But the report comes to no definite conclusion about the impact of fracking on drinking-water resources. The EPA says such conclusions are not possible because it does not have data on all the 20,000 to 30,000 wells that are drilled each year. It has information on just a fraction of the chemicals used in the process.

“In places where we know activities in the hydraulic fracturing water cycle have occurred, data that could be used to characterize hydraulic fracturing-related chemicals in the environment before, during and after hydraulic fracturing were scarce,” according to the EPA. “Because of these data gaps and uncertainties, as well as others described in the assessment, it was not possible to fully characterize the severity of impacts, nor was it possible to calculate or estimate the national frequency of impacts on drinking-water resources from activities in the hydraulic fracturing water cycle.”

The final report notably backs away from a June 2015 draft that said fracking did not cause “widespread and systemic” impacts on drinking-water resources. The EPA’s own Scientific Advisory Board, an independent panel of scientists, strongly criticized that finding because it was not backed by data in the report.

In its critique of the draft report, the SAB also said the report needed more information on local impacts, including on possible water contamination from fracking in Dimock, Pa.; Pavillion, Wyo.; and Parker County, Texas.

The EPA addressed the board’s concerns in the final report, providing details about specific instances of contamination. The agency also highlighted practices in the fracking process that could contaminate the local water supply.

The EPA says mechanical integrity failures of fracked wells have allowed gases or liquids to move to underground drinking-water resources. “The presence of multiple layers of cemented casing and thousands of feet of rock between hydraulically fractured rock formations and underground drinking-water resources can reduce the frequency of impacts,” the EPA said.

Storage of wastewater in unlined pits also provides a “direct pathway” for contaminants to reach drinking water, according to the report. Spills of hydraulic fracturing fluids in high concentrations also can lead to longer-term and more pervasive contamination, the EPA said.

Industry groups say they are using best practices to prevent contamination of drinking water and criticize the EPA for abandoning its previous conclusions.

“The agency has walked away from nearly a thousand sources of information from published papers, technical reports and peer-reviewed scientific reports demonstrating that industry practices, industry trends and regulatory programs protect water resources at every step of the hydraulic fracturing process,” says Erik Milito, director of upstream activities at the American Petroleum Institute.

Environmental groups encouraged the EPA to continue to study the issue. “There are persistent gaps in the data that must be closed in order to get a complete understanding of the full risks posed to drinking water from fracking activities,” said John Noel, coordinator for Clean Water Action’s National Oil and Gas Campaigns. “This study puts the science on our side and will allow communities to push back against the attacks we know are coming from the [new] administration.”