The EPA has found no evidence to suggest that hydraulic fracturing causes harm to drinking water. The controversial practice known as fracking has surged across much of the U.S. in recent years. Critics have charged that the activity leaks contaminants into nearby wells. Pictured is a pump that brings oil to the surface at a fracking operation in the Monterey Shale in California. Reuters/Lucy Nicholson

The Environmental Protection Agency says in a new report that it has found no evidence that fracking significantly threatens America’s drinking water supply. The agency’s latest statement, which comes after a four-year review that is the federal government’s most significant analysis of the issue of fracking and drinking water to date, disputes repeated claims made by environmentalists that injecting water into natural gas wells leads to groundwater contamination.

“We did not find evidence that these mechanisms have led to widespread, systemic impacts on drinking water resources in the United States,” the report concludes.

The EPA report, released Thursday, did acknowledge that there was clear potential for fracking activities to affect groundwater resources from operations both above and below the ground, but found that the number of cases where this actually happened was “small.” However, the agency also said that there is limited data available for before-and-after comparisons and acknowledged that its newest findings could be an underestimate.

In fracking, companies inject millions of gallons of highly pressurized water into wells, causing fissures in shale that are subsequently filled by deep veins of oil or natural gas, making it easier and cheaper to remove these resources. Fracking has been used since the 1940s but recently became more popular as a way to boost U.S. oil and natural gas production during a period of high foreign prices.

The report will likely shape both federal and state policies on the issue in the future. New York and Maryland have banned fracking activities due to health and safety concerns. In March, the Obama administration set the first federal safety regulations on fracking out of concern for groundwater contamination and air pollution, though the rules apply only to federal and tribal lands.