Specific ultra-violet absorbance as an indicator of mercury sources in an Adirondack River basin

Biogeochemistry, Aug 2012

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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Specific ultra-violet absorbance as an indicator of mercury sources in an Adirondack River basin

Douglas A. Burns 0 1 2 3 George R. Aiken 0 1 2 3 Paul M. Bradley 0 1 2 3 Celeste A. Journey 0 1 2 3 Jakob Schelker 0 1 2 3 0 P. M. Bradley C. A. Journey U.S. Geological Survey, Columbia, SC, USA 1 G. R. Aiken U.S. Geological Survey, Boulder, CO, USA 2 D. A. Burns (&) U.S. Geological Survey, 425 Jordan Rd., Troy, NY 12180, USA 3 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. - 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 sha (...truncated)


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Douglas A. Burns, George R. Aiken, Paul M. Bradley, Celeste A. Journey, Jakob Schelker. Specific ultra-violet absorbance as an indicator of mercury sources in an Adirondack River basin, Biogeochemistry, 2012, pp. 451-466, Volume 113, Issue 1-3, DOI: 10.1007/s10533-012-9773-5