Article pubs.acs.org/est
Fate and Transport of Ambient Mercury and Applied Mercury Isotope in Terrestrial Upland Soils: Insights from the METAALICUS Watershed Claire J. Oswald,†,‡,⊥,* Andrew Heyes,§ and Brian A. Branfireun∥,†,# †
Department of Geography, University of Toronto Mississauga, Mississauga, Ontario L5L 1C6, Canada School of Geography & Earth Sciences, McMaster University, Hamilton, Ontario L8S 4L8, Canada § Centre for Environmental Science (Chesapeake Biological Laboratory), University of Maryland, Solomons, Maryland 20688, United States ∥ Department of Biology and Centre for Environment and Sustainability, University of Western Ontario, London, Ontario N6A 5B7, Canada ‡
S Supporting Information *
ABSTRACT: The fate of mercury (Hg) deposited on forested upland soils depends on a wide array of biogeochemical and hydrological processes occurring in the soil landscape. In this study, Hg in soil, soilwater, and streamwater were measured across a forested upland subcatchment of the METAALICUS watershed in northwestern Ontario, Canada, where a stable Hg isotope (spike Hg) was applied to distinguish newly deposited Hg from Hg already resident in the watershed (ambient Hg). In total, we were able to account for 45% of the total mass of spike Hg applied to the subcatchment during the entire loading phase of the experiment, with approximately 22% of the total mass applied now residing in the top 15 cm of the mineral soil layer. Decreasing spike Hg/ambient Hg ratios with depth in the soil and soilwater suggest that spike Hg is less mobile than ambient Hg over shorter time scales. However, the transport of spike Hg into the mineral soil layer is enhanced in depressional areas where water table fluctuation is more extreme. While we expect that this pool of Hg is now effectively sequestered in the mineral horizon, future disturbance of the soil profile could remobilize this stored Hg in runoff.
1. INTRODUCTION In remote ecosystems, Hg concentrations have increased 2- to 5-fold over preindustrial levels.1 Transformation of inorganic Hg by bacteria to methylmercury (MeHg) and the subsequent biomagnification through aquatic food webs has resulted in human health concerns through fish consumption. The general consensus is that MeHg concentrations in fish respond to changes in atmospheric inputs of Hg to lakes and watersheds, but the magnitude and timing of the response will largely depend on the efficiency of the terrestrial system in delivering Hg to receiving waters and zones of methylation.2 A major uncertainty in our understanding of Hg fate and transport in watersheds is whether its distribution and mobility in upland soils changes over time. The Mercury Experiment to Assess Atmospheric Loading in Canada and the United States (METAALICUS), at the Experimental Lakes Area (ELA) in northwestern Ontario, was undertaken to investigate the role of the whole watershed in governing the response of Hg levels in fish to changes in atmospheric deposition.3−5 The fate and transport of newly deposited Hg was examined through the application of © 2014 American Chemical Society
enriched stable Hg(II) isotopes (spike Hg) to different watershed compartments (lake, wetland, and upland) over a six year period.5 This approach allowed for the discrimination of new spike Hg from historically-, and to a lesser extent newly deposited, Hg that was already stored in the watershed (ambient Hg). In a METAALICUS terrestrial soil pilot study, Hintelmann et al.3 examined the distribution of newly deposited Hg by increasing the inorganic Hg loading to a forested upland microcatchment at the ELA. While ∼8% of the spike Hg volatilized to the atmosphere within a few months, the majority was retained in vegetation (∼66%) and, to a lesser extent, soils (∼25%), with less than 1% lost in runoff. In soils, the spike Hg was transported up to 10 cm into the soil profile within a few months of application, however the mechanisms involved in its transport were not investigated. In the METAALICUS whole Received: Revised: Accepted: Published: 1023
September 24, 2013 December 23, 2013 January 2, 2014 January 2, 2014 dx.doi.org/10.1021/es404260f | Environ. Sci. Technol. 2014, 48, 1023−1031
Environmental Science & Technology
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watershed study, the first three years of experimental results showed that
