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Aqueous Elemental Mercury Production versus Mercury Inventories in the Lake Michigan Airshed: Deciphering the Spatial and Diel Controls of Mercury Gradients in Air and Water
Citation:
Lepak, R., M. Tate, J. Ogorek, J. DeWild, J. Hurley, AND D. Krabbenhoft. Aqueous Elemental Mercury Production versus Mercury Inventories in the Lake Michigan Airshed: Deciphering the Spatial and Diel Controls of Mercury Gradients in Air and Water. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 3(1):719-727, (2021).
Impact/Purpose:
Our study represents research where dissolved gaseous elemental mercury (Hg), dissolved Hg, and atmospheric Hg species were simultaneously measured in Lake Michigan over a broad spatial scale to better understand Hg fluxes and transformations in the surface waters (due to light) and at depth (due to microbes). This ambitious effort is the first of our knowledge to investigate the naturally occurring, lake-wide Hg removal mechanisms over such a broad range of conditions. The high frequency sampling also allowed us to observe an increase in atmospheric Hg from the urban plume of the south basin, and to better understand diurnal cycling of Hg. It is fortuitous that we chose Lake Michigan because it was also the location of some of the world’s first freshwater trace level Hg measurements in the early 1990’s and our results show that Hg concentrations have nearly halved in the past 25 years. This information is critical to understanding the fate of reactive Hg deposition to the lake and its potential for bioaccumulation
Description:
Atmospheric delivery mechanisms are the dominant pathways leading to mercury (Hg) contamination in the Upper Great Lakes, yet studies of gaseous Hg exchange between surface water and air are limited. Speciated atmospheric Hg, continuous surface dissolved gaseous Hg (DGM), profiles of DGM, and particulate and filter-passing total Hg were measured on a single cruise in Lake Michigan in 2013. Mercury measured in each component was at or near levels considered low in literature, and likely reflects pristine background conditions especially in offshore regions. In the atmosphere, reactive and particle-associated fractions were low (1.0 ± 0.5%) compared to gaseous elemental Hg (1.34 ± 0.14 ng m-3) and were elevated in the urbanized southern basin. DGM was consistently supersaturated and ranged 17.5 ± 4.8 pg L-1 in the main lake and 33.2 ± 2.4 pg L-1 in Green Bay. Diel cycling of surface DGM suggested increased flux during the day, likely due to increased winds. During stratified periods, epilimnetic DGM is largely formed from photochemical reduction and does not penetrate into hypolimnetic waters, thus hypolimnetic DGM likely results from biological Hg reduction. Low atmospheric delivery and enhanced removal mechanisms likely explains the low dissolved Hg in Lake Michigan.
