(a) Any department, agency, or instrumentality of the state or any political subdivision of the state is authorized to insert in the specifications of all contracts relating to the installation, extension, improvement, maintenance, or repair of any water or sewer facility a clause providing for the retention of amounts not exceeding 10 percent of the gross value of the completed work as may be provided for in the contract; provided, however, that no amounts shall be retained on estimates or progress payments submitted after 50 percent of the work on the project has been completed if in the opinion of the department, agency, or instrumentality of the state or any political subdivision thereof such work is satisfactory and has been completed on schedule. This will not affect the retained amounts on the first 50 percent of the work on the project which may continue to be held to ensure satisfactory completion of the project. If, after discontinuing the retention, the department, agency, or instrumentality of the state or any political subdivision thereof determines that the work is unsatisfactory or has fallen behind schedule, retention may be resumed at the previous level. Retainage shall be invested at the current market rate and any interest earned on the retained amount by such department, agency, or instrumentality of the state or any political subdivision of the state shall be paid to the contractor when the project has been completed within the time limits specified and for the price specified in the contract, or in any amendments or change orders approved in accord with the terms of the contract, as certified pursuant to subsection (b) of this Code section.

(b) Final payment of the retained amounts to the contractor under the contract to which the retained amounts relate shall be made after certification by the engineer in charge of the project covered by the contract that the work has been satisfactorily completed and is accepted in accordance with the contract, plans, and specifications. Payment to the contractor of interest earned on the retained amounts shall be made after certification by the engineer in charge of the project covered by the contract that the work has been completed within the time specified and within the price specified in the contract.

(c) At substantial completion of the work and as the governmental entity's authorized contract representative determines the work to be reasonably satisfactory, the governmental entity shall within 30 days after invoice and other appropriate documentation as may be required by the contract documents are provided pay the retainage to the contractor. If at that time there are any remaining incomplete minor items, an amount equal to 200 percent of the value of each item as determined by the governmental entity's authorized contract representative shall be withheld until such item or items are completed.