Dozens of toxic coal ash piles across the Southeast are in the path of what is forecast to be days of torrential rains and flash flooding from Hurricane Florence.

Environmental advocates are warning that the giant impoundments, often built beside waterways, are at risk of spills or collapsing.

They've seen what extreme rainfall can do: When Hurricane Matthew crossed North Carolina two years ago, it caused a breach in a cooling pond, and coal ash leaked from a nearby coal ash basin at a power plant on the Neuse River.

That was a Category 1 hurricane. Florence was headed toward the coastal Carolinas as a much more powerful storm, and it carried another threat: Meteorologists warned that Florence was looking a lot like Harvey, a slow-moving storm that parked itself over Houston last year and inundated parts of that city with 60 inches of rain.

The National Weather Service on Wednesday warned of "life-threatening, catastrophic flash flooding and significant river flooding" over portions of the Carolinas and Mid-Atlantic states from late this week into early next week as Florence arrives and moves inland.

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For the region's massive coal ash sites, days of torrential rain and swollen rivers could become a perfect storm.

"Unless you have been on a river or lake and seen these up close, it's tough to realize how high these are piles," said Sam Perkins of the Catawba Riverkeeper Foundation, which monitors waterways in some 5,000 square miles in the heart of the Carolinas. "Sometimes coal ash is piled 100 feet high."

EarthJustice, with its team of lawyers, has been pressuring the federal government and utilities to clean up problems with coal ash storage for years. It counts 71 coal ash surface impoundments at power plants in Georgia, South Carolina, North Carolina, Virginia and Maryland, most often right next to rivers.

These coal ash ash impoundments can be overcome by floodwaters or their levees can breach during heavy rain and flooding, spilling toxic waste laden with arsenic and heavy metals that could contaminate rivers and potentially drinking water supplies, said Lisa Evans, a senior attorney with EarthJustice.

There are also 29 ash dumps across those states—landfills that could pose landslide risks if they have open faces. And coal ash isn't the only risk: an Environmental Protection Agency official told CNN the agency was monitoring nine superfund sites in the path of the hurricane, and several nuclear plants in the region said Tuesday they had started to prepare for the storm.

Where Are the Greatest Coal Ash Risks?

The biggest coal ash threats from this storm are likely to be in North Carolina and Virginia, where utilities have been slower to clean up the ash piles, said Frank S. Holleman III, senior attorney with the Southern Environmental Law Center, which has pressed utilities to clean up its coal-burning wastes. South Carolina utilities are farther along on their ash cleanup efforts, he said.

"If Duke Energy and Dominion would simply remove its coal ash from all its unlined waterfront pits and move its ash to safe, dry, lined storage away from waterways or recycle it for concrete, then there would be nothing to fear from coal ash when a hurricane makes landfall in North Carolina or Virginia," Holleman said. Instead, he said, millions of tons of coal ash remain in the impoundments and at risk of catastrophe during hurricanes and floods.

Duke spokeswoman Paige Sheehan said the utility has made sure extra staff and equipment are ready to respond to any issues that may arise at its power plants and will inspect ash impoundments after heavy rains. Impoundments near the coast already have lower levels of water in them due to ongoing cleanups and can hold more water now, she said. She said the company's ash ponds were in various stages of being cleaned up.

Dominion Energy did not respond to a request for comment.

Holleman said several coal ash disposal sites in the coastal area at special risk, including one on the Cape Fear River near Moncure, North Carolina, that has a history of cracks in its dams, and the Lee power plant where coal ash leaked during Hurricane Matthew.

In Virginia, a recent scientific report commissioned by the Southern Environmental Law Center looked at a coal ash disposal site near Norfolk and found that about 2 million tons of coal ash stored in unlined pits along a branch of the Elizabeth River were "highly vulnerable" to hurricanes and sea level rise.

Environmental advocates are making plans to do flyovers of the region as soon as possible after Florence passes to conduct surveillance on the ash impoundments, said Donna Lisenby, a global advocacy manager for the Waterkeeper Alliance. This week, she looked at some of the maps that show how ash spills could spread during pond failures at power plants in front of Florence, and concluded: "It doesn't look good."

Coal Ash Disasters Are Destructive and Costly

The Southeast is infamous for its coal ash piles.

It wasn't even rain that caused two of the nation's worst coal ash disasters.

A stockpile of 5.4 million cubic yards of ash broke loose on a frozen December 2008 at the Tennessee Valley Authority's Kingston plant in Tennessee, destroying three homes, damaging dozens of others and requiring cleanup that cost more than $1 billion. And in February 2014, a pipe collapsed at a Duke Energy plant, sending 82,000 tons of coal ash into North Carolina's Dan River.

"These sites are dangerous on a good day when the sun is shining," Holleman said. "There is even greater risk when there are hurricanes and floods."