The phrase “coal ash” made headlines this week after a dam on a lake at the site of a power plant in Wilmington, North Carolina, was breached, allowing the hazardous ash into a river that supplies drinking water to much of the southeastern part of the state.

The plant that was shut down, owned by Duke Energy, had been a growing concern since last week after heavy rains associated with Hurricane Florence caused a coal ash landfill at the site to erode, spilling ash onto a roadway.

What Is Coal Ash?

Coal ash is the powdery substance that remains after burning coal.

What remains after coal is burned includes fly ash, bottom ash and so-called scrubber sludge, said Lisa Evans, chief counsel to Earthjustice, an environmental law organization.

The sludge, which is created from solutions sprayed inside exhaust stacks to capture the harmful chemicals that cause acid rain, sulfur dioxide and nitrogen oxide, falls to the bottom.

All of those remnants are mixed with water and then sent to vast retaining ponds or impoundments near the coal-burning power plants. An estimated 110 million tons of coal ash is produced each year, Evans said.

Why Is It Hazardous?

The remnants of burned coal include arsenic, boron, lead and mercury, which are known carcinogens and damage organs, among other health effects.

Is the Ash Really Held in Ponds?

No, according to Evans. That is the colloquial description of these basins, but it is really a misnomer given their size, she said.

“It’s a hard thing to get a word for because the EPA calls it a ‘surface impoundment,'” she said.

The largest of them is 1,300 acres in Pennsylvania, with a more typical size around 100 acres, she said. There are about 1,000 such ponds documented in the country, although the number is likely higher because not all have been inventoried, she said.

While there is a high concentration of these basins in coal states like Illinois, Indiana, Ohio, Pennsylvania and West Virginia, the largest concentration is in southeastern states such as Georgia, North Carolina and South Carolina, which are more prone to hurricanes.

What Is the Problem With These Basins?

Historically, these ponds have essentially been unlined pits that store “an alphabet soup of chemicals that are toxic to human health,” Evans said.

Data collected by the federal Environmental Protection Agency found that 95 percent of them had leaked, seeping ash into rivers and groundwater supplies.

How Is Coal Ash Storage Regulated?

EPA rules that took effect in 2015 required that the ponds be inspected for structural stability, have groundwater monitoring systems and be cleaned up if contamination is found.

The rules, however, did not address power plants that had been retired, allowed unlined impoundments to operate indefinitely and regulated the ash as nonhazardous solid waste, Evans said.

The coal industry and environmental groups sued federal regulators. The industry thought the rules were too stringent, and environmental groups thought they did not go far enough.

After President Donald Trump took office, the EPA entertained with “lightning speed” a petition to roll back the rules.

A federal appeals court, however, recently issued a ruling arising from the lawsuit, sending the EPA “back to square one” to revisit parts of the regulations related to unlined coal ash ponds and ones at retired plants, Evans said.