Acid mine drainage (AMD) is the outflow of acidic water from metal or coal mines. Mining activities typically involve large-scale earth disturbances, usually within rocks containing an abundance of sulfide materials. When surface water and/or groundwater come in contact with these pyritic rocks, along with air, oxidation of the sulfides occurs – generating acidic metal-bearing AMD. The pH of AMD can be extremely low (yes…negative pH levels are possible, as described below) – equivalent to or less than sulfuric acid.
Liquids that drain from coal stockpiles, coal washeries, and coal waste tips can be highly acidic; and this liquid effluent is analogous to AMD. In addition, other large earth-disturbing activities – such as construction sites, subdivision projects, and transportation projects – may create acid rock drainage (ARD).
What Are the Hazards of AMD?
AMD (as well as ARD) contains elevated concentrations of metals which – depending on the host rock – may include iron, zinc, copper and nickel, to name a few. AMD usually flows downhill to other receiving waters, potentially impacting them for miles. The high concentrations of metals and low pH are detrimental to the ecosystems and can cause large-scale fish kills. Downstream receptors, such as water plant intakes, could be impacted.
When the pH of AMD is raised past 3, either through contact with fresh water or neutralizing minerals, previously soluble iron precipitates as iron hydroxide – yielding a yellow-orange liquid, known as yellow boy. This precipitation may act to concentrate metal content in surface water sediments.
At some mines, AMD may be generated for decades or centuries – presenting serious long-term environmental hazards. If the responsible parties are identified; they may be found liable for massive, expensive cleanup efforts, potential Natural Resource Damage assessment fines, and possibly criminal prosecution.
How is AMD Cleaned Up?
AMD treatment technologies can be divided into two major categories: prevention and remediation. Prevention techniques mainly focus on inhibiting AMD formation reactions by controlling the source (air, water or both). This may be accomplished, for instance, by re-routing water around an AMD source.
Remediation techniques focus on the treatment of already produced AMD before its discharge into water bodies. Remediation technologies can be further divided into two broad categories: active and passive. Due to high cost and intensive labor requirements for maintenance of active treatment technologies, passive treatments are preferred and widely used all over the world. Conventional passive treatment technologies include constructed wetlands, anaerobic sulfate-reducing bioreactors, anoxic limestone drains, open limestone channels, limestone leach beds, and slag leach beds.
Active remediation includes methods such as lime neutralization (collection of the AMD and addition of a lime slurry) which is used to raise the pH – resulting in the precipitation of metals, which can be separated and potentially recycled into marketable materials. The remaining water may be decanted and recycled or discharged as clean water.
Iron Mountain Mine Oddities
As the Project Manager of the U.S. Environmental Protection Agency’s (USEPA’s) Iron Mountain Mine (IMM) Superfund site for five years, the author of this issue of HETI Horizons had a front-and-center view of some very interesting natural phenomena. IMM consists of approximately 4,400 acres and is located near Redding, California, in the foothills of the Klamath Mountains. It was actively mined from the 1890s until 1963, primarily for copper ore. Several distinct underground mines exist at the site; and a large open pit mine is located in the upper reaches of IMM.
Although State of California and USEPA regulatory actions began in 1976, it was not until 2000 that a regulatory settlement was reached with Aventis, the principal responsible party at IMM. The total value of this settlement – for past and future work – was more than $950 million. A cost cap policy and Scope of Work document was issued to facilitate the remedial activities; and it was within that framework that the author acted as Project Manager.
Currently, approximately 95% of the AMD at IMM is collected and gravity-transported via high density polyethylene pipe runs to the Minnesota Flats Treatment Plant located near the bottom of IMM. The AMD is treated with lime to produce a high-density sludge, which is sent to drying beds. The dried sludge is trucked to the open-pit portion of IMM near the top of Iron Mountain at the end of summer each year. Clean water from the treatment process is decanted and discharged to a nearby creek.
The IMM site is intensely faulted and fractured. This geologic setting, along with long-term intrusive mining activities and the relatively high average annual precipitation in Redding (approximately 35 inches), produces ideal conditions for the production of AMD – allowing copious amounts of air and water to come into contact with large areas of the sulfide deposit. In addition, in 2000, microbiologists discovered a new species of iron-oxidizing Archaea (along with plants and animals, one of the three primary forms of life on Earth) that thrives in the extreme conditions found in the mine – growing on the surface of exposed pyrite ore in pools of water so acidic that they were previously thought to be inhospitable to all forms of life. For this reason, the National Aeronautics and Space Administration (NASA) has studied the organism to aid in the search for life in harsh space environments. This organism greatly accelerates the rate of oxidative dissolution of pyrite (the main driver for AMD production). Subsequent research showed that the DNA of this bacteria changes every year – in effect, evolution on an annual basis.
Prior to cleanup and pollution control measures implementation, AMD discharge from IMM was the largest source of surface water pollution in the U.S. and accounted for one-fourth of the entire national discharge of copper and zinc to surface waters. The lowest naturally-occurring water pH for any world water body has been measured at -3.6 within the Richmond Mine at IMM. Water temperatures at IMM have been measured as high as 47˚C due to the exothermic nature of the AMD process. Historically, AMD travelled down the surface drainages and entered the Sacramento River. This was responsible for multiple extremely large-scale fish kills – at least 100,000 fish documented in 1955, 1963 and 1964.
Anecdotally, it is said that Native Americans called Spring Creek, one of the creeks running through IMM, “stinging creek” – which may be a reference to potentially naturally-occurring acidic conditions prior to mining activities. The USEPA disputes this claim.
There are many interesting stories regarding IMM. But the author will leave you with one of his favorites: NASA once sent a robot into the Richmond Mine at IMM to evaluate its performance in adverse conditions. It died!
HETI…A Resource for Environmental Services and Information
HETI professionals have extensive, in-depth experience dealing with a wide range of complex environmental issues for a variety of clients. We are available to provide information and services related to Acid Mine Drainage and mitigation methods. HETI staff can also offer expert technical assistance for a variety of mining operations – including precious metals, coal, uranium, and aggregate.