It shall be the duty of the board of trustees of any conservancy district organized under this Act to proceed diligently and without delay to prevent pollution of any stream or any other body of water within the confines of such conservancy district and to proceed at once to diligently cause any and all parties, persons, firms and corporations to cease any and all pollution of any such streams or body of water within such district; provided that the authority of the Pollution Control Board of the State of Illinois or its successor as may be fixed by law shall not be superseded.