Most people know that gasoline floats on water.
However, that is not always the case in the subsurface environment. We know that a given volume of gasoline contains less mass and therefore weighs less than does an equal volume of water. A beach ball contains less mass and therefore weighing less than the same volume of water will float. The first figure displays a leaking underground storage tank. Separate phase is depicted both floating on the water table and sinking some depth into the water table.