43 U.S.C. § 494

If any operation or maintenance charge is unpaid on the 1st day of the third calendar month after it became due a penalty of 1 per centum of the amount unpaid shall be added thereto, and thereafter an additional penalty of one-half of 1 per centum of the amount unpaid shall be added on the 1st day of each calendar month if such charge and penalties shall remain unpaid.