Researchers collected and analyzed more than 300 water samples from 11 lakes and rivers for the study, which was published today in the peer-reviewed journal Environmental Science & Technology. 

“In several cases, we found contamination levels that far exceeded U.S. Environmental Protection Agency guidelines for safe drinking water and aquatic life,” said Avner Vengosh, professor of geochemistry and water quality at Duke’s Nicholas School of the Environment.  

Some of the highest levels were found in coal ash pond effluents flowing to Mountain Island Lake, a primary drinking water source for Charlotte, and also to the French Broad River in Asheville. Hyco and Mayo lakes, two popular recreation lakes in the northern part of the state were also found to have high contaminant levels in the lake water.

Concern about the environmental impacts of coal-fired power generation has led to tighter regulation of the industry in recent years, but most measures have focused on reducing plants’ emissions into the atmosphere under the Clean Air Act, Vengosh said. 

“We are saving the sky by putting in more scrubbers to remove particulates from power plant emissions.  But these contaminants don’t just disappear.  As our study shows, they remain in high concentrations in the solid waste residue and wastewater the coal-fired power plants produce,” he said.  “Yet there are no systematic monitoring or regulations to reduce water-quality impacts from coal ash ponds because coal ash is not considered as hazardous waste”.

Contaminant levels in the new study varied by location and season, but some general trends emerged. Levels were higher, overall, in small bodies of water, where there is less natural dilution. The study predicts that during drought or unusually dry periods of summer, when water flow is low, the impact will be larger.  Contamination was found to be more persistent in pore water samples extracted from a depth of less than 20’’ in bottom lake sediments.

In Mountain Island Lake, a primary source of drinking water for Charlotte, pore water samples collected from lake sediment during the summers of 2010 and 2011 contained up to 250 parts per billion of arsenic – roughly 25 times higher than current EPA standards for drinking water, and nearly twice the EPA standard for aquatic life.

Samples collected during the summer of 2011 from coal ash waste flowing to the French Broad River in Asheville contained arsenic levels more than four times higher than the EPA drinking water standard, and selenium levels 17 times higher the agency’s standard for aquatic life.  Excessive levels of cadmium, antimony and thallium were also detected in the wastewaters.

Lake Julian, Lake Norman,  Lake Wylie, High Rock Lake, Belews Lake, and the Dan River were also part of the study. Jordan Lake was sampled as a reference site with significantly lower concentrations of any of the coal ash contaminants.   

Laura Ruhl, a PhD student in Vengosh’s lab, was the lead author of the study. Ruhl graduated Duke University this summer and is currently an assistant professor at University of Arkansas at Little Rock.  

One of the most striking findings of this study was that arsenic, a highly toxic chemical, is accumulating in the lake systems through retention to the lake sediments. “In spite of efforts in some coal-fired power plants to reduce the arsenic disposal, even a small quantity of arsenic release, could result in long-time accumulation” Ruhl said. 

The researchers found that installed technologies to cut power plants’ harmful atmospheric emissions increased the risk of downstream water contamination.  “Plants that used flue gas desulfurization often discharged wastewater with greater concentrations of selenium and other contaminants,” Ruhl said.   

Approximately 600 U.S. power plants generate about 130 million tons of coal ash and other coal-combustion residues (CCRs) annually, more than half of which is stored in settling ponds and landfills.  

“By volume, coal ash and associated waste represent one of the largest industrial waste streams in the United States today,” Vengosh said, “yet they still aren’t regulated as hazardous waste.  If we really want ‘clean coal,’ we need to do more to ensure that our air isn’t being protected at the expense of our water. Hopefully, this study is a step in that direction.”

Co-authors of the study included Gary S. Dwyer, senior research scientist, Heileen Hsu-Kim, assistant professor of environmental engineering and Grace Schwartz, a PhD student in environmental engineering, all of Duke. 

The research was funded by the North Carolina Water Resources Research Institute.

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Note: Avner Vengosh can be reached for additional comment at 919-491-6792 orvengosh@duke.edu.