The proceeds of the sale of bonds, held by any fiscal agent, trustee, or depositary of the commission, not currently required to meet acquisition or construction costs or expenses payable from the construction fund, or interest on the bonds, may be invested in bonds and other obligations eligible for investment of surplus county moneys, subject to such limitations as may be provided in the indenture. Any income or interest thereon shall be added to, and become a part of, the particular fund upon which such interest or income accrues.