@socalledunitedstates it depends on the contaminants involved. Stuff like creosote is dense and will collect in low areas so it is a threat to groundwater below but not so much for anything above. Other contaminants that are low density or highly water soluble could leach into soil in any direction.
To be safe though it might be best to line the bottom anyways. Also as long as the contamination is contained it is best not to disturb it (ie. Don't try to dig it all up).