Santee Cooper officials found the line of sludge in the morning.
It was at the site of the former coal-fired Grainger Generating Station in Conway, which still has tens of thousands of tons of toxic ash in ponds on site. Maintaining the ponds involves a pump system that empties water into the adjacent Waccamaw River, in part because drier ash is easier to excavate and haul away.
But when employees checked one pump the morning of Jan. 30, spokeswoman Mollie Gore said, they found that it had sucked out all the water in a section of pond No. 2 and started dredging up the sediment underneath. That sediment, Gore said, was likely a mix of ash and soil, and about a dumpster's worth ended up in the river.
Typically, employees check the pumps in a rotation at night, but for some reason, that piece of machinery was missed, she said. In the morning, what the utility found was a line of sludge on the wall of the dike separating the pond from the Waccamaw.
"Sometime overnight, it was during the night, the pump was not monitored and the area that it was pumping was being dewatered," Gore said.
The state-owned utility has since increased the frequency of its nighttime monitoring, and subsequent tests of the river did not show significant presences of toxic materials in coal ash, such as arsenic, lead and mercury. It also installed stone under the pump so it wouldn't be able to reach the mud underneath if the water levels again get too low.
But the episode serves as a reminder of the vulnerabilities of the site. It also stands in stark contrast to the utility's massive effort last fall to ensure that the ponds were not breached by the Waccamaw as it swelled during the historic flood brought by Hurricane Florence.
"We avoided a huge catastrophe during the flood," said Cara Schildtknecht, of Waccamaw Riverkeeper. "They moved heaven and earth to make sure there wasn't a breach, and things went well during the flood, and then this happens."
After slow-moving Florence, which proved a 1,000-year flood event for much of the watershed that includes the Waccamaw, Santee Cooper officials spent weeks shoring up the pits, which are near the Conway Marina.
As the river swelled to major flood stage, utility employees installed an extra water-filled dam to create more of a buffer for the ponds. They also pumped water back and forth from the river into the pits. The effort was aimed at maintaining equal pressure on each side of the dike so that the wall wouldn't rupture.
Ultimately, pond No. 2, which then contained 200,000 tons of ash, was not breached.
There was already an effort under way to remove as much of that material as quickly as possible. Since Florence, that work has been happening even faster, Gore said, and only about 50,000 tons remain today.
For decades, water flowed into the ponds and out of a permitted outfall to make sure the ash stayed in place. But the excavation has required getting rid of the water with pumps like the one that malfunctioned in January. Wet ash is heavier and more difficult to transport to its destination: the Winyah Generating Station near Georgetown.
From there, the material is used as an ingredient in concrete, Gore said, which is generally a safe way to make sure that the cancer-causing heavy metals in the ash are neutralized so they pose no danger to people.
Still, a slurry that likely contained some of that ash ended up in a trail of sludge down the dike wall, leading to the Waccamaw, in late January. All told, Santee Cooper estimates only 7.5 cubic yards of material escaped over that night, Gore said.
Schildtknecht said she's seen the test results from afterward and is confident the methods used to test the water are sound. Grand Strand Water and Sewer Authority, which draws water from the Waccamaw, didn't detect any harmful materials in its testing either, according to Chief Executive Officer Fred Richardson. Officials with the S.C. Department of Health and Environmental Control didn't respond to multiple requests for comment in time for publication.
As long as there's coal ash on the Grainger site, however, there's a possibility a flood — or human error — will land it in the Waccamaw.
"It just goes to show we need that coal ash removed," Schildtknecht said.