Managing Mineland Sulfate Release in the Saint Louis River Basin

PROJECT OVERVIEW
Over a century of iron mining in northeastern Minnesota has left numerous waste rock piles, open pits, and tailings basins that appear to be the dominant sources of sulfate in the St. Louis River. This sulfate has become a recent environmental concern due to the possibility that one of the byproducts of its increased presence, methylmercury, may lead to mercury contamination in fish and other wildlife. Through this appropriation, the Minnesota Department of Natural Resources and the University of Minnesota are evaluating the sources and fate of sulfate in the St. Louis River Basin in order to better understand its impacts and determine the best means for reducing or eliminating these impacts, particularly in environments where methylmercury is a byproduct of sulfate presence.

OVERALL PROJECT OUTCOME AND RESULTS
Taconite mining on the Iron Range sends an average of approximately 35 tons of sulfate per day down the St. Louis River. Another 15 tons per day arises from non-mining sources. Loading from both sources is episodic and depends on hydrologic conditions in the watershed. Most mining-related sulfate arises from the oxidative weathering of minor iron sulfide minerals present in the mined rocks. The predominant pathway for sulfate introduction into the streams is through pumping and overflow of water from taconite pits.

In some source regions, a large percentage of sulfate released near the mines was removed by natural reactions that convert sulfate back to insoluble sulfides ("sulfate reduction"). However, once the sulfate reached the open channel ways in streams, little, if any, additional sulfate was removed by sulfate reduction. Laboratory experiments demonstrated that sulfate reduction can also be stimulated artificially in mine waters by adding organic compounds and iron minerals and eliminating oxygen. However, water hardness and the production of hydrogen sulfide were difficult to control using the methods that were tested.

Methylmercury is a toxic compound that can form as a byproduct of biologic sulfate reduction. Widespread sampling and measurement of methylmercury reveal that its concentration is minimally impacted by sulfate concentration in the main stream or river channels. The dominant source of methylmercury to streams involves the slow passage of water falling on the land through reduced, organic rich materials that surround streams in this area. Except in a few instances, sulfate from mining, added directly to streams, has limited ability to access and impact methylmercury formed in this source region. Laboratory experiments conducted on estuary sediments also indicated that the rate of methylmercury addition to the water column is not directly controlled by sulfate concentration in the overlying water.

PROJECT RESULTS USE AND DISSEMINATION
The work by our group has been widely presented to outside groups including scientists and stakeholders. Plans are in works to publish all or parts of the above reports in peer reviewed journals over the next year.

Three reports and two MS theses were produced directly as a result of this research. Several reports were placed on the DNR's website in late October 2012. This website will be updated to reflect more recent reports by October 2013.