It could take decades, at least, to replace cheap, abundant fossil fuels with low-carbon energy sources. In the meantime, many scientists and government officials around the world think the next best option for keeping Earth's rising levels of atmospheric carbon dioxide (CO2) in check is to prevent the gas from escaping in the first place. This can be done by using a chemical solvent to separate it from the emitted byproducts of power plants and other high-polluting facilities like aluminum manufacturing plants and then burying (technically injecting) it deep underground—a process known as carbon capture and sequestration (CCS). Ideal storage areas include depleted oil and gas reservoirs, unmineable coal seams or deep saline formations, because they are all under sufficient pressure to force the greenhouse gas to stay put and are made of porous rock that can soak up CO2 like a sponge.

The Department of Energy estimates that deep saline formations in the U.S. could hold up to 12,000 gigatons of CO2, meaning they are a viable long-term solution because human activities currently emit around 33 gigatons of CO2 per year. Although burying billions of tons of CO2 underground may sound like a daunting, perhaps even dangerous task, engineers have a pretty good idea how to do it, and scientists have reason to think it can work safely on a large scale. The oil and gas industry began injecting various fluids underground in the 1930s; since that time, researchers have been working to understand the effects of the process on the geochemistry of storage sites and the risks it may pose to human safety. A handful of CO2 storage sites, including a Norwegian project beneath the North Sea initiated in 1996, are already active around the world, showing that the concept, on a small scale, can work.

One potential risk that has garnered a lot of research attention is that of an inadvertent leak—especially a hypothetical case in which CO2 seeps into drinkable groundwater supplies. This was the focus of a study published online November 11 in the journal Environmental Science & Technology.

The study authors acquired freshwater samples from four of the nation’s largest aquifers—the Aquia and Virginia Beach aquifers beneath Maryland and Virginia, respectively, the Mahomet Aquifer in Illinois and the Ogallala Aquifer in Texas—each of which overlies a potential sequestration site. Then, in the laboratory, the researchers exposed the experimental water samples to a flow of CO2 designed to simulate a slow leak and observed chemical changes that occurred over the course of more than 300 days. The CO2 caused the pH of the water in all the samples to drop 1–2 units as the gas reacted with the water to form carbonic acid. The drop in pH caused the rock in the samples to weather, increasing the concentration in the water of elements that had been previously part of the rock.

Although the specific chemical changes depended on the unique geochemistry of each sample’s respective site, the authors report that on the whole, CO2 caused concentrations of alkali and alkali earth elements, as well as manganese, cobalt, nickel and iron, to increase—in some cases by more than two orders of magnitude. Concentrations of aluminum, manganese, iron, zinc, cadmium, selenium, barium, thallium and uranium in some samples neared or exceeded maximum contaminant levels set by the Environmental Protection Agency (EPA). Additionally, in some cases the amounts of dissolved lithium, cobalt, uranium and barium kept increasing throughout the whole experiment, which the authors say shows the value of long-term investigations such as this one.

Even though these results may seem like cause for alarm, the truth is they aren’t surprising, says Julio Friedmann, the leader of the Carbon Management Program at Lawrence Livermore National Laboratory and an expert on CCS. "If CO2 gets into shallow freshwater aquifers, small amounts of trace metals will be freed from the rock volume," he says. "This is something we’ve understood." How a CO2 leak might affect drinkable groundwater was "one of the first questions the community asked 15 years ago," he adds. "It’s the place where we have put most of our experimental emphasis."

Friedmann says he was taken aback by reports that implied the study had identified a new class of risk, when in fact the threat of CO2 leaking into drinking water is well studied. Underground carbon sequestration is regulated under the Safe Drinking Water Act of 1974, which states that groups can't inject anything underground if there is even a small chance it might contaminate usable drinking water sources above the thresholds mandated by the EPA.

The drinking water act divides wells into different classes depending on the depth at which the material is to be injected, whether or not it’s hazardous and other distinguishing factors. On November 22, the EPA finalized regulations for Class VI wells, a category designed specifically for underground CO2 sequestration. According to the press release, "The rule requirements are designed to ensure that wells used for geologic sequestration of carbon dioxide are appropriately sited, constructed, tested, monitored, and closed."

It's in the context of monitoring these areas that the new Environmental Science & Technology results are valuable, says Robert Jackson, a professor of environmental sciences at Duke University and co-author of the paper. The ultimate goal, he says, is to develop a tool that could be used to test potential sites for geochemical signatures that may represent risk factors in the case of a CO2 leak and monitor the sites once they are active.

Regardless of regulations, leaks are bound to happen occasionally, says Jackson. A defective well or an unmapped fault may allow CO2 to escape at some point, and it would behoove regulators to develop a method to detect a leak in time to mitigate it before it became a bigger problem. Jackson says his study's results indicate that in some cases the groundwater chemistry may be able to serve as an early warning, as the concentrations of certain elements rose noticeably just two weeks after the samples were first exposed to the simulated leak. "The notion that you could look at manganese or some other element in groundwater and use that as an indicator of a leak before you could actually find CO2 itself—that's a powerful tool," he says.

Friedmann agrees that the new paper, combined with previously published data, adds to "the earliest parts of an infrastructure" that could be applied toward the development of an inexpensive monitoring tool.

There's still a lot more work to be done in this area, says Susan Hovorka, a senior research scientist at the University of Texas at Austin's Bureau of Economic Geology. Hovorka's research topic is the monitoring of CO2 sequestration wells. Ideally, she says, researchers could find a clear signal that would be consistent across all sites—a "magic bullet that would let us say, 'Aha! You guys are out of compliance, and this is how we know'." Unfortunately, such a signal has yet to emerge, and "this paper is one of a series that shows you get these subtle, murky, inconsistent indicators" that are site specific, she says.

The best data will come from actual field studies, which will likely increase as more underground storage sites become active. "It's always difficult to know, when you've got lab samples, how to scale them up to the whole aquifer," Hovorka says.

In the meantime, says Hovorka, it's important for the public (and the press) to weigh the risks of underground CO2 sequestration against those of continuing to allow CO2 to accumulate in the atmosphere. "Climate change is itself a really big risk to water," she says. "People aren't balancing that risk."