Florence's floodwater has already packed a significant punch in South Carolina, and when cleanup starts, the danger won't be over. 

That's because floodwater can carry a whole host of pollutants, from spilled hog waste in North Carolina to coal ash at the former Grainger plant in Conway — a site that Santee Cooper officials are watching closely as the nearby Waccamaw River continues to rise. 

Emergency officials and the Waccamaw Riverkeeper both advise avoiding contact with floodwaters. Once people return to their homes, they also are urged to take care when cleaning up. The South Carolina Emergency Management Division has advised owners of flooded homes to "assume everything touched by flood water is contaminated and will have to be disinfected." 

That potential for contamination also presents a challenge to local drinking water utilities and the ecosystems of the rivers themselves. Meanwhile, it's unclear how concentrated any potential toxins will be.

"It is hard to project whether any of these constituents are going to reach levels of concern because of the huge volume of water in the system available for dilution," said water scientist Susan Libes of Coastal Carolina University in Conway. "What is threatening to the water supply and aquatic life is concentration."

Libes, who runs a water quality monitoring lab at CCU, said recent samples of the Waccamaw River show that its water is more acidic with low levels of dissolved oxygen. Low oxygen leaves fish literally gulping for air. 

Libes hasn't been able to get back to her usual work, which includes testing ocean water along the Grand Strand's popular beaches. The university is closed down, and her lab staff evacuated before the storm. 

Instead, she's been taking test samples from her backyard dock, about 4 miles up the Waccamaw from Conway. 

Getting a more comprehensive understanding of what's in the water will take organization and significant funding, Libes said, which may be in short supply as government agencies focus on relief efforts. 

"We can all wave our hands and I can talk on and on, but unless we actually get out there and make some measurements, we will just be talking theory," Libes said.

Water OK so far, but sewer?

Water utility officials in Horry and Georgetown counties are testing their own systems and intake sites daily to monitor contaminants. 

"We have not seen any concerns as of yet," said Christie Everett, chief operating officer of Grand Strand Water and Sewer Authority. "We have water sampling locations throughout our distribution system." 

Of Grand Strand Water's two intake sites, neither is directly on the Waccamaw. One, which draws water from the Intracoastal Waterway and services customers along the beach, could be affected if there's a spill at the Grainger site. 

But the system has a series of strategically-placed wells to help make up demand in emergencies, Everett said.

Ray Gagnon, executive director of the Georgetown County Water and Sewer District, said his system is prepared to "super-chlorinate" its water to battle bacteria if needed.

Georgetown's Waccamaw Neck drinking water intake site is about 27 miles downstream of the Grainger coal ash pit. It takes in roughly 4 million gallons of water a day.

If the water quality is too poor, the utility could shut down its intake pipe there and use water in storage wells, which would allow the system to run "pretty much forever," Gagnon said. 

The utility faces another challenge from flooding, however. If the S.C. Department of Natural Resources' projections are proved correct, 45 of its sewer pump stations could be inundated in the coming days, according to Gagnon.

"We have hardened those facilities as best we can, but when an electric panel for a sewer lift station goes underwater, it tends to fry out everything," Gagnon said. "When that happens there will be no sewer service in those areas."

An underwater pump station also may leak raw sewage into the floodwaters, he said. 

Everett said that in Horry County there are 30 pump stations in flooded areas, which could mean a few inches of water on site or feet of water covering the station. 

She said she wasn't aware of any leaks.

Harm to wildlife

Flooding events can be disruptive to the ecosystems in and around rivers. Libes said it's possible that freshwater mussels in the Waccamaw will die. 

Some die-off of freshwater fish also can be expected because of strandings on swamped land or the lack of oxygen in the water.

Based on what fishery biologists saw after the historic 2015 flood in the Midlands and Lowcountry, most fish simply swim out the flood.

And little effect is expected on the saltwater species. Shrimp swim or get pushed downstream, where they're headed anyway. In 2015, shrimpers said they saw little difference in the catch following the flood. Most of the state's oyster beds lie south of the flooded areas in the Pee Dee region.

Even newly stocked juvenile fish — the biggest concern for biologists in 2015 — might not take severe losses. To find out, the S.C. Department of Natural Resources did a staged stocking of one- to three-inch long red drum in Winyah Bay near Georgetown, where the Great Pee Dee and Waccamaw rivers drain.

About 38,000 were stocked immediately after Hurricane Florence. Another 100,000 will be stocked when floodwaters recede, then another 100,000 in October.

The bay will be sampled over the next year to see how many of which stocking group make it and where they move, said DNR biologist Mike Denson.

It's too soon to give an overall assessment of other species, he said, "but we do know from the 2015 flooding event that healthy estuaries bounced back quickly."

Reach Chloe Johnson at 843-735-9985. Follow her on Twitter @_ChloeAJ.

Science and environment reporter. Author of Washing Our Hands in the Clouds.