An unknown percentage of the 45 million Americans who rely on private drinking water wells could face contamination risks from fracking, as the Trump administration attempts to roll back some of the nation’s few federal fracking rules.

About half of hydraulically fractured oil and gas wells in the United States are located within less than two miles of drinking wells recently drilled by homeowners, according to a study published last week in the Early Edition of Proceedings of the National Academy of Sciences.

“This is important to many wells, not just a few,” especially given that the study actually underestimated these numbers for each state, said study co-author Scott Jasechko.

Rural residents often lack access to regularly tested public water supplies. In the Appalachian states of Ohio, Pennsylvania and West Virginia, a large number of homeowners rely on their own private drinking wells — many of which are located near hydraulically fracked gas operations in the Marcellus Shale formation. The Safe Drinking Water Act, a federal law that protects public drinking water supplies, does not require routine testing of private groundwater wells.

Hydraulic fracturing is a process that pumps a liquid slurry of water, sand and chemicals into a well to break up underground rock formations rich in hydrocarbons.

Environmental and public health groups have shared widespread concer