A pair of environmental groups and the Confederated Salish and Kootenai Tribes are challenging the federal government’s approval of a measure Montana lawmakers passed last year that loosens water quality standards. In a lawsuit filed in federal district court in Great Falls on Monday, Upper Missouri Waterkeeper, Flathead Lakers and CSKT contend that the U.S. Environmental Protection Agency’s decision to approve House Bill 664 is “inconsistent with” the Clean Water Act. In a letter to its state counterpart, EPA wrote that the revised water quality standards “meet requirements to protect designated uses” under the Clean Water Act, the 54-year-old law U.S. Congress passed to clean up polluted rivers and lakes. “The law requires agencies to understand how gutting water pollution control standards would impact the state’s clean water resources, including threatened and endangered species, before making a decision. The tug-of-war over water quality standards that can garner the approval of environmental and industrial stakeholders without coming afoul of state and federal regulators has proven a very difficult nut to crack.