Photo: Fracking fluid (AP Photo)

In this March 25, 2014 photo, a machine mixes sand and water, left, before it is pumped underground during a hydraulic fracturing operation at an Encana Corp. well pad near Mead, Colo. Hydraulic fracturing, or "fracking,"is the practice of injecting water, sand and chemicals into source rock to crack it and create escape routes for oil and gas.