Pennsylvania regulators on Tuesday called on Marcellus Shale natural gas drillers to stop sending wastewater to 15 treatment plants, citing an increased risk of contaminating public drinking water.

The Department of Environmental Protection's action, while voluntary, will likely set the stage for a formal ban on the discharge of inadequately treated wastewater into the state's rivers.

"Now is the time to take action to end this practice," acting DEP Secretary Michael Krancer said in a statement Tuesday.

DEP's action comes as the U.S. Environmental Protection Agency and activists step up pressure on Gov. Corbett to increase regulation of the shale-gas boom, including the massive volumes of toxic wastewater produced by hydraulic fracturing.

"Amid growing concern by the public and increased scrutiny by the media, we are happy to see DEP finally take these critical steps to once and for all stop dangerous, undertreated Marcellus Shale wastewater from entering our waterways and drinking-water supplies," said Erika Staaf, a spokeswoman for the PennEnvironment advocacy group.

DEP's announcement came the day after Corbett, who has been criticized for his close ties to drillers and his refusal to support a gas-production tax, assured local officials he would not allow the industry to "poison the water."

"We need to protect the water," the governor, a Republican, said at a meeting of the Pennsylvania State Association of Township Supervisors. "But we must do it based on science, not emotion."

The Marcellus Shale Coalition, an industry trade group, said it welcomed DEP's announcement.

Kathryn Z. Klaber, the coalition's president, said that the industry was already recycling much of its wastewater and that drilling operators were "very confident they can in fact meet these additional limits."

Klaber said that the DEP's action was "timely" and that it was a "perfectly reasonable assumption" that the state would eventually put its voluntary order into enforceable regulations.

The DEP and the industry appear to have been influenced by complaints from public water suppliers in Western Pennsylvania, which say they are challenged by bromide levels whose concentrations have increased concurrently with the drilling boom.

The bromides themselves are not a public health risk - they account for a tiny part of the salty dissolved solids that create an unpleasant taste in water at elevated levels.

But bromides react with the chlorine disinfectants used by drinking water to form brominated trihalomethanes (THMs), a volatile organic compound. Studies have linked the prolonged ingestion of high levels of THMs to several types of cancer and birth defects.

Officials at several water authorities in the Pittsburgh area say their facilities have failed several tests for trihalomethanes in recent years.

Researchers at Carnegie Mellon University in Pittsburgh, who are in the final stages of a two-year study of trihalomethanes in the Monogahela River, have shared their preliminary data with federal and state regulators.

Bromide levels had been in the low to moderate level until last July, when the rates inexplicably spiked, said Jeanne M. VanBriesen, an engineering professor who heads the university study.

Since the spike, bromide levels have steadied at "slightly elevated" levels, she said.

She said the signature of the bromide concentrations suggests that the source is either water produced from oil and gas wells, or wastewater from pollution-control equipment of coal-fired power plants.

The level of trihalomethanes in public drinking water has also increased, she said, though she said that none has exceeded EPA limits of 80 micrograms per liter in a three-month average.

"I can't say the plants are in noncompliance," VanBriesen said. "But we saw levels that, if they persisted over a long period, would amount to violations."

VanBriesen said she was encouraged by the state's asking drillers to stop taking wastewater to treatment plants after May 19. "It's always nice to see science being used for something," she said.

Researchers expect bromide levels to decline after that date, reinforcing the presumption that gas drilling is responsible for the elevated levels.

"While there are several possible sources for bromide other than shale-drilling wastewater, we believe that if operators would stop giving wastewater to facilities . . . bromide concentrations would quickly and significantly decrease," the DEP's Krancer said.

Bromides, chlorides, and some heavy metals occur naturally in deep rock formations such as the Marcellus Shale, the massive deposit that underlies much of Pennsylvania and parts of several surrounding states.

In other regions where shale production has taken off, operators dispose of the wastewater in deep, federally regulated injection wells. But Pennsylvania's geology is insufficiently porous to accept large volumes of wastewater.

Marcellus drillers initially responded by trucking the wastewater to injection wells in Ohio and West Virginia or taking it to Pennsylvania treatment plants that are not designed to remove salty compounds, including bromides.

Regulators and the industry say they recognized several years ago that the disposal options were unsustainable, and the industry developed recycling techniques that allow the wastewater to be used in new wells.

Wastewater can also be treated in energy-intensive distillation plants that concentrate the toxins in a heavy brine, which is sent to injection wells.

Pennsylvania last year strengthened the discharge limits on total dissolved solids (TDS). But some plants were allowed to continue accepting the wastewater, as long as river levels did not exceed limits.

Now that practice appears to be nearing an end.

"While the prior administration allowed certain facilities to continue to take this wastewater, conditions have changed since the implementation of the TDS regulations," Krancer said.