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."It is a shame that this has to be published in the UK. Apparently, US outlets will not touch good news about 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.Fears that environmentalists, who are not scientists, have played on.
"You need strong rules and regulations on well integrity," said Prof Jackson.And they need to be enforced.
Pennsylvania has the longest history of fossil fuel well-digging in the world. And fracking is not a new process. Its success in extracting natural gas is a new development. Regulations in PA have existed for years about the need for well integrity. It is one of the reasons that the current governor of PA took so long to issue new regs. The State wanted to update their existing rules.