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By Valerie Volcovici and Timothy Gardner

WASHINGTON, June 4 (Reuters) - Fracking for shale oil and gas has not led to widespread pollution of drinking water, a U.S. Environmental Protection Agency draft report said on Thursday, although it warned some drilling activities could potentially cause health risks.

The study, requested by Congress and five years in the making, said fracking could contaminate drinking water under certain conditions, such as when fluids used in the process leaked into the water table.

The EPA said it found isolated cases of water contamination, but "the number of identified cases ... was small compared to the number of hydraulically fractured wells.

"We did not find evidence that these mechanisms have led to widespread, systemic impacts on drinking water resources in the United States," the study said.

Fracking is the common term for hydraulic fracturing, the technique of injecting sand, water and chemicals underground to crack open rock formations holding natural gas and oil. The practice is controversial and opposed by environmental groups alarmed by, among other things, risks to drinking water.

Dozens of clashing studies have examined whether fracking contaminates water, and the ambivalent conclusions of the latest report have allowed both advocates and critics to claim some vindication.

"The report contradicts the most prevalent claim from anti-fracking activists, which have made 'water contamination' the very foundation of their campaign against hydraulic fracturing," said Katie Brown, spokeswoman for the Independent Petroleum Association of America's Energy In Depth unit.

The expansion of fracking has played a major role in the U.S. domestic energy boom, and has enjoyed support from the Obama administration for bringing the country closer to energy self-sufficiency.

Energy groups said the EPA's findings back up earlier studies by the Energy Department and U.S. Geological Survey, with the American Petroleum Institute saying the study affirmed the sector's record of "continuous safety improvements."

Amy Mall, senior policy analyst at the Natural Resources Defense Council, told Reuters the report was the first time the EPA acknowledged the potential to poison drinking water.

"There are still significant gaps in the scientific understanding of fracking," Mall said in a statement. "This study is site-specific and limited, which makes it impossible to fully understand all the risks at this time."

Mark Brownstein, vice president of the Environmental Defense Fund, said fracking is just one risk. He cited growing concerns about the massive amounts of wastewater that must either be trucked away to surface pools or injected back underground.

In states such as Oklahoma, the latter practice has been tied to a jump in low-level earthquakes.

"Ongoing physical integrity of the wells and handling the millions of gallons of wastewater coming back to the surface after fracking, over the lifetime of each well, are even bigger challenges," Brownstein said. "Relentless focus on these issues by regulators and industry is critical."

Fears of contamination have had an impact. New York Governor Andrew Cuomo cited concerns about water safety as a major reason for his decision last December to ban fracking in the state.

That ban has come under assault in wake of the EPA report.

"I fully expect Governor Cuomo to reverse his previous decision to ban fracking, which was based upon controversial scientific studies and made to appease far left environmentalists," said Republican Chris Collins, who represents New York's 27th Congressional District.

He said the state deserves "the job opportunities and economic growth fracking has clearly produced in other states, including neighboring Pennsylvania."

The EPA said its study will give state regulators, local communities and companies "a critical resource to identify how best to protect public health and their drinking water resources."

It noted that other risks included inadequately cased or cemented wells that could leak gases and liquids underground.

The study is only a draft and must still be reviewed by the public and by the EPA's independent Science Advisory Board. It is due to be completed next year.

The results were welcomed by drillers such as Pete Brown, co-owner of Cimarron Production Co Inc, in Oklahoma.

Brown said his company has sunk 120 wells since it began fracking in 1972 and that state regulations implemented since then have prevented any contamination.

"I think the chances of (groundwater contamination) happening are less than one percent," Brown said. "I can't really fathom a reason why it would contaminate drinking water." (Reporting by Valerie Volcovici and Timothy Gardner. Additional reporting by Yeganeh Torbati; Editing by Bruce Wallace and Andre Grenon)