Pennsylvania regulators on Tuesday called on the natural gas industry to voluntarily change the way it disposes of potentially harmful drilling waste.

The state is asking drilling companies to stop sending the waste to a group of treatment plants that are not equipped to remove certain contaminants before discharging it into rivers.

Natural gas drilling is growing quickly in Pennsylvania, largely because of a drilling technique called hydrofracking that involves injecting chemical-laden water deep underground to release gas deposits.

The fate of that water — whether it is recycled, pumped back into the earth or sent to a treatment plant — has become a subject of greater scrutiny.