SWES researchers use compost, microbes to stabilize mine tailings
By Brooke Hamilton
Stacked tall around many of the mines in Arizona, barren, layered and usually a different color than the surrounding soil, are mine tailings. The exposed top layers of these mine tailings can be swept away by the wind and carried off into communities, posing a hazard to the people exposed to them.
Mine tailings aren't like normal soils, explained Raina Maier, a professor of environmental microbiology in the Department of Soil, Water and Environmental Science at the University of Arizona. The high salt content, heavy metals, lack of nutrients, lack of organic matter and other qualities contribute to the barrenness of the tailings’ surface.
“Because of its qualities, it remains barren,” she said. “So plants don't grow on this material.”
Maier is leading an ongoing five-year project at the Dewey Humbolt mining site near Prescott, Arizona. She and her team are seeking the most efficient methods of keeping the mine tailings in place using compost and microbes to make this previously hostile environment suitable for plants. Plants are one of the primary factors that keep soil moist and hold it in place, both of which help to keep this dangerous dust from getting into the air where people could breathe it.
“Our communities will be more and more affected by wind and water erosion of these materials,” Maier said. With increasing population, people are moving closer and closer to these mine tailings. With the increasing need for mined materials such as copper and other metals for the electronics industry, it’s unlikely that mining is going to go away anytime soon.
Meanwhile, abandoned mines can be even more hazardous than operating mines because they’re often left unmonitored, she indicated. Heavy metals like arsenic and lead are common in mine tailings, especially the older ones. While workers at operating mines will spray the tailings with water to keep the dust down, no one does this at abandoned mines.
The hazards are two-fold from the wind erosion. The dust being blown from the tailings into towns and cities can carry both small dust particles and toxic heavy metals. They can be inhaled directly from the dust in the air or accumulate in the dust in homes.
“Lead is a well-characterized neurotoxin, especially in kids that are exposed to high lead concentrations,” said Clark Lantz, professor and associate head of Cellular and Molecular Medicine at the University of Arizona, and deputy director of the Southwest Environmental Health Science Center at the university. Arsenic is also a big problem in Arizona, he noted. (For more on arsenic’s health effects, see related story.)
Another risk comes from the small size of the dust particles. When inhaled, they can exacerbate existing lung conditions like asthma or even heart disease, said Maier. To reduce the risk of these particles getting into the air, UA researchers are seeking to stabilize the tailings by making them suitable for plant growth. The amount of compost, the species of plants, and the presence of microbes all play roles in the success of the stabilization.
“We started our research in the greenhouse,” Maier said. SWES professor Jon Chorover is a co-investigator in the project. “We evaluate different composts and different compost rates and also different plants; and from there we've moved into the field, so we can translate what we do in the greenhouse successfully into the field.”
Compost is the first, and perhaps most crucial, factor to consider when attempting to grow plants in this hostile environment. Just like in a garden, compost makes the tops of these tailings a more habitable place for plants. Given time, the plants will start giving the mine tailings more soil-like qualities by adding organic matter, thus reducing the need to add compost in the future.
For the greenhouse studies, they were able to buy bags of compost from the store, Maier said, “but going out in the field, that isn't enough. You have to get huge volumes, so we actually bought compost from a dairy that takes their manure and composts it and then we can get it in truck loads.”
The species of plants also matter because they must be able to tolerate high amounts of salt in the soil and also must not accumulate toxic heavy metals in parts of the plant that could be eaten by animals. Maier and her colleagues tested plants and found that native plants like quail bush, mesquite and buffalo grass – a native not to be confused with the invasive buffelgrass – tend to be best suited to this job.
“We think that this mixture of grasses and shrubs and trees will give you different canopies and different rooting types that will do a better job in the long term of stabilizing the tailings.” remarked Maier.
To help keep the cost down and keep this an attractive option for mining companies to employ, Maier also works on finding the microbes, like bacteria and some fungi, that help plants grow best. So far they have found two or three in greenhouse studies that seem to be especially useful. Some of them even stabilize the metals by changing them into different forms so they aren't as toxic or don't get moved as easily by wind or water.
The old methods for keeping tailings in place involved things like covering them in a layer of gravel or concrete, both of which can be expensive and ugly. Also, this “cap” will eventually degrade.
“The mining industry is very interested in this [method], as are our regulatory agencies like EPA,” said Maier about using plants to keep the tailings in place. There is a lot of interest in revegetation because it's cheaper than a concrete cap or even a gravel cap ... it's more visually pleasing, and we actually think that in the long term the (heavy) metals will become more stable with a plant cap than if you have a different type of cap.”
Success is measured in a few different ways and involves more than just the team of students and scientists working with Maier.
To measure how successful the plants are, researchers measure the percent of ground directly covered by plants; the higher the percent coverage, the better the plants are doing. Maier's team assesses each site on a species-by-species basis as well, looking at what percent of the canopy cover is being provided by each of the six species planted at the field test site.
Maier is also working with a team from Atmospheric Sciences at the university to measure the numbers of particles coming off the mine tailings sites. This will provide them with real data about how the effectiveness of plants in keeping dust from the mine tailings from entering the atmosphere. The data for measuring the wind erosion are still preliminary.
Other researchers are examining the forms in which the heavy metals exist to see if they change into less harmful forms with the revegetation project in the field.
Although other projects in the country use vegetation to stabilize mine tailings, little research has been done in the unique climate of the Southwest.
“Very few rigorous studies have been done to show how long these vegetative caps can last,” Maier said. Her research group is now focusing on recording the long-term viability of this method in the arid Southwest for the next five years. Along the way, they’re tweaking their methods to get maximum efficiency. For instance, Maier hopes that the right combination of microbes might allow them to get by with less compost.
“I feel that the research that we're doing is important not only for our arid environment here, but globally for arid environments, and that's really cool.” Maier said. These methods could be applied in places such as Peru, Chile, northern Mexico and Australia as well.
This research is funded by the NIEHS, or the National Institute of Environment Health Sciences under the National Institutes of Health. Maier’s project is part of the 22 year-old UA Superfund Program.
Brooke Hamilton is a junior majoring in environmental biology at the University of Arizona and plans to attend medical school after graduating. She is interested in the connection between environmental and health issues.
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