Costs allocated to the purpose of municipal, industrial, and miscellaneous water service, commercial power, and the portion of recreation and fish and wildlife enhancement costs reimbursable under the Federal Water Project Recreation Act [
16 U.S.C. 460l–12 et seq.], shall be repaid within 50 years with interest. The interest rate used shall be determined by the Secretary of the Treasury, taking into consideration average market yields on outstanding marketable obligations of the United States with remaining periods to maturity comparable to the applicable reimbursement period during the month preceding the fiscal year in which the costs are incurred. To the extent that more than one interest rate is determined pursuant to the preceding sentence, the Secretary of the Treasury shall establish an interest rate at the weighted average of the rates so determined.