When you drill for natural gas, for every gallon of gas produced, some amount of wastewater gets created as well.
Sometimes it can be simple brine that can be disposed of in simple ways, such as using it to melt snow on Pennsylvania's roads in winter. Or it is used to keep the dust down in summer.
But the wastewater can also be pretty nasty stuff, which can't be cleaned up by water treatment plants. One option is to dump it down an old gas well, shooting it deep into the earth. It's a method used in thousands of wells across the country. Only five of those currently operate in Pennsylvania.