Background

Wisconsin is loaded with metal sulfide minerals. Native Americans mined the southwestern region of Wisconsin as early as the 1700s, and the first European settlers in Wisconsin, in the early 1800s, were trappers and miners. In fact the term “badgers” was originally applied to the miners, who dug shallow “badger-holes” to mine the minerals that were located just beneath the surface. In the early 1800s the miners were primarily after galena (PbS), since it can easily be smelted to lead, which was used for bullets, cannonballs, and a variety of metal utensils. Galena is actually the official state mineral of Wisconsin, and the galena miner, his pick-axe, and a stack of lead are featured prominently on the right side of the Wisconsin state flag (see Figure 1).

Figure 1. Picture of emblem on Wisconsin state flag.

The most common sulfide mineral, pyrite (FeS₂) is a very hard, heavy, shiny, yellow mineral, sometimes referred to as “fool’s gold”. Chemically, it is primarily composed of iron and pairs of sulfur atoms in the form of S₂^2– dumbbells. The crystal structure can be viewed as a face-centered cubic lattice of Fe²⁺ ions with 8 sulfur atoms within the unit cell. Each sulfur atom has three Fe²⁺ ions and one sulfur atom as nearest neighbors. (See Figure 2).¹ Other ions such as arsenic can substitute for sulfur in the crystal lattice forming a “solid solution” with up to ~6% arsenic concentration, so that “pyrite” also has other potentially hazardous metals locked into its crystal structure. At higher concentrations, a different mineral known as arsenopyrite, FeAsS, exists. A number of other sulfide minerals (based on lead, gold, silver, mercury, or copper, for instance) exist naturally in the environment. Examples include galena (PbS), argentite (Ag₂S), cinnabar (HgS), chalcopyrite (CuFeS₂), chalcocite (Cu₂S), and covellite (CuS). These metal sulfide minerals are part of a large deposit that extends through northeastern Iowa, northern Illinois, and parts of Wisconsin, especially the southwest, but including most of the state. In the southwest part of the state the sulfide minerals include large amounts of galena (PbS), which was heavily mined in the 1800s for its lead. A map of Wisconsin from the 1800s would show almost all the state’s population concentrated in the southwest part near the Mississippi, where the lead miners could mine and then ship their products down the river. When the lead ran out in the mid-1800s, the miners moved to other locations. However, their legacy is left behind in town names such as “Galena”, Illinois and “Mineral Point” just west of Madison.

Figure 2. Structure of pyrite.¹

Most of the sulfide minerals are located deep underground. However, several regions in central Wisconsin have significant problems with arsenic contamination arising from the natural geochemical distribution of arsenic-containing sulfide minerals, particularly pyrite and arsenopyrite. In the Fox Valley area, especially around Oshkosh and Appleton, a large amount of metal sulfide minerals containing arsenic and other metals are deposited at the boundaries of some sandstone formations. Most of these arsenic-containing metal sulfide minerals are found in a specific type of sandstone known as the St. Peter Sandstone Outcrop. The presence of these minerals has led to a large number of water quality problems in east-central Wisconsin, with naturally occurring levels of arsenic and other metals that far exceed health safety standards. One well drilled in the Green Bay area in 1977 produced water that in 1992 was measured to have a pH of 2.04 and an arsenic concentration of >4300 ppb, more than 400 times the EPA limits on arsenic concentration. Figure 3(a) on the next page shows a map with the location of private wells in Wisconsin having high arsenic concentrations (most in the outcrop in the Appleton-Oshkosh area); Figure 3(b) shows a map of the “Arsenic Advisory Area” in the Fox Valley established by the EPA.²

Figure 3. (a) Map showing private wells with arsenic levels greater than the 10 ppb. (b) Map showing the arsenic advisory area near Appleton and Oshkosh.²

