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Santee Cooper shore up a coal ash pond in Conway as the Waccamaw River rises on Monday, Sept. 17, 2018. Grace Beahm Alford/Staff

Floodwaters have started flowing into a pit of coal ash near Conway, Santee Cooper said Saturday, and they aren't expected to subside for days.

Santee Cooper, a government-owned power company, says the coal ash pit that is flooding shouldn't be an environmental concern. The utility had already cleaned out the area where water is flowing in.

But the Waccamaw River is expected to keep rising until it tops out early next week. When it does, it will threaten another pond of coal ash, one that holds some 200,000 tons of potentially toxic waste.

Santee Cooper has mounted an all-out defense of the second ash pit, installing a temporary dam around it and pumping in water to equalize the pressure on both sides of the earthen dike that surrounds it. It has also gathered massive bags of rocks to plug holes if the berm breaks.

The structure has been tested before: Floodwaters from Hurricane Matthew lapped up to the edge of the dike. It came close enough that Santee Cooper sped up its cleanup efforts to stave off another trial.

But that work won't be done until next year, and the deluge of rain from Hurricane Florence last week will send the Waccamaw River four feet higher than Matthew did, according to the National Weather Service. The river will keep pouring into the ash pond through late next week.

Florence has caused concerns about coal ash throughout its path and raised the prospect of environmental damage. Coal ash, which is the sludge left over from generating electricity, contains toxic metals like arsenic, cadmium and lead.

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