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. The study also found high contaminant levels in Hyco and Mayo lakes, two popular recreation lakes in the northern part of the state.
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," Vengosh said. "But these contaminants don’t just disappear. Our study shows they remain in high concentrations in the solid waste residue and wastewater the coal-fired power plants produce. Yet there is no systematic monitoring or regulations to reduce water-quality impacts from coal ash ponds because coal ash is not considered 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 inches 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 than the agency’s standard for aquatic life. Excessive levels of cadmium, antimony and thallium were also detected in the wastewaters.
One of the most striking findings of this study was that arsenic, a highly toxic chemical, is accumulating in the lake systems through retention in the lake sediments. “In spite of efforts by some coal-fired power plants to reduce arsenic disposal, even a small quantity of arsenic release could result in long-time accumulation,” said lead author Laura Ruhl, who completed her Ph.D. in Vengosh's lab this summer and is now an assistant professor at the University of Arkansas at Little Rock.
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 were Gary S. Dwyer, senior research scientist, Heileen Hsu-Kim, assistant professor of environmental engineering and Grace Schwartz, a Ph.D. student in environmental engineering, all of Duke.
The research was funded by the North Carolina Water Resources Research Institute.
CITATION: "The Impact of Coal Combustion Residue Effluent on Water Resources: A North Carolina Example," Laura Ruhl, Avner Vengosh, et al. Environmental Science and Technology, Oct. 15, 2012.Read the full article