Iowa Code § 50.9: Return of ballots not voted.

Ballots not voted, or spoiled by voters while attempting to vote, shall be returned by the precinct election officials to the commissioner, and a receipt taken for the ballots. The spoiled ballots shall be preserved for twenty-two months following elections for federal offices and for six months following elections for all other offices. The commissioner shall record the number of ballots sent to the polling places but not voted. The ballots not voted shall be destroyed after the end of the period for contesting the election. However, if a contest is requested, the ballots not voted shall be preserved until the election contest is concluded.