It may mean what will happen to a polluting material after a long time. For example, the insecticide DDT would be in insects, then in the birds that eat them. It would stay in the birds and harm their ability to lay eggs. Another example is chemicals from a mine left over after ore is removed. They are dug out of the ground and collected in piles. Rain falls on the piles, and washes the chemicals into the ground, where they appear in wells of drinking water.