How Climate Affects Mining Waste Tailings

It makes sense that weather affects mining tailings, the waste material stored at mining sites. Rain, snow, humidity, wind, heat and cold physically and chemically change tailings rocks and minerals. These conditions can cause waste to move to places that could pollute waterways or air, adversely affecting life.

But what do weather conditions do to tailings at a specific mine at a specific location? A two-year examination hopes to develop a model that mining companies can use to determine what happens when tailings weather.

“It helps them to predict the environmental risks in an environmental assessment … before they start,” explains Jon Chorover, head of the University of Arizona’s Environmental Science department. “This also relates to assessing legacy sites.”

Doctoral student Xenia De Gracia Medina, with support from UArizona’s Mineral Resources Research and Innovation Initiative, has started experiments that allow researchers to see what happens to tailings material when subjected to certain conditions specific to where the material was collected.

Lab experiments aim to reveal results of weathering.

Last year De Gracia Medina and others visited 12 old, also known as “legacy,” sulfide ore mining sites in Arizona, California, Montana, Nevada and Oregon. Freeport MaMoRan and BHP were among the industry partners who provided access to sites.

They collected tailings columns two meters deep at each site. The core samples come from both the top and bottom of the tailings. These samples have elevated levels of acidity, arsenic and lead, the result of their origin and weathering over their 50- to 100-year lifespans. Continued weathering may eventually release some of this arsenic and lead into water and air, adversely affecting the environment.

In De Gracia Medina’s experiments, the samples are packed into clear tubes with sensors and water samplers. The tubes were specially made for this experiment so that researchers can watch in real time what diagenetic alterations—that is, chemical and physical changes—occur when the samples are exposed to oxygenated water meant to simulate weathering conditions.

Each tube’s contents come from distinct climate areas. The water is applied at the top of the column and seeps through it to the bottom. Probes along the column length take samples of the tailings and the water solutions for chemical analysis.

“We want to know how the environmental conditions—precipitation, temperature, humidity—of each site influence in the presence of metals and metalloids at different depths in the tailings,” says De Gracia Medina, “and how the reactions of the minerals stored in them change the porosity of the medium.”

Over the next two years, De Gracia Medina and subsequent researchers will refine their understanding of the effects of tailings weathering. That includes rates of reaction and the nature of the physical and chemical changes. Eventually, “we can develop a model that predicts what are the weathering reactions,” Professor Chorover says. Mining companies could use those models to inform them on how to contain those processes so that the surrounding environment is protected from negative effects.

This work is part of a larger effort by the National Institute for Environmental Health Sciences to characterize the differences in tailings weathering across climates in the Western U.S. The overall study is part of the Superfund Research Program.

School of Mining and Mineral Resources makes this work possible.

The initiative’s funding for De Gracia Medina’s work comes from the UArizona’s School of Mining and Mineral Resources. Among other missions, the program aims to attract team research and education that is collaborative and interdisciplinary. This project supports the UArizona’s Superfund Research Center, which focuses on hazardous waste issues in the U.S. Southwest.

Panama native De Gracia Medina has long combined her interest in environmental science with associated challenges in mining. She holds bachelor’s degrees in environmental engineering and geological engineering from the Technological University of Panama. She worked at the Cobre Panama mine when she secured a Fulbright scholarship to study water quality in mine sites.

“The University of Arizona captured my attention because of its very well-ranked water resources program,” she says, “which led me to graduate with a master in hydrology.” She’s pursuing her doctorate in soil and water. Her wants to work for environmental companies that investigate hydro-biogeochemical processes in mining and find ways to reduce polluting effects.

“Humanity relies very much on minerals,” she explains, “and to obtain them there are wastes that need to be managed to reduce their environmental impacts and to create a more sustainable mining industry.”