PBS/AP

DURHAM -- Duke University researchers say the fish they caught in three North Carolina lakes that’ve taken power plants’ coal-ash waste showed relatively high levels of selenium, an element left over from the combustion process...

The results show that two of the lakes remain “burdened” by contamination even after the shutdown of the boilers that helped pollute them, three Duke professors and a doctoral student said in a new journal article.

And it’ll linger even as Duke Energy, the company that provides the bulk of the state’s electricity, cleans up its ash basin, the paper said. While the company will eventually remove the ash to a landfill, the water now in those ponds will be treated and released into the lakes.

That potentially makes the study of Mountain Island Lake, Mayo Lake and Sutton Lake one of the “benchmarks” future researchers can consult as they monitor the lakes’ recovery, doctoral candidate Jessica Brandt and the professors from Duke’s biology department and the Nicholas School of the Environment said.

The association of coal ash, selenium and lake pollution wasn’t particularly a surprise, as they’re all well-documented byproducts of Duke Energy’s power-generation work.

Selenium, a relative of sulfur’s on the periodic table, is an essential nutrient to life in small amounts but can be toxic in larger ones.

In fish, it’s thought to trigger birth defects, and is seen as a likely contributor to fish die-offs that happened in the 1970s in a couple of North Carolina lakes adjacent to power plants.

“In humans we don’t see the same effects, at least at the concentrations you’d see in the human eating of fish,” Brandt said.

The harm to fish nonetheless prompted a tightening of federal emission limits over the years, but the Duke University team results suggest that “selenium-imposed ecosystem stress could [still] be impacting fish population health.”

Brandt and her co-authors -- professors Emily Bernhardt, Gary Dwyer and Richard Di Giulio -- drew water and sediment samples from the three lakes, and from three other lakes not tied to power plants and thus suitable comparisons.

They also caught largemouth bass, bluegills and two types of sunfish from each lake, using “electroshock from a sampling boat” and a net, and took them back to the lab for dissection. From the animals they extracted “liver, muscle and gonad tissues” and analyzed them for trace chemicals.

In the fish, selenium concentration were highest in Sutton Lake, near Wilmington, and lowest in Mountain Island Lake, near Charlotte. Mayo Lake, near Roxboro, occupied the middle ground.

The Mountain Island fish samples generally had selenium levels within federal guidelines, but ones from Sutton Lake and Mayo Lake topped the guidelines in various ways. In each case, concentrations were “significantly elevated” compared to what the researchers saw in the fish from the lakes they used as “reference” controls.

Ash pollution in Mountain Lake and Sutton Lake came from power plants that have either shut down or stopped using coal in 2013. Mayo Lake, by contrast, “continues to receive effluent discharges” from an active coal-fired generation facility.

Brandt and her colleagues suspect the lakes’ varied hydrology contributes to the differences they saw.

Mountain Island Lake is a relatively free-flowing body of water, whereas the water in Mayo Lake has a slow, three-year turnover and Sutton Lake is “nearly a closed system” that gets water from pumps and loses it mainly to evaporation.

“This is the underlying reason why lakes are more of a problem than rivers,” Brandt said. “There’s more time for the contamination to settle.”

And while the paper, published in a journal called Environmental Science & Technology, spoke only of selenium, “we can run [tests] for a full suite of trace metals,” some “of which are more interesting in a human-health context,” she said.

The researchers “are interested” in what their tests show for arsenic, manganese and cadmium, and will “be looking at patterns” there, she said, adding that at least one more paper is due, likely in the fall.

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