HARRISBURG — Companies drilling in the lucrative Marcellus Shale natural gas formation in Pennsylvania face a Thursday deadline to stop taking salty, chemically tainted wastewater to riverside treatment plants as state regulators prepare to check for compliance in an effort to protect drinking water.

Many of the largest drillers say they have already stopped the practice and now either reuse nearly all of the wastewater that gushes from gas wells or truck it to Ohio for disposal in about 170 underground injection wells in that state.

Louis D’Amico, president of the Pennsylvania Independent Oil and Gas Association, said some companies might even have to stop drilling temporarily until they get a plan in place.

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 Pennsylvanians.

Gov. Tom Corbett’s administration last month asked drillers to stop using the 16 treatment plants, which aren’t equipped to remove all the pollutants from wastewater that gushes from gas wells.

The April 19 request was made after some researchers presented evidence that the discharges were altering river chemistry in heavily drilled Western Pennsylvania.