A new study suggests that the contamination of drinking water by shale gas is due to faulty wells and not hydraulic fracturing.
Researchers in the US analysed the gas content in 130 water wells in Pennsylvania and Texas.
They were able to trace the methane found in the water to problems with the casing or lining of wells drilled to extract the gas.
The report appears in the Proceedings of the National Academy of Sciences.
In many parts of the US, the migration of gas into drinking water has raised questions about the fracking process.
Previous research has detailed the scale of these difficulties without arriving at a satisfactory explanation of how the gas got into the water.
This new study focussed on areas which were well known for elevated levels of methane in drinking wells.
The researchers used noble gases to trace the path of methane as these inert chemicals are not affected by microbial activity or oxidation.
By measuring the ratios of the noble materials to the methane they were able to accurately determine the distance to the likely source.
The scientists analysed content from 113 wells in the Marcellus shale in Pennsylvania and 20 in the Barnett shale in Texas. They found eight clusters of wells with problems.
"The mechanism of contamination looks to be well integrity," said one of the authors, Prof Robert Jackson from Stanford University.
"In about half the cases we believe the contamination came from poor cementing and in the other half it came from well casings that leaked."
Cement is used in the oil and gas extraction industry to fill the spaces between the well casing and the sides of the well.
In one case the methane was linked to the failure of an underground well. In none of the investigated wells was there a direct link to fracking.
"These results appear to rule out the possibility that methane has migrated up into drinking water aquifers because of horizontal drilling or hydraulic fracturing, as some people feared," said Prof Avner Vengosh, from Duke University.
The researchers are concerned that the wells are failing because of the large volumes of water going through them at very high pressure. This is a critical part of the process of extracting gas from the shale rocks.
Separation of powers
They also point to the pressure that drillers are under to finish and move on to the next site. The historically low price of gas could also be affecting spending on well integrity as profit margins shrink.
The scientists believe that most of the problems they have identified can be resolved with better enforcement of existing regulations.
"You need strong rules and regulations on well integrity," said Prof Jackson.
"You need generous setbacks that protect homes and schools and water sources from drilling, sometimes farther than the drillers would want. You need enough inspectors on the ground to keep people honest and you need separation between the industry and the inspectors and you don't always have that in the US."
Other researchers say that the latest work shows that the process of fracking is safe - and that with proper regulation it could be a viable proposition in countries like the UK.
"It's important to put this work into a UK/EU perspective," said Prof Quentin Fisher from the University of Leeds.
"The licensing system in the US means that companies have to drill a massive number of wells very quickly. This is not the case in the UK/EU so far more care can be taken to ensure that leakage into groundwater does not occur."
The researchers say that further work in this field could help them predict the likely path of methane if leaks occur in areas where fracking takes place.
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