A comparison of results from a hydrologic transport model (HSPF) with distributions of sulfate and mercury in a mine-impacted watershed in northeastern Minnesota

Abstract

The St. Louis River watershed in northeast Minnesota hosts a major iron mining district that has operated continuously since the 1890s. Concern exists that chemical reduction of sulfate that is released from mines enhances the methylation of mercury in the watershed, leading to increased mercury concentrations in St. Louis River fish. This study tests this idea by simulating the behavior of chemical tracers using a hydrologic flow model (Hydrologic Simulation Program FORTRAN; HSPF) and comparing the results with measured chemistry from several key sites located both upstream and downstream from the mining region. It was found that peaks in measured methylmercury (MeHg), total mercury (THg), dissolved organic carbon (DOC), and dissolved iron (Fe) concentrations correspond to periods in time when modeled recharge was dominated by active groundwater throughout the watershed. This helps explain why the timing and size of the MeHg peaks was nearly the same at sites located just upstream and downstream from the mining region. Both the modeled percentages of mine water and the measured sulfate concentrations were low and computed transit times were short for sites downstream from the mining region at times when measured MeHg reached its peak. Taken together, the data and flow model imply that MeHg is released into groundwater that recharges the river through riparian sediments following periods of elevated summer rainfall. The measured sulfate concentrations at the upstream site reached minimum concentrations of approximately 1 mg/L just as MeHg reached its peak, suggesting that reduction of sulfate from non-point sources exerts an important influence on MeHg concentrations at this site. While mines are the dominant source of sulfate to sites downstream from them, it appears that the background sulfate which is present at only 1-6 mg/L, has the largest influence on MeHg concentrations. This is because point sourced sulfate is transported generally under oxidized conditions and is not flushed through riparian sediments in a gaining stream watershed system.

Global Meta‐Analysis on the Relationship Between Mercury and Dissolved Organic Carbon in Freshwater Environments

“…In total, 108 studies were excluded through that screening process. A total of 44 studies were kept from this search (Bergman & Bump, ; Berndt et al, ; Berndt & Bavin, ; Braaten et al, ; Bravo et al, ; Brazeau et al, ; Burns et al, , , ; Burton et al, ; Chaves‐Ulloa et al, ; de Wit et al, ; Demers et al, ; Denkenberger et al, ; Dranguet et al, ; Eckley et al, ; Eklöf et al, ; Ekstrom et al, ; Fagnani et al, ; Gerson & Driscoll, ; Girard et al, ; Guedron et al, ; Hamelin et al, ; Haverstock et al, ; Houben et al, ; Jeremiason et al, ; Leopold et al, ; Loftin et al, ; Luo et al, ; MacMillan et al, ; Moreno et al, ; Navratil et al, ; Noh et al, ; Oswald & Branfireun, ; Painter et al, ; Picado & Bengtsson, ; Poste et al, ; Pyhtila et al, ; Riscassi et al, ; Sherman & Blum, ; Tjerngren et al, ; Ukonmaanaho et al, ; Vidon et al, ; Voss et al, ). We included additional studies that have been cited by the abovementioned pool of studies more than five times specifically in reference to the relationship (or lack thereof) between Hg and DOC (eight studies; Brigham et al, ; Dittman & Driscoll, ; Dittman et al, ; Dittman et al, ; Driscoll et al, ; Eklöf et al, ; Porvari & Verta, ; Riscassi & Scanlon, ; Sørensen et al, ).…”

“…In total, 108 studies were excluded through that screening process. A total of 44 studies were kept from this search (Bergman & Bump, 2014;Berndt et al, 2016;Berndt & Bavin, 2012;Braaten et al, 2014;Bravo et al, 2017;Brazeau et al, 2013;Burns et al, 2012Burns et al, , 2013Burns et al, , 2014Burton et al, 2013;Chaves-Ulloa et al, 2016;de Wit et al, 2014;Demers et al, 2013;Denkenberger et al, 2014;Dranguet et al, 2017;Eckley et al, 2015;Eklöf et al, 2013;Ekstrom et al, 2016;Fagnani et al, 2012;Gerson & Driscoll, 2016;Girard et al, 2016;Guedron et al, 2014;Hamelin et al, 2015;Haverstock et al, 2012;Houben et al, 2016;Jeremiason et al, 2016;Leopold et al, 2012;Loftin et al, 2012;Luo et al, 2014;MacMillan et al, 2015;Moreno et al, 2016;Navratil et al, 2015;Noh et al, 2016;Oswald & Branfireun, 2014;Painter et al, 2015;Picado & Bengtsson, 2012;Poste et al, 2015;Pyhtila et al, 2012;Riscassi et al, 2016;Sherman & Blum, 2013;…”

Challenges and opportunities for managing aquatic mercury pollution in altered landscapes

“…In addition to direct emission of Hg to the air, mine operations can also impact Hg methylation in downstream aquatic systems by providing a substantial source of sulfate to receiving waterbodies (Berndt et al 2016 ; Bailey et al 2017 ) and by altering the surrounding hydrology as a result of dewatering around the mine pit/tunnels during operations and subsequent rewetting after closure (Willacker et al 2016 ; Eckley et al 2017 ). The drawdown and wetting cycles yield similar impacts on Hg methylation as that noted for reservoirs.…”

Net Ecosystem Production of a River Relying on Hydrology, Hydrodynamics and Water Quality Monitoring Stations

“…The HSPF model was used for predicting flows of a watershed that joins modeling of watersheds and streams [39][40][41]. In addition, the HSPF model can be used to incorporate the transport of pollutants and nutrients.…”