Waters on the majority of Indian reservations do not have water quality standards under the Clean Water Act to protect human health and the environment. Only 40 of over 300 federally recognized tribes withreservations have completed the process of obtaining EPA's approval to be treated in a manner similar to a state (TAS), and adopting standards for their waters that EPA has approved. EPA proposes to streamline how tribes apply for TAS for the water quality standards program and other Clean Water Act regulatory programs. The proposal would reduce the burden on applicant tribes and advance cooperative federalism by facilitating tribal involvement in the protection of reservation water quality as intended by Congress. Since 1991, EPA has followed a cautious approach that requires applicant tribes to demonstrate inherent authority to regulate waters and activities on their reservations under principles of federal Indian common law. The Agency has consistently stated that its approach was subject to change in the event of further congressional or judicial guidance addressing tribal authority under section 518 of the Clean Water Act. Having received such guidance, EPA proposes to conclude definitively that section 518 includes an express delegation of authority by Congress to eligible Indian tribes to administer regulatory programs over their entire reservations. This reinterpretation would eliminate the need for applicant tribes to demonstrate inherent authority to regulate under the Act, thus allowing tribes to implement the congressional delegation of authority unhindered by requirements not specified in the statute. The reinterpretation would also bring EPA's treatment of tribes under the Clean Water Act in line with EPA's treatment of tribes under the Clean Air Act, which has similar statutory language addressing tribal regulation of Indian reservation areas. This action would not revise any regulatory text. Regulatory provisions would remain in effect requiring tribes to identify the boundaries of the reservation areas over which they seek to exercise authority and allowing the adjacent state(s) to comment to EPA on an applicant tribe's assertion of authority. As a streamlining step, the proposed interpretive rule would have no significant cost.