U.S. government officials urged residents of a Wyoming farming community near natural gas drilling sites not to use private well water for drinking or cooking because of chemical contamination.

“Sample results indicate that the presence of petroleum hydrocarbons and other chemical compounds in groundwater represents a drinking water concern,” the Environmental Protection Agency said in a statement about tests of 19 water wells around the town of Pavillion.

The Wyoming investigation precedes a national study by the EPA into the safety of the drilling technique known as hydraulic fracturing or “fracking”, in response to concern in Congress and in some communities near gas rigs in many states that human health is threatened by the process.