The U.S. Environmental Protection Agency is investigating a well being drilled in western Pennsylvania for possible violations of air, water or hazardous waste rules.
The investigation at the undisclosed site began last September, the agency said last week.
"Legitimate concerns have emerged about potential environmental impact," spokeswoman Bonnie Smith said.
The EPA investigation was disclosed by the Pittsburgh Post-Gazette. It noted that Washington County south of Pittsburgh has more wells and more compressor stations to pump natural gas than any other county in the region.
Chesapeake Energy Corp,the Oklahoma-based firm is the No. 1 driller in Ohio.
Rig Count Interactive Map by Baker Hughes, an energy services company.
Shale Sheet Fracking, a Youngstown Vindicator blog.
The Ohio Environmental Council, a statewide eco-group based in Columbus.
Earthjustice, a national eco-group.
People's Oil and Gas Collaborative-Ohio, a grass-roots group in Northeast Ohio.
Concerned Citizens of Medina County, a grass-roots group.
No Frack Ohio, a Columbus-based grass-roots group.
Fracking: Gas Drilling's Environmental Threat by ProPublica, an online journalism site.
Pipeline, blog from Pittsburgh Post-Gazette on Marcellus shale drilling.
Allegheny Front, environmental public radio for Western Pennsylvania.