Pollution perhaps linked to gas drilling vanishing

"There is a general downward trend" in conductivity levels for 2012, Reilly said. "That's good news."

Steve Forde, a spokesman for the Marcellus Shale Coalition, an industry group, said its members had "been committed from the very outset" to complying with the state request to stop discharging wastewater at regular treatment plants. Now, most of the brine is either recycled or sent to deep underground wells for disposal.

Still, VanBriesen and Reilly cautioned that the Monongahela River is still far from being a pristine waterway. Coal-fired power plants and old flooded coal mines also discharge water into the Monongahela River that can contain bromides and other pollutants, and many other industries are based in the region.

The Marcellus Shale lies under parts of Pennsylvania, New York, Ohio, West Virginia and Maryland. The gas drilling procedure hydraulic fracturing, or fracking, has made it possible to tap into deep reserves of oil and gas but has raised concerns about pollution. Large volumes of water, along with sand and hazardous chemicals, are injected underground to break rock apart and free the oil and gas.

Regulators contend that water and air pollution problems are rare, but environmental groups and some scientists say there hasn't been enough research on those issues. The industry and many federal and state officials say the practice is safe when done properly.