49 CFR 176.70: Stowage requirements for marine pollutants.

(a) Marine pollutants must be properly stowed and secured to minimize the hazards to the marine environment without impairing the safety of the ship and the persons on board.

(b) Where stowage is permitted “on deck or under deck”, under deck stowage is preferred except when a weather deck provides equivalent protection.

(c) Where stowage “on deck only” is required, preference should be given to stowage on well-protected decks or to stowage inboard in sheltered areas of exposed decks.