As the nation’s coal-fired power plants work to create cleaner skies, they’ll likely fill up landfills with millions more tons of potentially harmful ash.

More than one-third of the ash generated at the country’s hundreds of coal-fired plants is now recycled — mixed with cement to build highways or used to stabilize embankments, among other things. But in a process being used increasingly across the nation, chemicals are injected into plants’ emissions to capture airborne pollutants.

That, in turn, changes the composition of the ash and cuts its usefulness. It can’t be used in cement, for example, because the interaction of the chemicals may keep the concrete from hardening. That ash has to go somewhere — so it usually ends up in landfills, along with the rest of the unusable waste.

Coal ash naturally contains arsenic and mercury, and if the elements leach into groundwater they can contaminate drinking supplies. The EPA says ash disposed of in landfills could pose significant risks when mismanaged, and there are gaps in state regulation.

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