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Scientists sampled lakebed sediment from this area where Highway 49 crosses Mayo Lake in Person County. The sediment was found to contain coal ash, which contributes to elevated levels of toxic elements, such as selenium and arsenic, in fish. Mayo Lake is a popular fishing spot. (Photo: Lisa Sorg)

This story has been updated with quotes from Duke Energy.

At least a half ton of coal ash is sitting on the bottom of five freshwater lakes in North Carolina, a new scientific study has found, highlighting the “legacy of inadequate storage,” of the toxic material in unlined pits.

The persistence of the ash in the lake sediment, some of it 70 years old, poses chronic risks to fish and aquatic ecosystems, the study authors concluded. And extreme weather caused by climate change could flush more residual ash into waterways.

“The magnitude of sediment contamination by coal ash and its consequential ecological risks are much more prevalent than previously realized,” the scientists wrote, “suggesting a problem of a national or global scale.”

The study, published in the peer-reviewed journal, Environmental Science and Technology, was a collaboration among scientists from Duke University, Appalachian State and Montclair State in New Jersey.

From July 2020 to August 2021, scientists probed a parfait of sediment from five freshwater lakes: Mayo, Hyco, Sutton, Belews and Mountain Island, the latter of which is a drinking water source. All of them are next to former or current coal-fired power plants operated by Duke Energy, which used them for cooling.

“These are recreational lakes,” said Zhen Wang, a Ph.D student at Duke Nicholas School of the Environment and the lead author of the study, in a press release. “Some of them, like Hyco Lake, were originally built for the coal plant, but over the years, it has become very desirable real estate where people build their dream homes. It looks very pristine and beautiful, but if you dig in, you find piles of toxic coal ash.”

They also sampled Lake Waccamaw in Columbus County, which isn’t near a coal-fired plant, for comparison.

Coal ash was found in all sediment core samples in the five lakes near the power plants; much lower levels were found in samples from Lake Waccamaw. 

“Regular tests of surface waters in lakes and streams adjacent to our plants show water quality remains safe,”  Duke Energy Bill Norton spokesman said. “Water providers around those lakes also confirm water supplies are safe, as shown by thousands of tests annually.”

Avner Vengosh is a co-author of the paper and a distinguished professor of environmental quality at Duke University. He said Duke Energy is ignoring the findings in the new paper. Data show that the concentration of toxic elements in the sediments exceed the quality guidelines designed to protect freshwater lakes. “So the levels of toxic and carcinogenic elements we found in the bottom lakes are violation of ecological protection,” Vengosh said.

There are three likely sources of lakebed contamination: runoff from the pits caused by heavy rain; underground leaks and seeps from the pits, and air deposition, when ash particles are blown over the lakes and land in the water, then sink to the bottom.

Levels of arsenic and selenium posed an increased risk for fish in all five lakes near power plants. Concentrations of other elements, including thallium, antimony also were high enough to harm fish. 

Norton told Policy Watch that the study “does not demonstrate any risk to people or wildlife, reinforcing what we’ve long known — water supplies are well-protected from coal ash impacts.”

Vengosh disputed that statement, noting that the science show that while coal ash is buried in the bottom sediments, trace elements can enter the water and “become available for biological uptake” — consumption by fish and other aquatic life.

Historically, there have been fish kills and lower numbers of fish in Sutton, Hyco and Belews lakes. Previous research has found that within less than a decade of a coal-fired power plant being built at Belews Lake, 16 fish species disappeared.

At Sutton Lake near Wilmington, in 2017 Duke University researchers found that in selenium in 85 percent of all fish muscle samples. The NC Wildlife Commission’s 2014 Sutton Lake Sportfish Assessment, reported “the conditions of largemouth bass, bluegill, redear sunfish and flathead catfish are all low,” in part because of metals entering the lake from the ash ponds.

The state Division of Public Health issues fish consumption advisories that recommend limiting the amount of certain fish species eaten per week, by an adult or by a child. 

DPH has issued state-wide fish consumption advisories based on mercury contamination – also a legacy of burning coal. There are also fish consumption advisories for arsenic and hexavalent chromium in many waterways, including in Brunswick and New Hanover County, the latter of which includes Sutton Lake.

Unlike the other lakes in the study, Sutton Lake is nearly a closed system. It gets freshwater from the Lower Cape Fear River only from an intake pump operated by the utility. There is no way for water to leave the lake except by evaporation, or as occurred during Hurricane Florence in 2018, flooding.

For the other four lakes with “open systems” in the study, floods and other extreme weather have washed ash downstream into rivers and streams, researchers found. They analyzed federal water level data and stream discharge rates, and found higher flow corresponded with relatively higher percentages of coal ash in the lakebed sediments.

Because power plants are commonly located near lakes, there is a flooding risk. In North Carolina, coal ash pits, such as Hyco Lake, lie within the 100-year flood plain, state maps show (The pits are contained by berms, but these too, can fail.) At Belews Creek, the 100-year flood plain laps less than one-tenth of a mile from the berm of the ash pit. 

By analyzing the chemical composition and physical structure of the ash particles, scientists could calculate the approximate time the material was deposited, the environmental regulations that were in place when the coal was burned, as well as its origin.

For example, in Hyco Lake, near the Roxboro plant, no ash was found in sediment cores from the “pre-impoundment” period – before the reservoir and unlined pits were built in 1966.

But at Mayo Lake, ash was present in pre-impoundment cores. Researchers speculate that ash could have been applied to farmland before it became the bed of the lake in the early 1980s.
At Sutton Lake, ash was found throughout the core samples. That suggests, the authors said, ash had been continuously released from the unlined pit and into the lake since 1972, when the Sutton plant was built. Those discharges have continued, even after 2013, when Duke Energy transitioned the Sutton plant to natural gas. 

The study authors are Ellen Cowan, Keith Seamur, Jessie Wilson and Randall Karcher of Appalachian State; Zhen Wang, Gary Dwyer and Avner Vengosh of Duke University; and Stefanie Brachfeld of Montclair State in New Jersey.