HARRISBURG — Pennsylvania’s environmental regulators believe that companies drilling in the huge Marcellus Shale natural gas formation have largely stopped taking salty, chemically tainted wastewater to riverside treatment plants that are ill-equipped to remove all of the pollutants, an official said Friday.

Department of Environmental Protection spokeswoman Katy Gresh said the agency’s staff has confirmed that the flow of millions of gallons of wastewater has dwindled to possibly just a handful of truck deliveries in the past two weeks.

“We know we have achieved a dramatic sea change here, but we are continuing to verify that we have complete compliance from all the operators all the time,” Gresh said.

The agency had set a May 19 deadline for the companies to voluntarily stop taking drilling wastewater to treatment plants that discharge into rivers — 16 plants, largely in Western Pennsylvania. The DEP took the step amid growing concern over harm to drinking water.

Many of the largest drillers have said they have stopped the practice and now either reuse nearly all the wastewater that gushes from gas wells or truck it to Ohio for disposal in that state’s underground injection wells.

Since drilling companies began using high-volume hydraulic fracturing in earnest in 2008 to extract natural gas from the shale, they have taken millions of barrels of the briny waste to treatment plants that discharge into rivers where utilities also draw drinking water for hundreds of thousands, if not millions, of Pennsylvanians.