Conway coal ash water fill.jpg (copy) (copy)

Santee Cooper employees pump water into a coal ash pit in Conway last week. The utility sought to equalize the amount of pressure on each side of its dikes as the Waccamaw River rose to record-breaking levels. Water could begin flowing into a pit with 200,000 tons of coal waste late Monday. Grace Beahm Alford/Staff

A pit of coal ash holding some 200,000 tons of toxic sludge in Conway could start taking on water Tuesday as the Waccamaw River sloshes over its banks.

Floodwaters had already started flowing into a second, cleaned-up ash pit that posed less of a threat to the environment, said Santee Cooper, the utility responsible for the ash.

The Waccamaw has been inching higher for days after Hurricane Florence dumped feet of rain over the area it drains that includes a slice of eastern North Carolina and the northeast corner of South Carolina that's the size of Rhode Island.

The river is expected to keep rising into Wednesday and it won’t subside below the coal ash pit’s barriers until Friday, according to the latest forecasts from the National Weather Service. The flood is already testing defenses set up by Santee Cooper, and the river has far surpassed its highest level in more than a century.

Santee Cooper, a government-owned utility, has set up a series of defenses for its ash ponds, which are just downstream of downtown Conway. Among other things, it installed a temporary dam to add a few feet of protection to dikes that were nearly submerged after Hurricane Matthew dumped rain on the area in 2016.

The waste is left over from decades of burning coal at the Grainger power plant and it has emerged as a top environmental concern in Florence’s wake along with other coal lagoons in the Carolinas. Coal ash contains toxic metals like arsenic, lead and mercury, and in North Carolina, regulators are worried that ash might already be flowing from a shuttered power plant.

The S.C. Department of Natural Resources banned boating near the ash pits on Monday, saying that water traffic could "severely degrade" the berms.

The Waccamaw will reach the top of the temporary dam in some parts when it reaches 21.3 feet in Conway, where it typically runs lower than 10 feet deep. It's expected to reach that level Tuesday morning. At 22 feet, the highest point projected by forecasters, it will reach the highest point on the dam.

If the forecast holds, Florence will top a river-level record set by Matthew by 4 feet. Santee Cooper says it sped up its cleanup efforts after Matthew highlighted the flooding risk, and before Florence, it had been on track to finish cleaning out the pits early next year.

The river's rise has happened slightly slower than expected, and forecasters have steadily nudged the floodwaters' peak further into the future.

That could be a good sign: Every delay carries the potential that the water will eventually begin to recede and be absorbed into the floodplain before it can reach the top of the dikes, said Cara Schildtknecht, director of the Waccamaw Riverkeeper, an environmental group.

“There is a very small chance that it might not happen,” she said. “That, obviously, would be the best possible solution for everybody.”

In the meantime, Santee Cooper spokeswoman Mollie Gore said the dam had held up well over the weekend, when the river started to lap up against it. Even if the dam is covered with water, it’s not certain that any coal ash will spill out.

Schildtknecht spent Monday morning on a boat with Santee Cooper workers taking samples of river water near the coal ash pits to test for signs of heavy metals. The samples will be useful to track changes in the river's water quality.

“They are doing a whole array of samples there,” she said.

Gray muck has been seen flowing into the Cape Fear River near Wilmington from a lake connected to coal ash dumps covered by the storm’s floodwaters. The flood breached the lake’s earthen dam in several points last week.

Duke Energy, which maintains those ash pits, has said that it doesn't believe the breach poses a significant threat of increased flooding to communities near the L.V. Sutton power plant.

But it has also acknowledged that it hasn’t ruled out the possibility of ash escaping the flooded dump and flowing through the lake into the river.

State inspectors in North Carolina collected water samples by boat on Sunday, and Mike Regan, who runs the state's environmental department, said aerial video of the site shows "potential coal ash" flowing into the river.

Glenn Smith of The Post and Courier and The Associated Press contributed to this report.

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Reach Thad Moore at 843-937-5703. Follow him on Twitter @thadmoore.

Thad Moore is a reporter on The Post and Courier’s Watchdog and Public Service team, a native of Columbia and a graduate of the University of South Carolina. His career at the newspaper started on the business desk in 2016.