Abstract
Net fluxes (change between upstream and downstream margins) for water, methyl mercury (MeHg), total mercury (THg), dissolved organic carbon (DOC), and chloride (Cl) were assessed twice in an Adirondack stream reach (Sixmile Brook, USA), to test the hypothesized importance of wetland-stream hydraulic and chemical gradients as fundamental controls on fluvial mercury (Hg) supply. The 500 m study reach represented less than 4%of total upstream basin area. During a snowmelt high-flow event in May 2009surface water, DOC, and chloride fluxes increased by 7.1±1.3%, 8.0±1.3%, and 9.0±1.3%, respectively, within the reach, demonstrating that the adjacent wetlands are important sources of water and solutes to the stream. However, shallow groundwater Hg concentrations lower than in the surface water limited groundwater-surface water Hg exchange and no significant changes in Hg (filtered MeHg and THg) fluxes were observed within the reach despite the favorable hydraulic gradient. In August 2009, the lack of significant wetland-stream hydraulic gradient resulted in no net flux of water or solutes (MeHg, THg, DOC, or Cl) within the reach. The results are consistent with the wetland- Hg-source hypothesis and indicate that hydraulic and chemical gradient (direction and magnitude) interactions are fundamental controls on the supply of wetland Hg to the stream.
Highlights
Identifying methyl mercury (MeHg) source areas and the controls on MeHg supply to streams is an environmental priority due to widely-reported correlations between elevated fluvial-MeHg concentrations and fish-Hg contamination [1,2,3], the leading cause of fish consumption advisories in streams throughout the United States (US) [4] and Canada [5]
To better understand the biogeochemical and hydrological processes that contribute to fluvial MeHg temporal variability in Sixmile Brook and in other Adirondack streams, two approaches were employed: 1) multivariate regression modeling of the encompassing Fishing Brook watershed[presented in 19] and 2) instantaneous, instream mass flux assessments in a 500 m study reach
Regression analysis of 43 fixed station samples collected in Fishing Brook downstream of Sixmile Brook during 2007-2009 suggested that seasonal temperature-driven effects on wetland Hg methylation and stream flow variations during the cold dormant season are principal controls on fluvial MeHg concentration patterns [19]
Summary
Identifying methyl mercury (MeHg) source areas and the controls on MeHg supply to streams is an environmental priority due to widely-reported correlations between elevated fluvial-MeHg concentrations and fish-Hg contamination [1,2,3] , the leading cause of fish consumption advisories in streams throughout the United States (US) [4] and Canada [5]. Strong correlations between wetland area and fluvial MeHg concentrations [6,7,8] , higher dissolved MeHg concentrations [7,911] and Hg methylation rates [7,12] in wetlands compared with adjacent stream habitats, and strong wetland-stream hydrologic connectivity [13,14,15,16] support the hypothesis that wetland areas are primary sources of fluvial MeHg in many systems [1,10-12,1719] This wetland-MeHg-source hypothesis, in turn, predicts that unfavorable wetland-stream hydraulic (water) and chemical (MeHg) gradients may limit fluvial MeHg concentrations and fluxes in the adjacent stream reach. Concentration gradients were assessed in Sixmile Brook during snowmelt and later during the growing season, to test the hypothesis that unfavorable hydraulic and chemical gradients (i.e., insignificant or from surface-water toward groundwater) limit fluvial MeHg supply in this and in similar systems in the Adirondacks region
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