The draft analysis by the Environmental Protection Agency, released after three years of study, looked at possible ways fracking could contaminate drinking water, from spills of fracking fluids to wastewater disposal.

"We conclude there are above and below ground mechanisms by which hydraulic fracturing activities have the potential to impact drinking water resources," the EPA said in the report. But, "we did not find evidence that these mechanisms have led to widespread, systemic impacts on drinking water resources."

The study was commissioned by Congress and represents the most comprehensive assessment yet of the safety of fracking, a technique that has led to a boom in domestic oil and gas production but also spawned persistent complaints about pollution. Fracking involves the injection of water, sand and chemicals underground to break apart shale rock and free trapped oil or gas.

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