Lake Sediments Record North Carolina’s Coal Legacy

By: Kimberly M. S. Cartier, EOS
October 14, 2022

Sediment cores taken from five lakes in North Carolina reveal the state’s history of coal ash pollution from power plants. Analysis of these cores, published in Environmental Science and Technology, explains that although coal ash deposition has declined in recent years, its legacy lives on in the contaminants the ash left behind to seep into the environment. These toxins could be affecting the health of local residents and ecosystems.

“These are recreational lakes,” Zhen Wang, an environmental geochemistry doctoral student at Duke University in Durham, N.C., and the lead author of the study, said in a statement. “Some of them…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.”

Decades of Pollution

Coal ash is the by-product of burning coal and is one of the most prevalent forms of industrial waste in the United States—around 130 million metric tons are produced every year. Ash contains toxic metals, including lead, mercury, arsenic, and selenium, which can cause respiratory diseases and cancers after short- or long-term exposure.

The team collected cores up to 70 centimeters deep from the lakes between July 2020 and August 2021. They also collected a core from Lake Waccamaw, a natural lake close to Lake Sutton, for comparison. “The oldest sediments we collected were aged to mid-1950s,” Vengosh said, “representing 60–70 years of sedimentation.” Some of the cores look back to before the nearby coal plants were installed and provide a good baseline to understand a plant’s environmental impact. For each core, the researchers analyzed the morphology, magnetic and geochemical properties, and strontium isotopes of the lake sediments and the embedded coal ash to track the history of ash deposition into the lakes.

The cores revealed three distinct phases of coal ash release into the lakes. The first phase, from the 1960s to 1970s, saw significant deposition of both coarse and fine-grained ash particles as ash was dumped right into the lakes. During the second phase, from the 1970s to 1990s, the enactment of the Clean Air Act meant that coal ash was stored in ponds next to the plant. The level of pollution into the nearby lakes decreased during this time, and larger particles were captured by air filters instead of entering the environment. The third phase, from the 1990s to the present, also saw a decrease in the quantity of coal ash deposited into the lakes as storage changed from wet coal ash ponds to dry landfills and a few plants shifted from coal to natural gas for energy production.

Using streamflow data from the U.S. Geological Survey, the team found that the lakes accumulated more coal ash during times of high streamflow and precipitation. This result suggests that stormwater runoff, flooding, and effluent discharge from coal ash ponds and dry landfills are some of the main mechanisms by which the ash enters nearby lakes. Ash can also enter the air, land in the surrounding landscape, and be washed into the lakes by rain. The researchers noted that climate change continues to heighten the severity of the storms striking North Carolina, including hurricanes like Florence in 2018, which bodes ill for the coal ash still stored at these plants.

This infographic displays a coal-fired plant next to a lake. The plant emits smoke from its stacks, which releases airborne ash that travels over the green surrounding landscape. Falling rain then washes that ash into the lake. Next to the plant is a black coal ash pond that releases ash into the lake via stormwater runoff and effluent discharge. A few fish swim in the lake, black ash accumulates in bottom sediments, and the contaminants are then mobilized into the ecosystem. An inset shows how the cores that were collected from the lake bottoms show the prereservoir surface and distinct phases of ash particle and trace element emplacement.
Research speculates that coal ash enters North Carolina lakes via three pathways: airborne ash that settles into the lakes and surrounding landscape, stormwater runoff from coal ash ponds, and effluent discharge from coal ash ponds. Once in the lake bottom sediments, the coal ash can release contaminants into the water, increasing their bioavailability. Credit: Wang et al., 2022https://doi.org/10.1021/acs.est.2c04717

A Toxic Legacy

The plants near Hyco, Mayo, and Belews Lakes continue to burn coal for power; the Sutton Plant has switched to natural gas, and the Riverbend Plant near Mountain Island Lake has been retired. But the toxic legacy of all of these energy plants continues. “Coal ash that was stored in the nearby coal ash ponds seems to continue [to be] displaced and transported into the lakes, specifically during times of major flooding like hurricane episodes,” Vengosh said. “The problem of coal ash legacy did not go away by switching to natural gas.”

Once in the lake bed, the coal ash breaks down and releases contaminants into the water. Those contaminants become bioavailable, which is a concern for the local ecosystems and the residents who live nearby. All five lakes are destinations for recreational boating, fishing, and camping, and Hyco Lake is also a residential area.

What’s more, Mountain Island Lake is a local drinking water intake, noted Amanda Strawderman, polluter accountability program director for the nonprofit Clean Water for North Carolina. This study raises pressing questions, she said. “To what degree could suspended particles of coal ash be taken into the municipal drinking water treatment system? Is this water being tested and remediated for toxic coal ash constituents? If remediation is taking place, to what extent are those 800,000 residents of the Charlotte/Mecklenburg area bearing the cost burden of that remediation through water rates?”

This study examined the environmental risks of bioaccumulation of coal ash pollutants, Vengosh said, but “it is well known that the assembly of contaminants in coal ash are highly toxic [to people]. There are reports that many workers who participated in the cleanup of the coal ash spill in the Tennessee Valley Authority [in 2008] became severely sick, with a high percentage of cancer. Many of them have already passed.”

“This alarming study reinforces the need to regulate generation and management of coal ash waste to understand where coal ash exists,” Strawderman added, “and strengthen protections for communities in North Carolina and beyond.”

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