Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. www.eenews.net
Carbon dioxide leaking from underground storage sites could contaminate drinking-water aquifers with dangerous levels of pollutants such as uranium under the wrong conditions, a new study finds.
If stored CO2 bubbled into aquifers made of rock with higher-than-average levels of naturally occurring metals, for example, the presence of the gas in liquid could create acidity, causing the pollutants to flow into the water, according to the research published in Environmental Science & Technology.
The study holds implications for carbon capture and sequestration, or CCS, which envisions grabbing carbon dioxide from coal plants and injecting the gas deep underground. The technology has never been proved at scale, although there are test projects sponsored by the Department of Energy and private companies. Many analysts consider the technology critical for slowing climate change, since coal fires almost half of U.S. electricity and spews about a third of national emissions.
Because likely injection spots for CO2 from coal plants sit deep in the ground below many of the nation’s water aquifers, the examination of how a minuscule amount of leaked gas would interact with fresh groundwater above is important, said study co-author Robert Jackson, an environmental sciences professor at Duke University.
“We need to be smart about where we locate CCS projects,” said Jackson.
“We’re not saying that in every place with a carbon dioxide leak you’re at risk of human health issues, but in some places you might be,” he said.
Zero tolerance for leaks
Jean-Philippe Nicot, a scientist at the University of Texas, Austin, said the study “added to the discussion” and mirrored other findings. Yet he said the researchers looked at a “worst-case scenario.”
“According to regulations, you should not have any leakage,” Nicot said. “They don’t state that clearly enough … that this is not going to happen every time,” he said.
U.S. EPA is finalizing regulations for sequestration wells under the Safe Drinking Water Act.
It is important that further research is done on C02 in aquifers to fully understand the risk to groundwater, added Sarah Forbes, an analyst at the World Resources Institute. “We need to be looking at the ‘What ifs,’” she said.
To reach their conclusions, Jackson and colleague Mark Little took rock samples from four drinking water aquifers in Maryland, Virginia, Illinois and Texas. They then placed the rocks in water and piped C02 through the liquid for almost a year. The goal was to simulate what would happen if stored gas from a coal plant seeped upward into aquifers. They assumed that far less than 1 percent of stored gas would leak under such a scenario.
“We exposed water to CO2 longer than many other studies,” said Little, a former postdoctoral researcher at Duke University. That is important for CCS, he said, since future stored gas is expected to remain underground permanently.
The researchers found that levels of uranium, iron and manganese rose above recommended federal levels for drinking water in some of their tests. If a similar process happened in the real world, problems for humans would most likely arise in cases where people depended on well water rather than municipal water for drinking, Jackson said.
A worst-case scenario
“I wouldn’t suggest that our study says anything necessarily about the quality of [future] water out of your faucet,” said Little.
Whether contaminants appeared in the water after the presence of C02 depended on several factors.
Rock without a natural buffer of calcium carbonate more freely allowed chemicals to flow into carbon dioxide-rich water, Jackson said. Rock with already high levels of heavy metals such as uranium also was more likely to leach those contaminants into the water, the researchers said.
The contamination levels varied from sample to sample, which is why rock testing of aquifers should be a key part of choosing CCS sites in the first place, said Jackson. “We can eliminate a lot of the risk that way,” he said.
That initial surveying, combined with routine monitoring of water after CO2 injections, would lower the risk of any water contamination greatly, Jackson said. Spikes in manganese, iron and calcium in water could be early warning indicators that C02 is leaking from below, since levels of those elements in the water rose quickly when exposed to the gas, according to the study.
Nicot said, though, that the study would not necessarily apply to conditions in an actual aquifer. Because the Duke researchers tested pure water, rather than aquifer water, they weren’t fully measuring all chemical reactions with CO2 that could occur in nature, he said. Pure water is more “aggressive” in dissolving pollutants, he said.
He also noted that aquifer water, unlike water sitting in a lab, flows and is replenished with new water continuously.
“That means you get dilution in nature,” Nicot said. “You are likely to get much lower concentrations of contaminants.”