California Code, Health and Safety Code - HSC § 13935

(a) All premiums and accrued interest received from the sale of the bonds shall be deposited with the district treasurer in a special bond service fund to be used for the payment of the principal of and interest on the bonds, and the remainder of the proceeds of the bonds shall be placed to the credit of the proper improvement fund and applied exclusively to the purpose and object recited in the proposition approved by the voters.

(b) When the purpose and object have been accomplished, any moneys remaining in the improvement fund shall be transferred to the special bond service fund. When the purpose and object have been accomplished and all principal and interest on the bonds have been paid, any balance of money then remaining shall be transferred to the general fund of the district.