A typical hydraulic fracturing operation on March 29 outside Rifle, Colo. In the background, yellow tanks hold water for the job at an Encana Oil & Gas (USA) Inc. gas well. Pump trucks are parked in front of the tanks. Workers control the flow of water, sand and chemicals into the well heads, left, from an operations trailer, center far right. Critics say fracking chemicals and hydrocarbons released can contaminate groundwater. Industry officials say an absence of documented, widespread problems proves the process is safe.