Within thirty (30) days after concluding all said hearings, but in any event within six (6) months of the filing of said petition with said department, the department shall make its determination of the lands to be included within the proposed district and state whether or not the persons or property within such proposed district will benefit directly or indirectly from the use or right to use the ground water supplies within such proposed district and the replenishment thereof, and this determination shall be filed with the board of supervisors of the principal county.