Plugging Up Old Mines

David Pacchioli
March 01, 1995

Pennsylvania, where coal has been mined for 200 years, is riddled with spoil piles, scarred earth, and abandoned tunnels.

"We're still finding them," says Louise Michaud, assistant professor of mining engineering. "A lot of these were old mom-and-pop operations, with no records of their locations."

There weren't environmental-impact regulations, either, when most of these mines were dug. "Fifty years ago, people weren't aware of the problems."

The problems still remain. Chief among them is acid mine drainage, which devastates life in many of the state's rivers and streams.

The rock of the Appalachian region is high in pyritic material, which contains sulfur. Exposed to air and water, it reacts to form sulfuric acid. "This reaction is one that occurs naturally all the time," Michaud notes. "But with mining we've increased it possibly a million-fold."

The run-off from exposed pyritic material can have a pH as low as 3, far below neutral, at 7. (8 and above is alkaline.) "Once you get under 5," Michaud says, "you begin to kill off aquatic life—fish and vegetation." Because of the effects of acid mine drainage, in combination with acid rain and the "natural" effect of the local mineralogy, the average pH for Pennsylvania streams is around 5.5.

Strict environmental regulations now control mine drainage and water quality. But drainage from abandoned sites is an ongoing problem, one that defies easy solutions.

Acid water can be treated. "You just dump in limestone or any other alkaline material," Michaud notes. But neutralization is an expensive, and open-ended, proposition.

Bioremediation, a better long-term approach, attempts to adapt natural systems, such as wetlands, to clean-up needs. Under the right conditions, constructed wetlands act as buffer zones, employing naturally occurring bacteria to convert potential pollutants into more stable, and relatively innocuous, mineral forms.

"When we first started using them," Michaud says, "some people thought wetlands would solve all our problems.

"They are good for samll seeps," places where a mine discharge can be pinpointed. "But there are basic limitations. They can't handle a pH below 4. They can't handle really high levels of metal. And they can't handle large volumes of water."

Much current research, Michaud adds, has focused on finding complementary approaches. She and others at Penn State are working on one: Filling up old mines with alkaline cement.

"Think of this room as an underground tunnel," she says, motioning to the walls of her office. "If you can fill a void like this, or even coat its surfaces, you can block off reactive sites and prevent the sulfur-based reactions from occurring in the first place." And by leaving the surfaces of tunnel walls alkaline, one can neutralize the groundwater that flows through.

But where to acquire all that cement? Michaud pulls out a small glass vial, half-full of a dark-gray powder, and hands it across the desk. A typed label answers the question: "Fly ash."

"Fly ash," Michaud explains, "is a major waste product of coal-burning power plants. They produce millions of tons of it." Typically, this waste is trucked off-site for disposal. With recent changes in landfill regulations, however, disposal of fly ash has become an increasingly expensive proposition.

As it happens, Michaud continues, fly ash is generally alkaline in composition. Furthermore, she says, "Some of it is naturally possolanic," that is, it will bind as cement when you add water. She pulls out another sample: a small gray brick.

"The idea is to mix fly ash with water and pump it down into the mine, where it will set up as cement."

Michaud is heading a $250,000 project, funded by the state and federal government and the mining and power industries, to determine whether fly-ash injection can work.

"The hydrology of the site is very important," she says. "Everything has to fit with groundwater flows."

Then there are specific technical problems to be solved: The cement itself, for instance, "has to have the right properties, the right set-up time. There may be heavy metals in fly ash that must not leach out." A group at Penn State's Materials Research Lab, headed by materials scientist Mike Silsbee, is working on the optimal cement formula.

The logistics of injection are tricky, too, Michaud says. "You're talking about old mines. There's no access to them. Everything has to be done blind, from the surface. You could just fill up the mine voids with cement, but what about the cost involved?" The biggest expense, she estimates, will be for drilling the 400 to 500 foot holes necessary to reach old mine workings.

Michaud must rely on computer models to pull these complexities together. Next year she intends to set up pumping trials at a mine site.

Already, the promise of a technology that could solve two environmental problems at once has attracted considerable attention. "The Bureau of Mines and the state Department of Environmental Resources have called me," she notes, and she has been approached about setting up a similar study in Poland.

The focus for now is on remediation—undoing the lingering damage of the past. Ideally, however, Michaud says, fly-ash injection is something that could eventually be incorporated directly into mining practice.

"You could go along, mine an area, and fill in your voids as you go."

Louise H. Michaud, Ph.D., is assistant professor of mineral engineering in the College of Engineering, 121 Hosler Building, University Park, PA 16802; 814-863-1641. Michael R. Silsbee, Ph.D., is research associate and assistant professor in the Materials Research Laboratory. Derek Elsworth, Ph.D., associate professor of mining engineering, and Chris Eaker, a graduate student in environmental pollution control, are also involved in the fly-ash injection project. The project has received funding from the National Mine Land Reclamation Center, the Pennsylvania Energy Develoment Authority, PENELEC, the Florence Mining Co., the U.S. Bureau of Mines, and the Energy and Fuels Research Center at Penn State.

Last Updated March 01, 1995