Fifteen percent of the modification costs shall be assigned to project purposes in accordance with the cost allocation in effect for the project at the time the work is initiated. Non-Federal interests shall share the costs assigned to each purpose in accord with the cost sharing in effect at the time of initial project construction: Provided, That the Secretary of the Interior shall recover costs assigned to irrigation in accordance with repayment provisions of Public Law 98–404.

Repayment under this subsection, with the exception of costs assigned to irrigation, may be made, with interest, over a period of not more than thirty years from the date of completion of the work. 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 reimbursable period during the month preceding the fiscal year in which the costs are incurred, plus a premium of one-eighth of one percentage point for transaction costs. 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.