Methane levels were 17 times higher in ground water near areas where shale-gas "fracking" wells had been drilled in Pennsylvania, compared with areas where no gas drilling had occurred, a new study has found.

Duke University researchers analyzed methane gas in 68 private ground-water wells across five counties in Pennsylvania and New York. The study cited "evidence for methane contamination of drinking water associated with shale-gas extraction."

In shale-gas extraction, water is mixed with chemicals and sand and is injected at high pressure deep into shale formations, which then releases natural gas.

The peer-reviewed study, which is being published in the Proceedings of the National Academy of Sciences, is one of the first to conclude that hydraulic fracturing is polluting ground water. And it’s likely to be used as ammunition in court by those opposing drilling in sensitive watersheds.

The hydraulic fracturing approach has dramatically increased available US reserves of natural gas by unlocking gas that was previously trapped in shale formations from the mid-Atlantic to Texas to Colorado. But environmentalists and local residents have long claimed that fracking pollutes ground water with methane as well as with chemicals in the injection fluids.

The Duke researchers said that the presence of methane likely was due to its escape from faulty drill casings.

While the study found high methane levels, it did not find any evidence that the chemicals injected at deep levels to fracture the shale had moved upward to pollute relatively shallow ground water.

"We found no evidence for contamination of drinking-water samples with deep saline brines or fracturing fluids," the study found. "We conclude that greater stewardship, data, and possibly regulation are needed to ensure the sustainable future of shale-gas extraction and to improve public confidence in its use."

Even so, the study was immediately attacked by natural-gas industry lobby groups. They zeroed in on the lack of base-line data to further corroborate that the higher methane levels were caused by drilling and were not naturally occurring.

“It’s amazing these guys are this comfortable making these kinds of conclusions with a data set this small, no random sampling, and no baseline information to speak of whatsoever," said Chris Tucker, a spokesman for Energy in Depth, in a statement. Energy in Depth is a drilling industry lobby group.

Researchers said that while they lacked base-line data, the higher levels of methane within one kilometer of nearby gas wells were so strongly correlated statistically that it would be difficult to conclude that they were caused by anything else.

"At least some of the homeowners who claim that their wells were contaminated by shale-gas extraction appear to be right," Robert Jackson, lead author of the study, said in a statement.

Environmentalists hailed the study, which comes on the heels of a Cornell University study that found that the overall process of collecting shale gas produced more greenhouse gases than many had expected. More research is needed, the environmentalists said.

"It comes as no surprise that natural gas is not as clean as the industry pretends," says Deborah Goldberg, an attorney with Earthjustice, an environmental group. "The gas industry has made it virtually impossible to do base-line testing because in order to do that, researchers need to know what they're testing for – not just methane, but the variety of other contaminants being injected into the ground."

Chemicals from the fracking fluids, she adds, are likely to appear decades from now as they work their way up from deeper levels.

Hydraulic fracturing for natural gas is being conducted in shale formations in 32 states, according to an Earthjustice tally. New York has blocked the controversial practice pending results of an environmental review later this year. The Environmental Protection Agency is conducting a study whose results are not expected until 2012.