Filling the Pit

By Dana Bauer

Locals call the 400-acre plot of scarred land just south of Hazelton, Pennsylvania, the “Big Gorilla.” Piles of leftover coal hulk over the brown earth. A 90-foot-deep pit filled with acidic water covers almost 17 acres. Hundreds of feet below the pool, abandoned tunnels snake along a coal seam. Acid mine drainage flows into the nearby Little Schuylkill River.

Abandoned mine sites, remnants from the days when anthracite coal made the region an economic powerhouse, mar northeastern Pennsylvania. At this one, a team of Penn State geologists is working with a local coal plant to reclaim the land and reduce acid mine drainage. They’ve started by filling the 135-million-gallon surface pool with coal ash.

“We’re taking an area that’s not productive. Then we’re taking coal ash, a residual material from the coal plant that is usually landfilled, and we’re using it to make the land useful,” says graduate student Caroline Loop.

The plant near the Big Gorilla burns culm — a rocky by-product of anthracite coal mining — in a fluidized bed combustor. “It’s called fluidized because limestone is sprayed in and it floats above the burning coal,” explains Barry Scheetz, professor of materials science and one of Loop’s advisers. The limestone absorbs the sulfur in the culm and reduces sulfur-dioxide emissions. “The culm has a lot of non-combustible material in it,” Loop explains. Burning it creates ash — a reddish powder of quartz and clay with a high calcium oxide content.

Dumptrucks holding 40-ton loads move the ash from the coal plant to the mine pool. When the ash is poured into the pool, it flows like a river along the bottom. “The CaSO4 in the ash sucks up the water in the pool and forms a plaster. The mixture sets up like cement,” says Scheetz. A cemented platform forms at the top of the pool while the ash slurry continues to flow underneath. “The cemented ash is dense enough for a dumptruck to drive across it,” Loop says. “We’re creating pseudo rock.”

As part of her master’s thesis in environmental pollution control, Loop has been studying how the chemistry of the water in the mine system changes as the coal ash is added to the pool.

The pH of the acid pool was originally 3.6, about the same as vinegar. After two and a half years of dumping ash, the CaO in the ash changed the pH value to 11.6 — closer to the pH of household ammonia. Alkalinity went up and the metals that were already present in the surface pool — aluminum, manganese, and iron — formed metal oxides that sank to the bottom. Calcium carbonate, CaCO3, has been precipitating from the water, forming a white rim around the walls of the pool.

Loop has been running computer models, simulating the mixing of water and ash, to determine the chemical species that will originate over a long period of time.

She says there is no indication so far that the change in water chemistry in the surface pool has affected the water in the deep mines beneath. “There’s only one outlet that drains the mine system, so it’s easy to test the outflow,” says Loop. “The pH hasn’t changed. It’s still really low, around 4. A normal pH range for healthy rivers and streams ranges from 5 to 8.” Loop thinks the basic water from the surface pool is consumed by the large reservoir of acidic water in the deep mines.

Loop is also tracking trace metals, including arsenic and mercury, that are naturally present in both the acid pool and the ash. She suspects that the metals are trapped in the cemented ash, or settled at the bottom of the pool with the metal oxides. “We’re not detecting trace metals in the water that remains in the surface pool, or in the outflow waters,” she says.

Loop’s doctoral studies in geosciences will focus on understanding what happens to the trace metals and precipitates in the acid pool.

In the meantime, the coal plant continues to dump ash into the pool. Loop estimates that about 30 percent of the pool is filled with cemented ash. “We’ve poured almost 600,000 tons into it,” she says. “It will probably take about five years to fill it completely.” When it is filled, the owner of the coal plant plans to cover the land with four feet of topsoil and reseed it.

Caroline Loop is a graduate student in environmental pollution control and geosciences; loop@psu.edu. She won first prize in the physical sciences category at the 2000 Graduate Exhibition. Her advisers are Barry Scheetz, Ph.D, professor of materials science and civil and nuclear engineering in the College of Earth and Mineral Sciences and the College of Engineering, 107 Materials Research Lab, University Park, PA 16802; 814-865-3539; se6@psu.edu; and William White, Ph.D, professor of geosciences in the College of Earth and Mineral Sciences, 210 Materials Research Lab; 865-1152; wbw2@psu.edu. Funding for this project was provided by the Northeastern Power Cogeneration Plant and the Pennsylvania Department of Environmental Protection.