The most extensive government review of U.S. “fracking” practices has found no evidence of widespread damage to drinking-water supplies, while also warning of the potential for contamination from the controversial technique used in oil and gas drilling.
The draft study by the Environmental Protection Agency linked fracking to a few cases of water pollution but said the problems appeared so far to be isolated. It cautioned that a number of fracking-related activities carry a future risk of polluting wells and aquifers used for drinking and farming.

The long-awaited report appears unlikely to cool the national debate over the drilling practice that has spurred huge increases in U.S. oil and gas production in the past five years. Opponents and supporters of fracking instantly seized on portions of the report that supported their view.

The report could, however, help ease immediate concerns about damage to water supplies in the areas where fracking, or hydraulic fracturing, is underway. New York recently banned fracking in part because of fears of risks to groundwater.

“We did not find evidence that these mechanisms have led to widespread, systemic impacts on drinking water resources in the United States,” said the report’s executive summary, released on Thursday. It said that, while there are documented cases of contamination, the problem is “small compared to the number of hydraulically fractured wells.”

The EPA acknowledged that it was hampered in its assessment by inadequate data, preventing experts from reaching firm conclusions about whether contaminants in an individual well came from fracking or another source. Some critics of the report said poor data skewed the agency’s conclusions.

“This study was hobbled by the oil industry’s refusal to provide key data,” said Kassie Siegel of the Center for Biological Diversity, an environmental group. “The EPA found disturbing evidence of fracking polluting our water despite not looking very hard.”

The report’s conclusions stressed that numerous fracking-related activities “have the potential” to affect drinking water. “Water sources may be vulnerable to impacts, and these vulnerabilities should be considered,” EPA science adviser Thomas A. Burke told a news conference.

The study specifically cited the possibility of spills of wastewater stored above ground and the leaking of pollutants from poorly constructed well heads and casings. Some of the most significant risks to water supplies could occur far from the oil and gas wells themselves, it said.

“A truck carrying wastewater could spill, or a release of inadequately treated wastewater could have downstream effects,” the report said.

Fracking, or hydraulic fracturing, is the controversial practice behind the explosive growth of oil and gas production in the United States over the past five years. It involves the injection of liquids into underground rock layers at high pressure to extract oil and gas trapped inside.

The report’s mixed verdict stems from a nearly five-year effort by the EPA to analyze technical data from thousands from fracking operations and nearby aquifers in states around the country. The study was ordered by Congress in 2010 and has been dogged by controversy over the scope and scale of the research.

Burke called the draft report “the most complete compilation of scientific data to date,” encompassing 950 sources of information, such as scientific papers and technical reports, as well as original, peer-reviewed research conducted by the agency itself.

“It greatly increases our understanding of potential impacts,” he said.

Energy trade groups and environmentalists reached starkly different conclusions about the report’s central message. The American Petroleum Institute called the study a validation of the energy industry’s contention that fracking poses little risk to drinking water.

“After more than five years and millions of dollars, the evidence gathered by EPA confirms what the agency has already acknowledged and what the oil and gas industry has known,” said Erik Milito, the institute’s Upstream Group director. “Hydraulic fracturing is being done safely under the strong environmental stewardship of state regulators and industry best practices.”

Michael Brune, the Sierra Club’s executive director, said the report “confirms what millions of Americans already know: that dirty oil and gas fracking contaminates drinking water.” Brune criticized the report for failing to adequately consider the full range of impacts to local communities. “The EPA must conduct a comprehensive study that results in action to protect public health,” he said.

The draft will be finalized after a 85-day comment period and a formal review by the EPA’s Science Advisory Board.