As most forms of energy in the US have been going up in price, natural gas has gone in the opposite direction. This is largely the result of a new extraction technique called "fracking," in which fluids under high pressure are used to fracture rock formations deep underground, releasing large volumes of gas that would otherwise be trapped in small pockets. Because of relaxed regulations in some states, the process of fracking boomed before anyone had a clear perspective on its environmental consequences.

Most of the concerns about fracking have focused on the fluids involved in the process, which tend to exit the wells heavily contaminated with dissolved metals and radioactive material. But those may not be the only worries. A study, released yesterday by PNAS, now shows that fracking may be contaminating local groundwater with enough methane to pose a risk of explosion.

The problem isn't with municipal water systems, which are carefully monitored for signs of contamination. Instead, lots of fracking takes place in rural areas, where isolated houses may rely on shallow private wells for their water. Pennsylvania alone is estimated to have over a million private wells and, in some ares of the state, fracking activity has risen 27-fold in the last two years alone. To check whether the drilling activity was having an impact on local well water, a team from Duke University sampled water from 68 private wells, looking for signs of contamination.

The wells included the Catskill, Genesee, and Lockhaven formations in New York and Pennsylvania; some were in close proximity to drilling and others over 5km away. The trends were immediately clear: those within 1km of an active drilling site were much more likely to have high levels of methane, on average 17 times higher than those sites more distant from active drilling. That average covers a broad range, too. Some sites were indistinguishable from the typical inactive well, while others had concentrations of methane between 19.2 and 64 mg/l, enough to pose an explosive hazard, and high enough to qualify for hazard mitigation under the Department of the Interior's rules.

Some wells in areas more distant from active drilling did have appreciable levels of methane, so the authors looked into whether the contamination was likely to be from local biological activity or the release of deeper deposits of the gas. Based on isotope ratios and the presence of ethane and propane, they conclude that the methane from areas of active drilling is almost certainly from the drilling itself.

On the plus side, methane seemed to be the only contaminant from the drilling process that made it into these wells. There were no signs that brine from deep sources or radioactive material had made it into the local water supplies.

How did the methane get there? The simplest answer would be that the casings of the new wells are simply not sealed properly, allowing gas to leak out. Alternately, there are many old and abandoned drill sites in the area, and fracking may have opened a pathway for gas to escape into these wells. Finally, the authors suggest that the fracking process may open up natural fractures in the overlying rock formations. At this point, it's impossible to distinguish between these possibilities.

This isn't the first recent study to suggest that fracking enabled the escape of methane into the environment. A Cornell professor set of a fair bit of controversy by publishing a study that indicated that fracking released so much methane that its impact on greenhouse gas levels was worse than that of burning coal. Although some of the assumptions in that paper have come under sharp criticism, the two papers suggest that inadvertent methane release is an issue worth giving careful attention.

The authors of the new paper provide a second reason: lawsuits. "Based on our groundwater results and the litigious nature of shale-gas extraction," they conclude, "we believe that long-term, coordinated sampling and monitoring of industry and private homeowners is needed."

PNAS, 2011. DOI: 10.1073/pnas.1100682108 (About DOIs).