Specific ultra-violet absorbance as an indicator of mercury sources in an Adirondack River basin
Douglas A. Burns
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George R. Aiken
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Paul M. Bradley
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Celeste A. Journey
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Jakob Schelker
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P. M. Bradley C. A. Journey U.S. Geological Survey,
Columbia, SC, USA
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G. R. Aiken U.S. Geological Survey, Boulder,
CO, USA
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D. A. Burns (&) U.S. Geological Survey, 425 Jordan Rd., Troy,
NY 12180, USA
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J. Schelker Swedish University of Agricultural Sciences
, Umea,
Sweden
The Adirondack region of New York has been identified as a hot spot where high methylmercury concentrations are found in surface waters and biota, yet mercury (Hg) concentrations vary widely in this region. We collected stream and groundwater samples for Hg and organic carbon analyses across the upper Hudson River, a 493 km2 basin in the central Adirondacks to evaluate and model the sources of variation in filtered total Hg (FTHg) concentrations. Variability in FTHg concentrations during the growing seasons (May-Oct) of 2007-2009 in Fishing Brook, a 66-km2 sub-basin, was better explained by specific ultra-violet absorbance at 254 nm (SUVA254), a measure of organic carbon aromaticity, than by dissolved organic carbon (DOC) concentrations, a commonly used Hg indicator. SUVA254 was a stronger predictor of FTHg concentrations during the growing season than during the dormant season. Multiple linear regression models that included SUVA254 values and DOC concentrations could explain 75 % of the variation in FTHg concentrations on an annual basis and 84 % during the growing season. A multiple linear regression landscape modeling approach applied to 27 synoptic sites across the upper Hudson basin found that higher SUVA254 values are associated with gentler slopes, and greater riparian area, and lower SUVA254 values are associated with an increasing influence of open water. We hypothesize that the strong Hg-SUVA254 relation in this basin reflects distinct patterns of FTHg and SUVA254 that are characteristic of source areas that control the mobilization of Hg to surface waters, and that the seasonal influence of these source areas varies in this heterogeneous basin landscape.
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Organic matter regulates mercury (Hg) cycling in the
environment through numerous mechanisms
including: ligand binding, effects on dissolution and
precipitation, by serving as a microbial substrate, through its
central role in redox processes, and as an indicator of
hydrologic conditions (Gerbig et al. 2012). The strong
association of Hg and organic matter in surface waters
has long been recognized (Lindberg and Harriss 1974;
Mierle and Ingram 1991), and numerous studies of Hg
in streams and lakes of mid- and northern latitudes of
the northern hemisphere have generally shown strong
correlations between dissolved Hg concentrations and
dissolved organic carbon (DOC) concentrations, and
between particulate Hg concentrations and particulate
organic carbon concentrations (POC) (Grigal 2002;
Yin and Balogh 2002; Dennis et al. 2005; Brigham
et al. 2009; Riscassi and Scanlon 2011). The
association of Hg with organic matter is sufficiently strong in
many surface waters, that measures such as DOC or
absorbance at 254 nm (UV254) have been suggested as
surrogates for Hg concentrations, because Hg analyses
are expensive and may be unstable in automated water
samplers (Dittman et al. 2009).
Interactions between Hg and organic matter are
numerous and diverse, reflecting the complexity of
natural organic matter (Ravichandran 2004; Gerbig
et al. 2012). Strong binding of Hg by sulfur-bearing
thiol groups has been shown, and is believed to be a
strong driver of Hg association with dissolved organic
matter (DOM) (Benoit et al. 2001; Skyllberg 2008).
Hg may also be bound to organic matter as HgS
nanoparticles that pass through 0.45 lm pore size
filters, but this form may not be common in surface
waters unimpacted by mining activities (Gerbig et al.
2011).
In natural waters, measures that reflect in part, the
relative aromaticity of DOM such as the hydrophobic
acid (HPOA) concentration or UV254, are often more
strongly related to dissolved Hg than a general
measure such as DOC or TOC (Shanley et al. 2008;
Dittman et al. 2010; Riscassi and Scanlon 2011).
Among several possible reasons for this association is
that most reduced sulfur (and therefore, most of the
strong Hg binding sites) is typically found within the
HPOA fraction of DOM (Ravichandran 2004), which
is more aromatic than the non-HPOA fraction.
Additionally, the more aromatic fraction of organic matter
may better stabilize HgS nanoparticles
(Ravichandran et al. 1999; Gerbig et al. 2011). Thus, the
association of Hg with the aromatic fraction of organic
matter may be rooted in the complex chemistry of Hg
interactions with natural organic matter. In addition to
geochemistry-based reasons why Hg is more strongly
associated with aromatic organic matter, this
association may also originate through mobilization to
surface waters and mixing of Hg and DOM from
principal runoff source areas such as humic-rich
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