Characterizing mercury concentrations and fluxes in a Coastal Plain watershed: Insights from dynamic modeling and data

Abstract

[1] Mercury (Hg) is one of the leading water quality concerns in surface waters of the United States. Although watershed-scale Hg cycling research has increased in the past two decades, advances in modeling watershed Hg processes in diverse physiographic regions, spatial scales, and land cover types are needed. The goal of this study was to assess Hg cycling in a Coastal Plain system using concentrations and fluxes estimated by multiple watershed-scale models with distinct mathematical frameworks reflecting different system dynamics. We simulated total mercury (Hg T , the sum of filtered and particulate forms) concentrations and fluxes from a Coastal Plain watershed (McTier Creek) using three watershed Hg models and an empirical load model. Model output was compared with observed in-stream Hg T . We found that shallow subsurface flow is a potentially important transport mechanism of particulate Hg T during periods when connectivity between the uplands and surface waters is maximized. Other processes (e.g., stream bank erosion, sediment re-suspension) may increase particulate Hg T in the water column. Simulations and data suggest that variable source area (VSA) flow and lack of rainfall interactions with surface soil horizons result in increased dissolved Hg T concentrations unrelated to DOC mobilization following precipitation events. Although flushing of DOC-Hg T complexes from surface soils can also occur during this period, DOC-complexed Hg T becomes more important during base flow conditions. TOPLOAD simulations highlight saturated subsurface flow as a primary driver of daily Hg T loadings, but shallow subsurface flow is important for Hg T loads during high-flow events. Results suggest limited seasonal trends in Hg T dynamics.

Nonlinear and threshold‐dominated runoff generation controls DOC export in a small peat catchment

“…Similar relationships were developed by Golden et al . [] in a coastal plain watershed in the U.S. Therefore, we focused on DOC simulation and the most parsimonious model representation possible in a rather homogeneous and small peat catchment assuming that the same driving mechanisms operate for DOC, Pb, and As similar to work by Schelker et al .…”

Climate change and watershed mercury export: a multiple projection and model analysis

“…McTier Creek Watershed study site in South Carolina, USA (after Golden et al ). [Color figure can be seen in the online version of this article, available at wileyonlinelibrary.com.…”

“…The mass balance of Hg at the watershed outlet is estimated using the equations where C s is the concentration of Hg in watershed soils (µg m −3 ); L is the Hg load (µg d −1 ); L p is the Hg atmospheric deposition load on pervious land (µg d −1 ); L f is the Hg atmospheric deposition load on forest land (µg d −1 ); L d is the litter decomposition Hg load on forestland (µg/d); K r is the reduction rate constant (d −1 ), where reduced Hg is assumed to immediately volatilize and is considered a loss from the watershed; K l is the leaching loss constant (d −1 ); K ro is the runoff loss constant (d −1 ); K e is the erosion loss constant (d −1 ); V s is the watershed soil volume (m 3 ); A c is the grid area (m 2 ); and z d is the watershed soil mixing depth (m). A full description of GBMM's equations and sensitivity analysis , final input parameters for McTier Creek Watershed for the hydrology module , and parameters for the sediment and Hg modules in McTier Creek are detailed elsewhere.…”

“…The change in soil mercury areal density, or (g m −2 ) is calculated using a forward Euler finite difference approximation for a predetermined (typically daily) time step where total is the Hg(II) load (g m −2 d −1 ], is the demethylation source (Equation ) or sink (Equation ; g m −2 d −1 ), J in is the daily soil Hg(II) (Equation ) or MeHg (Equation ) influx (g/m −2 /d −1 ), is the reduction sink (g m −2 d −1 ), is the methylation sink (Equation ) or source (Equation ; g m −2 d −1 ), J out is the daily soil Hg(II) (Equation ) or MeHg (Equation ) outflux (g m −2 d −1 ), and is MeHg deposition (g m −2 d −1 ). Additional details on the VELMA model can be found elsewhere .…”

“…The purpose of the present study is to provide insight into possible responses of watershed hydrological and total Hg (THg) fluxes from the landscape to the watershed outlet (i.e., watershed THg fluxes) and in‐stream THg concentrations to a unified set of future climate change projections in a coastal plain basin using multiple watershed models calibrated to baseline empirical data. We apply 3 watershed models (Visualizing Ecosystems for Land Management Assessment Model for Hg [VELMA‐Hg], Grid‐Based Mercury Model [GBMM], and the TOPography based constituent LOADing model [TOPLOAD]) and 2 statistically downscaled global precipitation and temperature models (ECHAM4/HOPE‐G [ECHO‐G; herein referred to as ECHO] , Community Climate System Model, Ver 3 [CCSM3] ) using scenario A2 from the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (IPCC SRES) , configured to McTier Creek Watershed, South Carolina, USA . This approach allows us to bracket the relative response of watershed THg fluxes to climate change and assess the uncertainty associated with the projections.…”