Workers move a section of well casing into place at a Chesapeake Energy natural gas well site near Burlington, Pa. Fracking uses water mixed with sand and chemicals to break underground rock and release large amounts of gas. (Associated Press)

Workers move a section of well casing into place at a Chesapeake Energy natural gas well site near Burlington, Pa. Fracking uses water mixed with sand and chemicals to break underground rock and release large amounts of gas. (Associated Press)