Constituents of concern
There are number of potential pollutants (predominately, though not exclusively, related to water quality) associated with open-pit taconite iron mining. These include:
Acid Mine Drainage. When sulfide minerals are exposed to oxidizing conditions (e.g., brought to the surface and exposed to oxygen and water), sulfuric acid is produced. This acid release has the potential to reduce pH, deposit precipitates, and mobilize metals in downstream water resources. Pyrite and other iron sulfides are a commonly encountered minor constituent in the rocks of the Penokee Range, and pyrite has also been found to be abundant (20%) at some locations. However, acid mine drainage is a complex function of a number of geochemical and environmental factors, and the potential for it occurring at a given site is best assessed using laboratory methods (e.g., humidity cells) that simulate chemical weathering.
Metals. Many metals are toxic to organisms at elevated concentrations and some metals bioaccumulate in aquatic food webs. Aquatic organisms are often particularly sensitive to metal pollution. Because trace metals often occur as impurities in mineral formations, metals of concern are also best identified using simulated chemical weathering. Aluminum is of particular interest, as it is abundant in the Ironwood formation and has been linked to fish mortality in acidified waters. Magnetite taconite also commonly contains Mg, Ti, V, Cr, Mn, Co, Ni, Cu, Hg, and Zn.
Mercury. Taconite processing is often a major source of mercury emissions, released primarily during the induration process (heating of taconite pellets). Historically, mercury emissions to the atmosphere were considered to be widely dispersed. Recent studies suggest, however, that local and regional deposition is more important than previously thought. Mercury methylation (methylmercury is the form taken up by organisms) and bioaccumulation occurs in the aquatic environment. Elevated concentrations in fish tissue are a well-documented human health concern.
Sulfate. Sulfate, which can potentially be leached from mine tailings and waste rock, plays a
role in the production of acid mine drainage and also has a stimulatory effect on
methylmercury production in the environment. There is some evidence that elevated
sulfate concentrations in surface waters is toxic to wild rice, and Minnesota has a 10 mg/L
sulfate standard for its protection. Minnesota is currently in the final stages of a two-year
in-depth study on the effects of sulfate on wild rice, and may revise their standard based on
Selenium. Selenium is geochemically similar to sulfur, and thus is expected to occur in small
amounts in sulfide minerals (and may be similarly released due to chemical weathering).
Selenium is highly bioaccumulative, and thus dissolved concentrations are not reliable
indicators of potential selenium environmental toxicity. Selenium pollution has been
documented and is being addressed at the taconite iron mines in Michigan.
Mineral fibers. The iron formations of Wisconsin contain fibrous amphibole minerals, and
asbestiform grunerite has been found at the site of the proposed Gogebic Taconite project.).
The Northshore mine on the Eastern end of Minnesota’s iron range is also a documented
source of several amphibole minerals. Asbestos is a human health hazard; extended
exposure to airborne fibers can cause mesothelioma and other cancers.
Nutrients. The rock present in the Penokee-Gogebic contains a relatively large concentration
of phosphorus (the phosphorus content of the iron-bearing formation is approximately 660
mg of phosphorus per kg of rock.). Weathering of waste rock and processing of ore may
liberate phosphorus, which is the primary driver of eutrophication in Wisconsin’s surface
Suspended solids. Taconite processing produces a large amount of fine-grained rock. This
and other mine-site characteristics (e.g., increased cover of impervious surfaces) have the
potential to increase downstream turbidity.
Organic compounds. While potential pollutants associated with the ore and waste rock are
typically of greater concern due to the large volume of these materials processed, a number
of fuel, lubricants, solvents, and process chemicals are also used on site, and may potentially
be present in stormwater or process water outfalls.