The NYT is reporting on a letter sent by House Democrats alleging that some service companies have illegally used diesel fuel in their frac fluids: "Oil and gas service companies injected tens of millions of gallons of diesel fuel into onshore wells in more than a dozen states from 2005 to 2009, Congressional investigators have charged. Those injections appear to have violated the Safe Water Drinking Act, the investigators said in a letter to the Environmental Protection Agency on Monday."
For a copy of the letter, see here. According to industry (in the NYT article): "Oil and gas companies acknowledged using diesel fuel in their fracking fluids, but they rejected the House Democrats’ assertion that it was illegal. They said that the E.P.A. had never properly developed rules and procedures to regulate the use of diesel in fracking, despite a clear grant of authority from Congress over the issue."