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A study of tainted drinking water in areas where natural gas is produced from shale shows that the contamination is most likely due to leaky wells rather than the process of hydraulic fracturing used to release the gas from the rock.

The study looked at seven cases in Pennsylvania and one in Texas where water wells had been contaminated by methane and other hydrocarbon gases. Both states have extensive deposits of gas-bearing shale that have been exploited in recent years as part of a surge in domestic energy production. Some environmental groups have suggested that hydraulic fracturing, or fracking, could cause the gas to migrate into drinking water aquifers.

Shale-gas producers commonly drill a deep vertical well that is then extended horizontally in several directions into the rock, like spokes from a hub. In fracking, water and chemicals are injected at high pressures into these spokes, creating fissures and releasing the natural gas trapped within.

But in their analysis, published Monday in The Proceedings of the National Academy of Sciences, the researchers found no evidence that fractured shale led to water contamination. Instead, they said cement used to seal the outside of the vertical wells, or steel tubing used to line them, was at fault, leading to gas leaking up the wells and into aquifers.

“In all cases, it basically showed well integrity was the problem,” said Thomas H. Darrah, a researcher at Ohio State University and the study’s lead author. The gas that leaked, he added, most likely came from shallower gas-rich pockets that the vertical wells were drilled through on their way to the shale formations, rather than from the shale itself.

“The good news is, improvements in well integrity can probably eliminate most of the environmental problems with gas leaks,” Darrah said.

Richard J. Davies, a professor at the University of Newcastle in Britain, said the study confirmed what he and others had shown in earlier research, that the fissures created by fracking were generally not long enough to affect aquifers.

“It is good to know which parts of the fracking process are the ones we need to worry about,” Davies said. “It’s unlikely to be the fracking itself. It’s more likely to be poor well construction.”

Well integrity is a widespread problem in the oil and gas industry, with one often-quoted statistic suggesting that 15 percent of all cement sealing of wells may be imperfect, said Scott Anderson, who studies energy production issues for the Environmental Defense Fund.