Study Charts How Underground CO2 Can Leach Metals into Water

Study is the first to observe, for at least a year, the effects of a CO2 leak on groundwater

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It’s not common for a solution to carbon emissions to also pose a contamination danger for drinking water supplies, but new research indicates that if CO2 stored deep underground were to leak in even small amounts, it could cause metals to be released in shallow groundwater aquifers at concentrations that would pose a health risk.

In a study published in Environmental Science & Technology, authors Mark Little and Robert B. Jackson studied samples of sand and rock taken from four freshwater aquifers located around the country that overlie potential carbon capture and sequestration (CCS) sites.

The scientists found that tiny amounts of CO2 drove up levels of metals including manganese, cobalt, nickel, and iron in the water tenfold or more in some places. Some of these metals moved into the water quickly, within one week or two. They also observed potentially dangerous uranium and barium steadily moving into the water over the entire year-long experiment.

“We did the study to try and build a framework to help predict where problems with groundwater might arise if CO2 leaked,” Jackson told SolveClimate News. “The chemistry of the water provides us with an early warning of the potential leaks before the leaks occur, and that by itself if a very useful tool,” Jackson added.

The technology for capturing and storing CO2 emissions from coal plants and industrial facilities is not yet commercially available. Still, the Obama Administration and other governments consider capturing carbon dioxide and sequestering it underground a vital technology that will allow the world to continue using coal as fuel while reducing the impacts of climate change. This new study sheds further light on how fresh water contamination from the technology could potentially occur.

Similar to Ocean Acidification

When the CO2 buried deep underground escapes into groundwater, it forms carbonic acid, a chemical reaction very similar to the process that occurs when the oceans absorb CO2. But the problems created by the carbonic acid in groundwater are quite different from the reactions that occur in the ocean, Little said.

Scientists have already observed that atmospheric CO2 is causing ocean acidification that is harming corals, shellfish, lobsters, and other marine animals at the bottom of the sea. The increased acidity caused by CO2 dissolved in water underground can cause metals to leach out of surrounding sand and rock.

Borrowed from agencies such as the US Geological Survey, the sediment used in the study was from 17 locations within four project sites: Acquia and Virginia Beach in the Virginia and Maryland tidewater region; Mahoment in Illinois; and Ogallala in the southern high plains of Texas. The scientists dried the sediment samples and placed them in bottles, then piped a stream of 99.8% pure CO2 to each bottle for 320 to 344 days.

Jackson and Little used their observations of the leaking CO2 to develop selection criteria, based on the metal contamination seen in the water, to help owners and operators choose CCS sites that are less likely to contaminate nearby freshwater aquifers. They also identified four geochemical markers to help monitor sites and discover when CO2 has leaked and caused metals to move into the groundwater.

Jackson, Nicholas Professor of Global Environmental Change at Duke University’s Center on Global Change (co-author Little was a postdoc fellow at the time of the study), said the research is unique because of its length: it is the first to observe, for at least a year, the effects of a CO2 leak on groundwater.

Scientists have already conducted short-term experiments of two-weeks to one month and found that CO2 in very small amounts can escape along rock faults and old petroleum wells into near-by groundwater and release harmful metals such as arsenic and uranium into the water.

Once CO2 reaches a freshwater aquifer, the quality of the drinking water is site specific, and depends on an array of factors including the size of the leak and the types of bacteria in the water, Little said. “By no means would all sites be susceptible to problems of water quality,” Jackson added.

Other researchers are trying to determine how a very large leak might affect the subsurface environment, while the Department of Energy (DOE) and private investors are beginning studies of potential groundwater contamination in the field, rather than in a lab as Jackson and Little did.

EPA’s Rule

The paper was published just as EPA finished a rule designed to protect potential drinking water sources from contamination following a CO2 leak. Announced on November 22, the rule is written for the owners and operators of potential CCS wells. It’s designed to ensure that the wells are appropriately sited, constructed, tested, monitored, and closed, according to EPA.

Sally Benson, director of the Global Climate and Energy Project at Stanford University, said EPA’s rule should protect groundwater because it will make it difficult to inject CO2 too close to a possible drinking water source. She also said the new study doesn’t present any surprises and is not likely to put an obstacle in the way of those CCS projects in the planning stages.

“Really, it gets down to making sure projects are designed carefully and that the project has monitoring so that one has early warning of any CO2 movements,” Benson added.

But drinking water utilities aren’t convinced that EPA’s rule will protect water sources from metal contamination resulting from the bubbling up of CO2, which is sure to occur in small amounts at least.

Cynthia Lane with the American Water Works Association (AWWA), a nonprofit research and advocacy organization representing researchers and water utilities, said this rule doesn’t include specific site selection criteria. Rather, the rule leaves many of the decisions about site selection and permit approval up to each state.

“It is not as protective as we might like,” said Lane. “We are concerned about the quality of drinking water. There is a definite shift in certain parts of country to use saline or more brackish water for drinking.”

Groundwater protections should be in place for areas in the southwest, such as Las Vegas, where utilities are having a difficult time finding water sources, Lane said. “They are using anything that is wet no matter what the saline content is,” Lane added.

After observing the CO2 percolating through aquifer sand and sediment for a year, Jackson said the study strongly suggests to him that long-term monitoring for CO2 leakage into freshwater aquifers should be part of every CCS project.

The CO2 caused concentrations of manganese, cobalt, nickel, and iron to increase by more than 100 times the original levels (or 2 orders of magnitude), and potentially dangerous uranium and barium increased throughout the entire experiment in some samples. In general, they found that iron and manganese concentrations increased within 100 days. The response of other potentially harmful metals was more varied.

“We don’t want a private homeowner with a well that is not regularly monitored by the local utility to suddenly have elements in their groundwater that they don’t even know about.”

The two researchers are now collecting data on sites that are under consideration by DOE and private consortiums.

“Our next step is to do incubations under a variety of conditions,” said Jackson. “I think we could contribute to a list that indicates why certain sites are better than other sites.”

Photo: Artur Jan Fijałkowski