SALLADASBURG, Pa. - An unknown amount of the fluid used in natural gas drilling spilled onto a roadway and into a creek after a collision involving two tractor-trailers in Central Pennsylvania, state police say.

The Williamsport Sun-Gazette reports ( one truck rear-ended the other on Route 287 early Monday afternoon in Mifflin Township, shutting down the roadway for several hours.

Police say the second truck was pushed through a stop sign and down an embankment.

The truck overturned and was leaking fluid used in a type of natural gas drilling known as hydraulic fracturing or "fracking."

Environmentalists and other critics worry fracking, which involves blasting the chemical-laced water into the ground, could poison water supplies. But the natural gas industry says it's been used safely for decades.