A new federal rule barring waste fluids out of shale oil and gas operations from being sent to public sewage plants for treatment and disposal might have inadvertently swept up Pennsylvania’s traditional oil and gas industry’s wastewater as well.

The U.S. Environmental Protection Agency’s rule was meant to make sure a discarded practice stays dead. Marcellus Shale companies operating in Pennsylvania had voluntarily agreed in 2011 to stop using public treatment plants to manage their wastes. Shale oil and gas operators in other regions of the country never embraced municipal sewage plants as a disposal option.

Wastewater from unconventional oil and gas extraction often contains high concentrations of salt, as well as chemicals, metals and radioactive materials, the EPA said in its justification for the rule. Public sewage treatment plants are not designed to remove those pollutants, which can flow through to streams untreated or interfere with the plant’s normal function.

The new rule, published on June 28, was not meant to apply to conventional oil and gas operations, which generally use vertical wells to tap shallower formations and produce smaller amounts of waste fluids than unconventional wells. The EPA plainly said that its final rule “does not include pretreatment standards for wastewater pollutants associated with conventional oil and gas extraction facilities,” although the agency reserved the right to develop such standards.

But guidance documents published with the final rule describe in more detail what the EPA considers a shale or tight oil and gas formation — those layers of rock that are covered by the rule. The list includes three geological designations in the Appalachian basin — the Devonian, the Tuscarora and the Clinton-Medina formations — that have been tapped by conventional drillers for decades.

The Pennsylvania Independent Oil and Gas Association, a Wexford-based trade group, flagged the issue in July and urged companies to push the EPA to clarify its intentions.

Unlike shale operators, many of the state’s traditional oil and gas drillers continue to use public treatment plants to manage the waste fluids that travel up their wells along with oil and gas.

According to industry-reported figures published by the Pennsylvania Department of Environmental Protection, public treatment plants accepted about 337,000 barrels of waste fluids from the state’s conventional wells in 2014.

PIOGA spokesman Dan Weaver said the geological formations listed by the EPA encompass many of the oil- and gas-bearing layers targeted by conventional producers in Pennsylvania.

“You’re talking thousands of wells that have already been drilled into these that now are falling into that category,” he said.

If the EPA’s rule is as broad as the list implies, it would be “a huge impact” to some operators because they would have to find other, more expensive ways to manage their wastewater on short notice, he said.

“It’s devastating to our traditional operators up in the western part of the state,” he said.

An EPA spokesman said the agency “has heard the concerns from industry stakeholders and is considering this information.”

Mr. Weaver said industry representatives and the EPA have made progress in their discussions, and are working through the issue together.

“Clearly, I don’t believe it was their intent to include the conventional operators,” he said. “It’s just the fact that there’s some wording in there and now they’re looking at it.”

A complicating factor in the discussions is time: The EPA rule is scheduled to take effect next Monday. The industry also faces a Nov. 9 deadline if it wants to challenge the rule in court.

“We’re willing to pursue this to whatever degree it takes. We’re just hoping it doesn't take litigation,” Mr. Weaver said. “We’re hoping that both parties will be able to sit down together and clearly discuss this.”

--  Laura Legere: llegere@post-gazette.com.