DURHAM, N.C – Mixing heavy metal-laden coal ash into acid mine drainage may sound like an odd recipe for an environmental solution, but a new Duke University-led study finds that the mixture can neutralize the drainage’s dangerously low pH and help reduce harmful impacts on downstream ecosystems — if the right type of ash is used.

Using the wrong type of ash might not neutralize the drainage and could release arsenic, selenium, boron, and other toxic contaminants into local waters at levels that exceed safe standards for drinking water and ecological health, the study shows.

The findings come at a time when mixing coal ash and acid mine drainage is becoming an increasingly accepted remediation practice in parts of the Appalachian coalfields and other regions dealing with decades-old legacies of contamination from underground coal mining.

At several sites in West Virginia, for instance, instead of dumping coal ash in landfills, companies are putting it in abandoned mines with the intent of stemming the flow of contaminated drainage at its source. They’re finding a beneficial use for the large volume of toxic ash generated by the region’s coal-fired power plants.

“There’s a perception, based on some valid but narrowly focused past studies, that you can take these two harmful things and put them together to make them less harmful,” said Avner Vengosh, Distinguished Professor of Environmental Quality at Duke’s Nicholas School of the Environment. “Our study, which looks at outcomes for a broader range of contaminants using coal fly ash from five different regions, shows that’s not always the case.”

“You might make the problem better, or you might create a whole new problem. It depends on which coal ash you use,” Vengosh said.

To test the efficacy of the practice, Vengosh and his team conducted 49 separate laboratory experiments on coal fly ash samples from the three major coal-producing U.S. regions — the Appalachian Basin, Illinois Basin, and Powder River Basin — as well as coal fly ash from two of India’s major production regions, the Gondwana and Northeastern Tertiary basins.

They mixed each sample with a simulated acid mine drainage solution at solid-to-liquid ratios typically used in remediation, and then measured the results after 24 hours and five weeks.

Only the fly ash from the Powder River Basin, located in the western U.S., produced the desired result of effectively neutralizing acid mine drainage without causing significant secondary contamination.

Fly ash from the Appalachian and Illinois basins efficiently neutralized the acidity of the mine drainage and removed some heavy metals, as the past studies showed, but also triggered chemical reactions that caused increased leaching of other contaminants, including arsenic, selenium, boron, thallium, molybdenum, chromium, and antimony.

Ash from the two Indian basins produced less secondary contamination but, because it contains less organic matter and fewer oxides than ash from the three U.S. basins, was less efficient at taming the acidity of the mine drainage.

“Like humans, not all coal ash is alike. Ash from each region has a different chemistry and creates different reactions when mixed with acid mine drainage,” Vengosh explained.  “At most sites in the eastern U.S. coalfields they are currently using local coal fly ash from the Appalachian Basin, which is the wrong type. They need to use Powder River ash instead.”

Vengosh and his colleagues published their peer-reviewed findings August 22 in the journal Fuel.

Rachel Weinberg, a 2021 Master of Environmental Management graduate of Duke’s Nicholas School, was a member of Vengosh’s lab during her studies and served as lead author of the study as part of her master’s project. She is now a stormwater consultant at Raftelis in Cary, N.C.

Co-authors on the new paper were Nicholas School PhD student Zhen Wang; recent PhD graduate Rachel Coyte, now a postdoctoral researcher at The Ohio State University; and Debabrata Das of the Centre for Advanced Study of Geology at Panjab University in India.

Funding came from the Duke University Energy Initiative, Earthjustice, and India’s Ministry of Coal.

CITATION: “Water Quality Implications of the Neutralization of Acid Mine Drainage with Coal Fly Ash from India and the United States,” Rachel Weinberg, Rachel Coyte, Zhen Wang, Debabrata Das and Avner Vengosh. Fuel, August 22, 2022. DOI: 10.1016/j.fuel.2022.125675