U.S. EPA took a step today toward resolving a thorny drilling issue that has challenged the credibility of both the agency and the oil and gas industry when it proposed guidelines for hydraulic fracturing with diesel. The issue stems from an exception in 2005 law that environmentalists call the "Halliburton loophole." It exempted hydraulic fracturing from regulation by EPA under the Safe Drinking Water Act, unless diesel fuel is in the mix.