Workers adjust piping during a short pause in water pumping during a natural gas hydraulic fracturing operation at an Encana Oil & Gas (USA) Inc. drilling site outside Rifle, in western Colorado. The first experimental use of hydraulic fracturing was in 1947, and more than 1 million U.S. oil and gas wells have been fracked since, according to the American Petroleum Institute. The National Petroleum Council estimates up to 80 percent of natural gas wells drilled in the next decade will require hydraulic fracturing. Photo: AP
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