By James McCarten THE CANADIAN PRESS WASHINGTON- Indigenous leaders in Canada and the U.S. are turning up the pressure on Ottawa for an investigation of toxic mining runoff from B.C., despite high-level bilateral promises of an agreement by this summer to “reduce and mitigate” its impact. Tribal and First Nations leaders were in Washington, D.C., last week to plead their case at meetings of the International Joint Commission, the body that oversees the 114-year-old treaty governing waters that straddle the Canada-U.S. border. They said they are encouraged, but not convinced, by the commitment Prime Minister Justin Trudeau and President Joe Biden made in March to reach an agreement in principle by summer to address pollution in the Elk-Kootenai watershed. “Mining pollution across our homelands jeopardizes our culture,…