(1) The county commissioners may by resolution make such changes in the boundaries of a district as they shall deem reasonable and proper but may not delete any portion of the proposed area which will create an island of included or excluded lands.
(2) They may not delete any portion of the proposed area which is contributing or may reasonably be expected to contribute to the pollution of any watercourse or body of water in the proposed area.
(3) For the purpose of subsection (2), it will be conclusively presumed that an area which is within 1,500 feet of a proposed or existing sanitary sewer is contributing to the pollution of a watercourse in the proposed area.