The past four years of U.S. environmental regulation was marked by a crackdown on emissions that angered coal miners and power companies. Over the next four, the new head of the Environmental Protection Agency will have to decide whether to take on an even larger industry: Big Oil.
Following Lisa Jackson's resignation on Wednesday, her successor will inherit the tricky task of regulating a drilling boom that has revolutionized the energy industry but raised fears over the possible contamination of water supplies.
The controversial technique at the center of the boom, hydraulic fracturing, involves injecting millions of gallons of water laced with chemicals deep into shale rocks to extract oil and gas. It has become a flashpoint issue, putting the EPA -- charged with safeguarding the nation's water -- in the middle of a fight between environmentalists and the energy industry.