A coalition of four senators Wednesday sent a bipartisan letter to the Environmental Protection Agency (EPA) expressing their "growing concerns" about the broad approach that the agency seems to be taking to regulate fluids used in hydraulic fracturing (fracking), not just diesel fuels.

"Specifically, we are concerned about EPA's intention to regulate hydraulic fracturing under the Safe Drinking Water Act (SDWA) when diesel fuel is used. From a practical standpoint, a key issue is whether EPA's actions will cause unnecessary confusion and open the door for states to lose their primacy for UIC [Underground Injection Control] permitting programs," wrote Sen. Lisa Murkowski of Alaska, the ranking Republican on the Senate Energy and Natural Resources Committee; and Sen. James Inhofe of Oklahoma, the ranking Republican on the Senate Environment and Public Works Committee; as well as Sens. Mary Landrieu (D-LA) and John Hoeven (R-ND).

"My greatest concern lies with the extremely broad approach that EPA appears to be taking in defining diesel fuel," Murkowski said. "For example, it appears EPA could define 'diesel fuel' as any product that contains similar carbon chain characteristics. Diesel fuel...is similar, on a molecular level, to mineral and vegetable oils. Mineral oil is increasingly being used in fracking fluids, and according to the FDA [Food and Drug Administration], is safe enough for human consumption. Treating and regulating it the same as diesel fuel therefore appears to make little sense," the senators said.

The "EPA's definition of 'diesel fuel' will be the linchpin of its regulatory action. Some are urging EPA to broadly define the term to include a wide range of hydrocarbons and petroleum distillates. Whether EPA pursues guidance or regulation, we believe that a vague definition would overreach EPA authority, open the door for increased litigation and create a significant burden on the 40 states that will be responsible for implementing and enforcing any regulations under the SDWA," the senators said.

The SDWA exempts fracking fluids (with the exception of diesel fuel fluids) from federal regulation and keeps it in state hands. Murkowski is concerned that too broad a definition of diesel fuel by the EPA could bring non-diesel fuel fluids under the regulatory wing of the agency. EPA currently is developing UIC permitting guidance for fracking activities that use diesel fuels.

Before Congress passed the Energy Policy Act in 2005, the EPA believed that the SDWA was never meant to regulate fracturing, the senators said. And "for five years following its passage, EPA took no action to implement the discretionary authority that it had been granted [with respect to diesel fuels]. Last year, however, EPA reversed its position. This began with a website notice that it would consider all hydraulically fractured wells where diesel is used for Class II wells under the UIC program, and was followed by the initiation of a process to develop guidance for implementing its SDWA authority. We believe the UIC Class II wells for which EPA is developing guidance are a strange fit for hydraulic fracturing regulation," they said.