As congressional Republicans and industry advocates assailed U.S. EPA for a suite of new air regulations last year, the Obama administration responded with peer-reviewed studies and statistics showing that protecting air quality provides an economic boost by improving public health and protecting natural resources.

But in the fight over water regulations, EPA and its allies find themselves lacking ammunition.

“There’s nothing comparable in the water sector,” said Steve Polasky, a professor of ecological-environmental economics at the University of Minnesota.