A gas flare burns at a fracking site in rural Bradford County Pennsylvania

 
A gas flare burns at a fracking site in rural Bradford County Pennsylvania
A gas flare burns at a fracking site in rural Bradford County, Pennsylvania in this January 9, 2012 file photograph. Fracking involves the high-pressure injection of sand, water and chemicals into shale to crack it open and allow the flow of gas or oil. It has sparked an energy boom in several regions of the United States and driven natural gas prices to 10-year lows. But environmental concerns about fracking also have grown, from complaints of contaminated groundwater in Pennsylvania and Wyoming to earthquakes in Ohio last year that a state agency linked to fracking. REUTERS/Les Stone/Files (UNITED STATES - Tags: ENVIRONMENT ENERGY)