Regulators ordered a Montgomery County wastewater-treatment operation last year to halt accepting liquids from a Marcellus Shale natural gas driller after discovering that hundreds of truckloads had been improperly imported into the Delaware watershed.

The Delaware River Basin Commission, after learning that 1.8 million gallons of wastewater had been trucked to Hatfield Township, ordered a private industrial treatment facility and the Hatfield Township Municipal Authority to cease accepting the fluids from northern Pennsylvania.

Much of the liquid - primarily contaminated water from the hydraulic fracturing process - was discharged into Neshaminy Creek after it had been treated, a process that removes some solids, but does not purify the waste.

George Stark, spokesman for Cabot Oil & Gas Co., the Texas gas driller that produced the wastewater at its Susquehanna County well sites, said the contractor that treated its wastewater sent it to the Philadelphia area without Cabot's knowledge.

"As soon as we discovered the water was being sent to the Delaware River basin, the operation ceased," Stark said.

The DRBC did not fine Cabot or PSC Environmental Services L.L.C., the private company that treated the wastewater at its Hatfield plant before discharging it to the township sewer system.

"In our minds, the most important issue was that they stop taking the imported material," said Clarke Rupert, a DRBC spokesman.

But the Delaware Riverkeeper Network, an environmental group that has led efforts to restrict gas-drilling activity in the watershed, said the unauthorized discharges underscored its doubts about the effectiveness of regulators.

In a letter Tuesday to the DRBC, Maya K. van Rossum, the head of the riverkeeper organization, urged the commission to take "swift and strong" enforcement action against the driller.

"Because of this blatant violation of DRBC's rules and regulations, and the precedent that gets set for the future, it is important that the DRBC exercise all due enforcement authority for this violation," van Rossum wrote.

The environmental group learned of the Hatfield Township discharges only after it was contacted recently by a reporter from the Associated Press, which reported the incident this week.

The DRBC said it learned of the discharges in May, after PSC had been treating the wastewater for nearly a year. It sent letters to Cabot and the treatment plant June 1 ordering them to cease, said Rupert.

Peter Dorney, executive director of the municipal authority, said it stopped accepting PSC's treated drilling waste immediately after it was notified by regulators.

Dorney said the Marcellus wastewater was only a small portion of the six million gallons a day that passes through the municipal plant in Colmar. The additional drilling waste did not cause the plant to exceed its effluent quality, he said.

"We didn't violate any of our limits," Dorney said.

Wastewater from Marcellus drilling operations contains some chemicals used in the hydraulic fracturing process. But the primary contaminant that worries regulators is the high volume of total dissolved solids - TDS - which can make the wastewater many times saltier than seawater.

In August, the Pennsylvania Department of Environmental Protection began enforcing stricter regulations limiting TDS discharges to 500 milligrams per liter. The Delaware River Basin Commission is considering regulations that would prohibit discharges altogether in the Delaware.

DEP Secretary John Hanger said that no Pennsylvania streams - including Neshaminy Creek - exceeded the limits because of natural gas drilling.

"There isn't one place in the commonwealth that isn't meeting that standard," he said.

Hanger took issue with media reports that implied shale drillers were discharging untreated wastewater into rivers because of lax regulation.

"The state's booming Marcellus drilling industry is creating challenges, but Pennsylvania has stepped up to meet those challenges at every turn," he said. "We've doubled the number of oversight staff and now have arguably the nation's most aggressive oversight program."