts to human health from fish consumption to also address wildlife impacts. Sample sites were identified as being urban or non-urban. All sample mercury concentrations were above the 3.33ugkg(-1) (ppb) quantitation limit, and an estimated 25.4% (±4.4%) of the 51663 river miles assessed exceeded the U.S. EPA 300ugkg(-1) fish-tissue based water quality criterion for mercury, representing 13144±181.8 river miles. Estimates of river miles exceeding comparable aquatic life thresholds (translated from fillet concentrations to whole fish equivalents) in avian species were similar to the number of river miles exceeding the human health threshold, whereas some mammalian species were more at risk than human from lower mercury concentrations. A comparison of means from the non-urban and urban data and among three ecoregions did not indicate a statistically significant difference in fish tissue Hg concentrations at p<0.05.
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