Media playback is unsupported on your device Media caption Rising prices has led to a sharp increase in shale gas extraction schemes

Shale gas drilling operations increase the risk of nearby drinking water becoming contaminated with methane, a study has suggested.

Researchers found, on average, methane concentrations 17 times above normal in samples taken near drilling sites.

Growing demand for energy has led to a sharp increase in shale gas extraction around the globe, prompting concerns about the impact of the technology.

The findings appear in the Proceedings of the National Academy of Sciences.

"We found surprising levels of methane in home-owners' wells that were close to natural gas wells, " co-author Rob Jackson, Nicholas Professor of Global Change at Duke University, North Carolina, explained.

"We found that within a kilometre of an active gas well, you were much more likely to have high methane concentrations," he told BBC News.

The team from Duke University collected samples from 68 private water wells in the north-eastern states of Pennsylvania and New York.

Fire water

"We found some extremely high concentrations of methane: 64 milligrams of methane per litre of drinking water, compared with a normal level of one milligram or lower," Professor Jackson observed.

Image caption Professor Jackson said he had witnessed contaminated water being set alight

"That sort of concentration is up at a level where people worry about an explosion hazard."

Videos are available on the web that appear to show people setting fire to water pouring out of a tap, and Professor Jackson said that he had witnessed such an spectacle himself.

He agreed that the main concern at present was not from drinking the water, but from the risk of an explosion.

However, he added that the team were calling for a medical review of chronic, low-level exposure to methane.

"I could not find any peer reviewed literature on the health effects of low level methane on people," He observed.

Media playback is unsupported on your device Media caption The BBC's Roger Harrabin shows how to get fuel from hard rock

Professor Jackson said that the simplest explanation of how the gas ended up in people's water supplies was down to "leaky gas well casings".

"If there are cracks or imperfections in the gas well, especially in the vertical section nearer the surface, then methane and possibly fracking fluid/waters," he said.

"That is the simplest and most likely explanation.

"There are other possibilities; some people have proposed that methane can migrate to the surface through fissures that are opened in process of fracturing the rock. To me, that is less likely."

Professor Jackson was keen to point out that the study's drinking water samples revealed no evidence of contamination from fracking fluids, of which about five million gallons are used to unlock the gas in each well.

As well as the PNAS study, the team has also published a paper outlining a number of research recommendations, highlighting areas they feel needs to be done in order to gain a stronger scientific insight into the impact of the technique.

These include:

Initiate a medical review of the health effects of methane

Construct a national database of methane and other chemical attributes in drinking water

Evaluate the mechanisms of methane contamination in drinking water

Refine estimates for greenhouse gas emissions of methane associated with shale gas extraction

Systematically sample drinking water wells and deep formation waters

Study disposal of waste waters from hydraulic fracturing and shale gas extraction

In the UK, MPs are expected to publish a report shortly that considers the impact of shale gas extraction and what role it can play in terms of delivering future energy security.

The Energy and Climate Change Select Committee inquiry will also assess what are the risks and hazards associated with drilling for shale gas?

The combination of rising energy prices and concern about future energy supplies has seen the technology being embraced by nations around the globe.

"Ten years ago, people did not really know about this source of gas," said Professor Jackson.

"The boom in the United States started in the Barnett Shale (found in Texas, and considered to be the largest onshore gas reserve in the US) and it has only been in recent years that we have realised how much gas is out there and economically available.

"it is the combination of energy economics and the emergence of new drilling and fracturing technologies that have made it cheap enough to do."