Cal. Pub. Util. Code § 70246

All premiums and accrued interest received on the sale of bonds shall be placed in the fund to be used for the payment of principal of and interest on the bonds. The remainder of the proceeds of the bonds shall be placed in the district treasury to the credit of the proper fund, and shall be used exclusively for the objects or purposes for which the bonds were voted; provided that when such objects and purposes have been accomplished any moneys remaining shall be transferred to the fund to be used for the payment of principal of and interest on the bonds, and that when all principal of and interest on the bonds shall have been paid, any balance of money then remaining shall be transferred to the general fund of the district.