If, after examining documentary evidence and hearing testimony, the regulating district board shall modify such apportionment, the revised apportionment shall not become effective until approved by the Department of Environmental Conservation and a copy thereof is served and filed in the same manner as upon the completion of the same in the first instance at which time the apportionment shall be final and conclusive. If the regulating district board adopts a resolution approving the apportionment without modification, the apportionment shall be final and conclusive.