A bipartisan group of U.S. senators, led by Iowa Sen. Charles Grassley and Minnesota Sen. Amy Klobuchar, last week demanded that the Environmental Protection Agency (EPA) end the practice of providing so-called hardship waivers that exempt refineries from blending enough biofuel to comply with the Renewable Fuel Standard (RFS).

The senators also want the EPA Administrator Scott Pruitt to provide information on the waivers that have already been issued and to outline the agency’s plan to make the waiver process more transparent.

“There is no...