Gasoline and oil also float on water, so shouldn't get beneath the water table. Probably depends on the soil as to how fast it evaporates. I would guess fast in the U.S. coastal plains (ex Florida), where the sand is permeable and the water table close to the surface. In older days, people in Florida would pour oil on ponds and creeks to spread and suffocate mosquito larvae. Many herbicides still have oil to disperse it on leaf surfaces.
