Flooding isn’t the only threat posed by the Waccamaw River. What’s in the water?

Horry County is waiting to see how high the Waccamaw River will rise by this time next week, with the potential for it to cut off key access routes days after Hurricane Florence struck. But local residents should also consider what’s in the water.

The Waccamaw basin covers 765 square miles in Horry and Georgetown counties, and Santee Cooper is already struggling to keep the river away from 200,000 tons of coal ash at a former power plant site in Conway. Coal ash holds metals that, in high doses, can be toxic to people and wildlife.

Upstream, in hog-intensive North Carolina, lies another potential threat. The environmental group Waterkeeper Alliance maps more than 40 confined animal farms in the N.C. portion of the Waccamaw basin.

By noon Wednesday, four lagoons — open pits of water and hog waste — had structural damage after Florence, the N.C. Department of Environmental Quality reported. Thirteen lagoons were spilling water over the earthen dikes that contain them, and nine lagoons were flooded. Fifty-five lagoons were full or nearly so.

The agency did not identify the location of those troubled farms.

The two known breaches of hog waste lagoons occurred outside the Waccamaw basin, in Sampson and Duplin counties, North Carolina Department of Environmental Quality spokeswoman Megan Thorpe told the Sun News on Wednesday. The condition of the hog lagoons in the Waccamaw basin was unclear.

About 5,500 hogs and 3.4 million chickens and turkeys also died during the storm, the state agriculture department reported.

Environmental officials in both Carolinas haven’t yet responded to Sun News requests for other known contamination events in the Waccamaw basin, such as large sewage or chemical spills.

Even without hog farms upstream, floodwater can become a soup of bacteria, sewage and chemicals — ingredients swept from the ground as water rises.

“Limit contact with it, because you never know what’s in it,” said Horry County spokeswoman Mikayla Moskov.

The county’s flood disaster page says flood waters carry the risks of infectious diseases, chemical exposure and injury. They also might harbor animals, insects and reptiles. The county advises residents with open wounds to especially avoid flood waters or cover wounds with waterproof bandages.

The blackwater Waccamaw River basin drains into South Carolina from Columbus County, N.C., just north of the state line. The Waccamaw Riverkeeper staff, a program of the Winyah Rivers Foundation that is based at Coastal Carolina University, recruits volunteers to help monitor the river’s water quality.

The volunteers help Riverkeeper Cara Schildtknecht understand the river’s normal conditions. Heavy rain in the watershed typically reduces oxygen in the river as organic material decomposes, Schildtknecht said, sometimes leading to fish kills. Bacteria levels from contaminants washed into the river typically rise during high water, she said.

“But we might see (bacteria) go down because there’s so much water moving down the river,” she said of Florence.

Kim Lyerly, a Duke University professor of pathology and immunology, told Raleigh’s News & Observer on Tuesday that hog waste, and other contaminants in floodwaters, could cause health problems in children, the elderly or people with certain medical conditions.

N.C. State University agricultural engineering professor John Classen told The News & Observer that most of the water from overflowing hog lagoons is cleaner than the bacteria-laden sludge at their bottom. Flooding from sewage treatment plants or chemical industries might pose greater risks, he said.

Staying safe around flood water

The Environmental Protection Agency has this advice for people who make contact with flood water:

• Wash hands frequently with soap, especially before eating and drinking.
• Don’t let children play in flood water or play with toys that have been in the water.
• Report cuts or open wounds and report all symptoms of illness. Keep vaccinations current.

Bruce Henderson: 704-358-5051; @bhender