Environmental groups said in court Monday that the Tennessee Valley Authority is essentially storing toxic ash from an aging Tennessee coal-fired power plant in a colander, letting pollutants seep into a major river in violation of the Clean Water Act. In the bench trial that began Monday in federal court in Nashville, TVA responded that the Tennessee Clean Water Network and Tennessee Scenic Rivers Association can’t prove the federal utility is polluting the water supply in violation of the law or its permits at the Gallatin Fossil Plant, 40 miles outside Nashville.