Abandoned Mine Sites on Forest Service Lands
The Forest Service administers 193,000,000 acres of National Forest System federal lands nationwide, mostly in the west. According to a recent Forest Service Report (GTR-260), these lands include between 27,000 and 39,000 abandoned mines. Of these mines, an estimated 2,000 sites present significant environmental or human health problems due to a release, or threat of a release, of a hazardous substance, pollutant, or contaminant where the Forest Service intends to use its CERCLA authority. The estimated cost to address these sites, not including natural resource damage restoration, is approximately $2.1 billion. The 25,000 to 37,000 abandoned mines which do not exhibit significant environmental or human health problems due to a release, or threat of a release, of a hazardous substance, pollutant, or contaminant, and where the Forest Service will not use its CERCLA authority, will cost an estimated, additional $2.3 billion to mitigate, not including natural resource damage restoration. Most of the abandoned mine sites administered by the Forest Service were metals mines, although in the eastern United States, the Forest Service also administers thousands of acres formerly mined for coal.
Acid Mine Drainage
Acid mine drainage (AMD), also called “acid rock drainage,” is fairly common at many types of mines, including both metal mines and coal mines. In general, water and oxygen react with sulfide minerals to produce acid mine drainage. Pyrite (iron sulfide, or FeS2) is the most common sulfide mineral, and is frequently abundant at both metal and coal mines. The reactions produce hydrogen ions (H+) which equates to lowering the pH of the water, or increasing the acidity of the water. Also, as the pH of the water decreases, more metals (iron, copper, zinc, etc.) can be dissolved from the rocks ans go into solution in the water. Waters with a pH below approximately 4.5 to 5.0, and waters with excessive amounts of various metals, can be toxic to aquatic organisms. AMD from metals mines may contain a wide array of dissolved metals in addition to iron (including arsenic, cadmium, copper, lead, mercury, zinc and many others) at concentrations that may be harmful to humans and the environment, while AMD from coal mines typically contains elevated levels of iron and aluminum, although other metals including manganese and zinc may also be present.
As noted in the Environmental Situation and Data section, at the Ore Hill site, the pH 3.3 acid mine drainage contains high levels of aluminum, cadmium, copper, iron, lead, and zinc, and at least one mile of stream is impacted.
The chemical processes of AMD formation are complex but well understood – the following was edited from an Office of Surface mining website, so was originally written to refer to coal mine acid mine drainage, but numerous sources of information on ADM formation and chemistry are available on the web.
Chemistry of Pyrite Weathering
A complex series of chemical weathering reactions are spontaneously initiated when mining activities expose metal-bearing materials to an oxidizing environment. The mineral assemblages exposed by mining are not in equilibrium with the oxidizing environment and almost immediately begin weathering and mineral transformations. The reactions are analogous to "geologic weathering" which takes place over extended periods of time (i.e., hundreds to thousands of years) but the rates of reaction are orders of magnitude greater than in "natural" weathering systems. The accelerated reaction rates can release damaging quantities of acidity, metals, and other soluble components into the environment. The pyrite oxidation process has been extensively studied.
The following equations show the generally accepted sequence of pyrite reactions:
2 FeS2 + 7 02 + 2 H2O -> 2 Fe2+ + 4 SO4 + 4 H+ (Equation 1)
4 Fe 2+ + O2 + 4 H+ -> 4 Fe3+ + 2 H2O (Equation 2)
4 Fe3+ + 12 H2O -> 4 Fe(OH)3 + 12 H+ (Equation 3)
FeS2 + 14 Fe3+ + 8 H2O -> 15 Fe2+ +2 SO42- + 16 H+(Equation 4)
In the initial step, pyrite reacts with oxygen and water to produce ferrous iron, sulfate and acidity. The second step involves the conversion of ferrous iron to ferric iron. This second reaction has been termed the "rate determining" step for the overall sequence.
The third step involves the hydrolysis of ferric iron with water to form the solid ferric hydroxide (ferrihydrite) and the release of additional acidity. This third reaction is pH dependent. Under very acid conditions of less than about pH 3.5, the solid mineral does not form and ferric iron remains in solution. At higher pH values, a precipitate forms, commonly referred to as "yellowboy."
The fourth step involves the oxidation of additional pyrite by ferric iron. The ferric iron is generated by the initial oxidation reactions in steps one and two. This cyclic propagation of acid generation by iron takes place very rapidly and continues until the supply of ferric iron or pyrite is exhausted. Oxygen is not required for the fourth reaction to occur.
The overall pyrite reaction series is among the most acid-producing of all weathering processes in nature.
The pyrite weathering process is a series of chemical reactions, but also has an important microbiological component. The conversion of ferrous to ferric iron in the overall pyrite reaction sequence has been described as the "rate determining step". This conversion can be greatly accelerated by a species of bacteria, Thiobacillus ferroxidans. This bacteria, and several other species thought to be involved in pyrite weathering, are widespread in the environment. T. ferroxidans has been shown to increase the iron conversion reaction rate by a factor of hundreds to as much as one million times.
The activity of these bacteria is pH dependent with optimal conditions in the range of pH 2 to 3. Thus, once pyrite oxidation and acid production has begun, conditions are favorable for bacteria to further accelerate the reaction rate. At pH values of about 6 and above, bacterial activity is thought to be insignificant or comparable to abiotic reaction rates. The catalyzing effect of the bacteria effectively removes constraints on pyrite weathering and allows the reactions to proceed rapidly.