Mine Tailings Restoration Method uses high grade biosolids to revegetate land By Susan McGinley ine tailings and biosolids don’t sound like they go together, Mbut combining the two in the proper ratio actually produces a medium that can support living plants. (The prod- uct of wastewater treatment of sewage is known as biosolids.) Ongoing research conducted through the University of Ari- zona, National Science Foundation Wa- ter Quality Center (W-Q-C) offers a so- lution for covering unsightly bare mine tailing piles while at the same time dis- posing of sewage sludge without affect- ing groundwater quality. Tests con- ducted at commercial sites in southern Arizona show that the remediation method does not release heavy metals or excess nitrogen into the soil. I. Pepper I. Mine tailings, the waste material left over from processing ore, are essentially tons of crushed Mine tailings unamended. rock with zero percent Why mix mine tailings with biosolids In cooperation with ASARCO and way beyond that percentage at these in the first place? Because each has Pima County Wastewater Management sites.” The biosolids proved to be ben- what the other lacks. Mine tailings, the Department and with the approval of eficial in adding essential plant nutri- waste material left over from process- the Arizona Department of Environ- ents, amending the soil with organic ing ore, are essentially tons of crushed mental Quality, Pepper and soil scien- matter, and enhancing soil microbial rock with zero percent organic matter. tist Ed Glenn began their research at the populations. The resulting vegetation Deposited in ten-foot “lifts,” they are Mission Mine in Green Valley in De- stabilized the mine tailings and re- extruded like a thick toothpaste that cember 1998. Biosolids were added at duced wind erosion. dries out, according to Ian Pepper, di- the rate of 80 dry tons per acre to a 3.75- “This is important, because the raw rector of the W-Q-C. Thousands of acre site that was seeded initially with tailings are very prone to air pollution these raised, barren piles dot the South- native Southwest grasses. The area was from dust,” Pepper says. Although he west. monitored for plant growth and for the was pleased with these benefits, Pep- “Nothing grows on them; they look presence of nitrates and metals. In De- per admits he was concerned that the like the surface of the moon,” he says. cember 2000 the first biosolid amend- tremendous amount of organic nitro- “We wondered if we could take ment was added to a 20-acre site, also gen going into the ground from the biosolids that are primarily all organic at the Mission Mine, as a real-time dis- biosolids could cause seasonal nitrate materials and add them to mine tail- posal from a wastewater treatment nitrogen (a pollutant) buildup in the ings — the perfect inorganic matrix — plant in Pima County. Approximately groundwater. During the monsoons the to make an instant ‘soil.’ The answer is 150 dry tons per acre were applied to moisture causes mineralization and yes, you can. As soon as you add the that site, then seeded and monitored. rapid nitrification of nitrogen, particu- biosolids to the mine tailings, the mi- Neither site was irrigated, yet today, larly in surface soil. Yet the research- crobial population jumps from 400 per grasses and shrubs cover the once stark ers found the nitrates didn’t leach, es- gram in the tailings to about 10⁷ or 10⁸ landscape. pecially during the cooler winter million per gram in the created soil. “Given the statistics, natural indig- months. This microbial population is a prereq- enous Sonoran Desert is only about 20 “Heterotrophic bacteria feed on some uisite for growth.” percent cover,” Pepper notes. “We go of this. We speculate that massive 18 The University of Arizona College of Agriculture and Life Sciences The Water Quality Center The University of Arizona, Na- tional Science Foundation Water Quality Center (W-Q-C) investi- gates physical, chemical and micro- bial processes that affect the qual- ity of surface and subsurface wa- ters including potable supplies. Housed in the UA College of Agri- culture and Life Sciences Environ- mental Research Laboratory in Tuc- son, the center includes an inter- disciplinary group of biologists, chemists, physicists, hydrologists, and engineers who work together to resolve water quality problems. Undergraduate and graduate stu- I. Pepper I. dents also participate in conduct- ing research, publishing and present- Mine tailings three years after biosolids amendment. ing papers, and thus gain broad in- dustry perspective and industrial job opportunities. Funding for the amounts of denitrification were occur- also the benefits and hazards of apply- center is supplied by companies and ring, releasing the nitrogen as gas,” ing liquid digested sludge for agricul- agencies interested in specific wa- Pepper explains. “There was an over- tural crop production. Since 1983 ter quality issues, and by the Na- all loss of nitrogen from the system biosolids have been put on agricultural tional Science Foundation. without detrimental effects on under- land in Marana, north of Tucson, but Research focal areas include wa- ground aquifers.” farmland there is becoming scarce, and ter security; the fate and remediation of commercial and in- Another potential problem was the this method offers an alternative av- dustrial contamination; agrochemi- possibility that the mine tailings still enue of disposal. The W-Q-C is also ex- cal products and practices that in- contained high metal concentrations, amining bioaerosols, chemical odors, fluence water quality; municipal pathogen transport through soil, and particularly copper and molybdenum, waste treatment and reuse; mining; methods for upgrading of class B that could leach into the groundwater; and potable water quality. biosolids to class A. biosolids are characteristically low in “It is the dynamic relationship metals. Again, there were protective The Arizona Mining Association, min- between the private sector and the factors: the lack of irrigation inhibited ing companies, California’s Los Angeles university, and the integration of the leaching, and Pepper found that and Orange County Sanitation Districts, university with industry and govern- biosolids have a tendency to tie up and several agribusinesses have shown mental agencies that makes this complex metals within the mine tail- an interest in this remediation work and NSF Water Quality Center unique,” ings, making them less bioavailable. It the potential for its application on a more says Ian Pepper, director of the should be noted that neither site was widespread basis. Center. “In addition, it is the com- monitored for the presence of patho- bination of university expertise and gens because the treated sludge used corporate funding that leads to sci- in the research was rated class A “ex- CONTACT entific discoveries that can enhance ceptional quality,” or pathogen-free. Ian Pepper water quality for the community at This study was designed to evaluate (520) 626-3328 large.” [email protected] not only the revegetation aspects of adding biosolids to mine tailings, but For more information: wqc.arizona.edu 2002 Agricultural Experiment Station Research Report 19.