The Interior Department is walking back its 2015 pledge to regulate toxic pollution generated by the use of explosives at open-pit coal mines.

The Office of Surface Mining Reclamation and Enforcement, part of the Interior Department, said it doesn’t believe it has the legal authority to regulate air pollution associated with blasting at surface coal mines, according to a Federal Register notice scheduled for publication July 30.

WildEarth Guardians petitioned the office in 2014 to regulate nitrogen oxide emissions produced from explosives used at coal mines. The office during the Obama administration instead decided to go...