The Environmental Protection Agency said in a new report, after a study conducted over six years and involving $29 million issued that it did not know whether fracking posed a risk to drinking water.
"Because of the significant data gaps and uncertainties in the available data, it was not possible to fully characterise the severity of impacts, nor was it possible to calculate or estimate the national frequency of impacts on drinking water resources" from fracking activities, the EPA said in the report that raised more questions than it answered.
The report removed a finding from a draft issued last year suggesting fracking had not caused "widespread, systemic" harm to drinking water in the US. The draft had been hailed by industry groups as proof of the safety of fracking, while environmentalists pointed to the report's identification of cases where fracking-related activities polluted drinking water.
Fracking involved pumping huge volumes of water, sand and chemicals underground to split open rock formations to allow oil and gas to flow. The practice had led to an ongoing energy boom but had raised widespread concerns that it might lead to groundwater contamination, increased air pollution and even earthquakes.
The new report, was welcomed by environmentalists as proof that fracking threatened drinking water, while industry groups complained that the Obama administration had yielded to political pressure on its way out, a reversal of the earlier reactions.
According to commentators, the new version was far more worrying than the first, which found ''no evidence that fracking systemically contaminates water'' supplies.
''EPA. scientists chose not to include that sentence. The scientists concluded it could not be quantitatively supported,'' said Thomas A Burke, the EPA.'s science adviser, and deputy assistant administrator of the agency's Office of Research and Development, AP reported.
The report, the largest and most comprehensive of its kind to date on the effects of fracking on water supply, comes as president-elect Donald J Trump had vowed to expand fracking and scrap existing regulations on the process.