Sec. 9. (a) The proportion set aside to the depreciation account shall be expended in making good depreciation in the waterworks or in new construction, extensions, or additions to the property of the waterworks.
(b) Any accumulations in the depreciation account may be invested by the board, and if invested, the income from the investment shall be carried in the depreciation account. The board may invest the funds in accordance with IC 5-13-9.