A comparison of results from a hydrologic transport model (HSPF) with distributions of sulfate and mercury in a mine-impacted watershed in northeastern Minnesota.
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.
Minnesota Department of Natural Resources, 500 Lafayette Road, St. Paul, MN, 55155, USA. Electronic address: mike.berndt@state.mn.us.
Michael E
ME
Berndt
University of Minnesota, Earth Sciences Department, 310 Pillsbury Drive SE, Minneapolis, MN, 55455, USA.
Wes
W
Rutelonis
Minnesota Pollution Control Agency, 520 Lafayette Road, St. Paul, MN, 55155, USA.
Charles P
CP
Regan
D004784:Environmental Monitoring / Q000379:methods; D060587:Groundwater; D006801:Humans; D007220:Industrial Waste; D008628:Mercury / Q000737:chemistry*; D008906:Mining; D008910:Minnesota; D008956:Models, Chemical; D013431:Sulfates / Q000737:chemistry*; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000737:chemistry*
D007220:Industrial Waste; D013431:Sulfates; D014874:Water Pollutants, Chemical; D008628:Mercury
HSPF model; Methylmercury; Mining; Minnesota; Sulfate