Unless otherwise provided in the Act authorizing construction of the project, the delivery and distribution of municipal and industrial water supplies shall be deemed to be an authorized project purpose under sections 421a to 421h of this title, and where appropriate, an allocation of loan funds acceptable to the Secretary shall be made between irrigation and municipal and industrial purposes. Loan repayment contracts shall require that the borrower pay interest on that portion of the unamortized loan obligation (including interest during construction) allocated in each year to municipal and industrial purposes at the rate provided in the Act authorizing the project, or absent such an authorized rate, at a rate determined by the Secretary of the Treasury as of the beginning of the fiscal year in which the contract, or contract amendment entered into pursuant to section 421f of this title, is executed, on the basis of the computed average interest rate payable by the Treasury upon its outstanding marketable public obligations which are neither due nor callable for redemption for fifteen years from date of issue, and by adjusting such average rate to the nearest one-eighth of 1 per centum.