The sources of oil pollution that most of us think about are the major spills that make news headlines. The two most major sources of oil pollution in Canada are 1. Municipal discharge, and 2. Now, there are different techniques and stricter regulations for bilge washing, so that it doesn't make such a big mess. Once the oil enters the water, many chemical, physical and biological processes take place that change the composition and distribution of the oil.
Oil usually stays at the surface of salt water because it has a density that is less than seawater.
However, some heavier oils are denser than freshwater and may sink to the bottom. Oil that sinks to the bottom of a water column is very difficult to clean up. Less dense oil also spreads more quickly, forming a sheen on top of the water that looks like a rainbow from above. Increased temperature also causes oil to spread quickly, while cold water causes the oil to solidify. There are many different ways to clean up after an oil spill.