Recently in Wisconsin, public attention was drawn to the Crandon area, where there had been proposals to mine a sulfide ore deposit, primarily to obtain copper (found as copper sulfides) and zinc (found as zinc sulfide), along with trace amounts of silver and gold. The potential environmental problems associated with crushing and exposing large amounts of sulfide minerals, together with other environmental and economical concerns, led to a great deal of opposition to the mine and eventually resulted in the termination of the project. More information can be found on the Wisconsin Department of Natural Resources (DNR) website.³

Sulfide minerals create similar environmental problems elsewhere. The most serious problem is in Bangladesh, where more than 65 million people are exposed to high levels of arsenic due to the arsenopyrite naturally present in the sub-surface soil. In Bangladesh, the sulfide minerals were not considered a problem until the 1970s. An exploding population and poor sanitation led to contamination of the surface water, such as lakes and rivers. When the UN encouraged use of wells instead of surface water as a well-intentioned attempt to improve water quality in Bangladesh, the water quality improved temporarily, but eventually the heavy use of wells caused a drop in the level of the water table. Arsenic-bearing pyrite minerals that were previously completely under water (and therefore protected from oxygen and other oxidizers such as ferric ion) were now repeatedly exposed to oxygen and then water, leading to rapid release of arsenic into the underwater aquifers. While millions of people were exposed to very high levels of arsenic, it has only been in the last few years that the source of the contamination has been identified as sub-surface arsenopyrite and related arsenic-bearing minerals. Even today, the population of Bangledesh continues to use this contaminated water. You can find more information about this problem from numerous sources.^{4, 5}

The problems associated with metal sulfide minerals often go by the name of “acid mine drainage”, or “AMD”. This term refers to the fact that the oxidation of sulfide minerals ultimately produces sulfuric acid, as described in more detail below. Water trickling through a mining area, for example, will react with the sulfide minerals, releasing sulfuric acid and lowering the pH. This highly acidic runoff flows to rivers and streams, where it impacts fish and other plant and animal life. Despite the term “AMD”, however, the most serious environmental consequences of sulfide minerals arises from the release of metals such as arsenic, lead, silver, and copper into the water system. Many metals are toxic even at very small concentrations. For instance, the EPA standard for the maximum allowable amount of arsenic in drinking water is 10 ppb. The “AMD” term is often used to denote both the acid generation and the release of high concentrations of metals into the environment. More information on the problems of arsenic in drinking water can be found on the EPA and the Wisconsin Water Library websites.^{6, 7}

Although the sulfide minerals are present naturally and slowly react under ambient conditions, mining activities accelerate their reactions and have the potential to create very significant, long-lasting environmental problems. Assessing the risk versus benefit of these activities is difficult in part because the underlying science is actually quite complex, making it difficult to quantitatively predict the environmental impact of these activities in the real environment. Additionally, it must be remembered that many of the raw materials must be obtained from somewhere, and mining operations not conducted in the United States may be conducted in other locations, such as the “Minas Gerais” region of Brazil, with much poorer controls on discharges into the environment. Thus, while opposition to the Crandon mine, for example, helps to protect the environment in Wisconsin, it may have a secondary effect of leading to much worse environmental contamination elsewhere, particularly in developing countries such as Brazil and China.

There exists a great deal of rhetoric regarding surrounding the environmental science of mining run-off. Opponents of mining and other activities with potential for damaging the environment often overstate the problems and point to worse-case scenarios, while mine operators/owners underestimate the likely environmental damage. What is nominally a scientific issue rapidly becomes politicized, and the public is left trying to figure out who (if anyone) to believe. As scientists, one of our roles is to provide unbiased scientific data and models that can be used to provide a rational, scientific basis for understanding and predicting the potential effects of human activities on the environment. A second, and often underutilized, responsibility is to communicate this information to the general public. Most people do not have sufficient scientific background to understand the detailed science. However, especially because environmental issues are rarely black-and-white, in order to make informed decisions the public needs to be able to discern between fact and political hyperbole.