Mine Tailings

Mine Tailings

What are mine tailings?
Mine tailings are large piles of crushed rock that are left over after the metals of interest like lead, zinc, copper, silver, gold and others, have been extracted from the mineral rocks that contained them.

The mineral separation process, especially in older mining operations, is only partially efficient. As a result, after the crushing and grinding (milling) processes, some of the metal-containing minerals are left behind as small tailings particles.

Why are mine tailings an emerging issue?
According to a U.S. Environmental Protection Agency (USEPA) Region 9 assessment of state data, there are approximately 420,000 abandoned mines in the states of California, Arizona and Nevada. A total of 13,243 of those are considered “abandoned mines with potential environmental hazard”. There are even larger numbers of sites in northern Mexico.

Saginaw Hill area, Tucson, Arizona. For more information about the Saginaw Hill area, click on the image above. Photo Credit: Janick F. Artiola.

In arid and semi-arid areas of the world, including regions of the western United States and the northern region of Mexico, mine tailings and their associated metal contaminants, such as arsenic, lead, and cadmium, are prone to wind (eolian) dispersion and water erosion. Wind dispersion occurs because the fine tailings particles (silty sand-like material) and their associated contaminants are easily suspended into the atmosphere by wind and dispersed throughout the environment as dust particles. Spread of metal toxicants in association with tailings particles, through a combination of wind dispersion and water erosion, has been shown to result in measurable elevated levels in wildlife and humans even significant distances from the tailings site.

These problems are extensive and can persist for decades because these sites have low pH and lack normal soil stabilization processes. As a result these sites do not develop normal soil structure or support the establishment of a plant cover. Many of these mining areas have been economically developed within the last century, and as a result, are now located next to large urban populations. Thus, there are both human and ecological health impacts that can occur from exposure to dust that is blown from these sites. These issues are of particular concern with regard to children in nearby communities and for sensitive riparian or wildlife refuge areas.

Map of UA withthe Pacific SW highlighted

EPA Region 9 (in green) serves the southwestern United States (Arizona, California, Nevada, and Hawaii), along with 147 federally recognized tribes in the Pacific Southwest. Image credit: www.epa.gov/region09/reg9bck.html

Diagram Credit: Monica D. Ramirez, 2007. Go to the end of this page to download the informational brochure entitled: What are Mine Tailings?

What is Dust?
Dust is a generic term used to describe particulate matter, a mixture of fine solid particles and liquid droplets, suspended in the atmosphere. Dust is formed when these fine particles are taken up into the atmosphere by wind or other physical disturbances. Aside from wind blowing on mine tailings, there are other mining activities that can also generate dust. Dust is typically classified according to its particle size (see illustration below):

PM10 refers to particles 0.01mm in size or less (coarse particles)
PM2.5 refers to particles 0.0025 mm in size or less (fine particles)

Particle size is an important factor influencing the dispersion and transport of dust in the atmosphere and the effects of dust on human health.

What is the UA SBRP doing to address this emerging issue?
Current UA SBRP research (Project 10) is investigating easy, low-cost ways to revegetate mine tailings to reduce both wind dispersion and water erosion. Revegetation is done using native plants and optimized to allow the minimum amount of site preparation, fertilizer application, and maintenance. This process is called phytostabilization - the establishment of a vegetation cover using native plants. Key for phytostabilization is that the plants do not accumulate toxic metals in their shoot tissues. Phytostabilization would replace traditional methods such as removing or disposing of the tailings or capping with either clay or asphalt.

What type of field studies are being conducted by the UA SBRP project team?
Study sites include those that range from severely and moderately impacted. These sites vary in pH from neutral to extremely acidic and in metal content from background to greater than 2% (e.g., for lead and zinc).

The Klondyke Tailings State Superfund (also referred to as an Arizona Water Quality Assurance Revolving Fund (WQARF)) site (see photo) in Aravaipa Valley, Graham County, Arizona has an acidic pH and high metal content, and is therefore considered highly impacted. Mine tailings here have the consistency similar to flour.

In a greenhouse study using a native, salt and drought tolerant plant species quailbush, Atriplex lentiformis germination, growth and metal uptake was evaluated using two Klondyke mine tailing samples at pH 3 and pH 6, amended with 25%, 15%, 10%, 5% and 0% compost. For a complete summary of the UA SBRP methodologies and results at the Klondyke Tailings site, please download:

Phytostabilization of Acidic Mine Tailings in Aravaipa Valley, Graham County, Arizona-Klondyke State Superfund Site (pdf) or the publication entitled: "Phytostabilization Potential of Quailbush for Mine Tailings: Growth, Metal Accumulation, and Microbial Community Changes" (pdf).

The Boston Mill mine tailings site (see photo), adjacent to the San Pedro River in southern Arizona, in contrast to the Klondyke Tailings site, has a neutral pH and high metal content. It is considered moderately impacted. A mixture of native forb and grass seeds have been applied to establish a vegetative cover on 1.5 acres at the San Pedro River mine tailings site. The site will be monitored for two years following seeding to measure growth and to determine shoot metal uptake in selected plants. For a complete summary of the UA SBRP methodologies and results at the Boston Mill site, please download:

Phytostabilization of Neutral Mine Tailings within the San Pedro River National Conservation Area in southern Arizona (pdf) or the publication entitled: "Bacterial Community Changes during Plant Establishment at the San Pedro River Mine Tailings Site" (pdf) .

The UA SBRP Project 10 research team has identified several native species that are good candidates for phytostabilization of mine tailings in semi-arid regions of the U.S. Southwest and northern Mexico. In combination, organic matter amendments may be required depending on the extent of pH, metal, and microbial community stress that exists in a given site. Attributes of the native plants include drought and metal tolerance and low accumulation of toxic metals such as As, Cd, or Pb into shoot tissues.

Plants used in study
Atriplex lentiformis, an example of one of the plants used for the
phytostabilization studies.

For more information regarding the research objectives, go to:

Phytostabilization of Mine Tailings in the Southwestern United States: Plant-Soil-Microbe Interactions and Metal Speciation Dynamics (PI: Raina Maier)

For more general information about mine tailings and dust, download the informational brochure entitled:

What are Mine Tailings? (pdf) or Que son los Jales Mineros? (pdf)

All contents and materials © 2008, Arizona Board of Regents. All rights reserved.

Illustration Credit: Monica D. Ramirez, 2007. Go to the end of this page to download the informational brochure entitled: What are Mine Tailings?

Klondyke WQARF site

The Klondyke Tailings State Superfund site in Graham County, Arizona. Click on image for more information. Photo Credit: Raina Maier.

San Pedro mine tailing site

The Boston Mill mine tailings site, adjacent to the San Pedro River in southern Arizona. Click on the image to learn more about the demonstration project at the Boston Mill mine tailings site, funded by the Bureau of Land Management. Photo Credit: Chris Grandlic.