Controls on streamwater dissolved and particulate mercury within three mid-Appalachian forested headwater catchments

Riscassi, Ami

Abstract: This dissertation examines streamwater dissolved and particulate mercury dynamics (HgD and HgP, respectively) to gain an improved understanding of factors that control Hg transport from the terrestrial to stream environment. The three study sites, Piney River, Staunton River, and Paine Run, are each forested, headwater systems situated within Shenandoah National Park, VA, and are distinguished by chemical (acidic to neutral pH) and physical soil (sands to clays) characteristics. At each of the three sites, streamwater HgD, HgP, dissolved organic carbon (DOC) concentrations, turbidity, as well as suspended sediment at one location, were evaluated over a range of discharge conditions within an 18-month period with a focus on frequent sampling during high-flow periods. Total Hg streamwater export ranged from 1.26-3.71 μg m-2 yr-1, representing 4-19% of the Hg deposited through atmospheric deposition. Hg fluxes were dominated by the particulate fraction at each site. Brief high-flow rainfall/snowmelt events, corresponding to approximately 1% of the time, were found to export the majority (80%) of the annual Hg flux. HgD was strongly coupled with DOC at all sites confirming the known association with organic carbon. Both soil solution chemistry as well as soil particle size distribution were found to be controlling factors on the mobilization of HgD. More acidic systems mobilize less aromatic DOC, which relates to less Hg per unit DOC. Soil composition was also found to influence HgD export, with coarse sandy soils having a lesser capacity to retain Hg and keep it out of solution resulting in more Hg per unit DOC. Soil organic matter content appears to be a first-order determinant of the amount of Hg per unit organic carbon based on all available stream data, which include locations throughout the U.S. and Sweden. HgP was strongly positively correlated with the organic fraction of suspended sediment, and due to a consistent organic fraction of the suspended solids, was also well correlated with total suspended sediment (TSS). Stream turbidity measured with an in situ sonde also had a strong correlation with TSS, enabling commutative association with HgP. Turbidity-HgP relationships established for each site allowed for improved quantification of HgP mass fluxes. Dissertation organization:This dissertation is organized as four self-contained papers, all related to streamwater Hg transport in headwater systems subject to Hg inputs via atmospheric deposition within Shenandoah National Park. Chapter 2 examines the viability of using an automated device to sample streamwater during high-flow events for low-level Hg analysis. In Chapter 3, particulate Hg (HgP) and suspended sediment dynamics are evaluated to gain a better understanding of the factors that mobilize HgP downstream. An approach to estimate HgP with turbidity, measured at high frequency with the use of an in situ sonde, is also presented. Chapter 4 investigates the controls on dissolved Hg (HgD) concentrations at three sites which are distinguished by their streamwater chemistry and soil characteristics, factors that have been shown to affect HgD transport in laboratory studies. Chapter 5 presents annual HgP and HgD fluxes computed at each site for the 2010 water year. A summary of the entirely of this work and likely implications from the results are presented in Chapter 6.

Controls on streamwater dissolved and particulate mercury within three mid-Appalachian forested headwater catchments

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