Bioremediation is the clean up of oil, toxic chemicals, or other environmental pollutants. Often it impossible to remove all contaminated material, so it is then contained. An example of a need for containment would include uranium, an inorganic pollutant. An organic pollutant, hydrocarbon, can be completely degraded by microorganisms, eventually becoming CO2. Petroleum in an oil spill is degraded by microorganisms. Because oil and water do not mix an oil spill will cause a slick layer of the water. Microorganisms will stick to this layer and degrade the oil eventually dispersing the slick. Some chemicals have never been seen by the environment, called xenobiotics. Xenobiotics include pesticides and dyes amongst other chemicals. Some xenobiotics are so chemically different that they take years to degrade, if at all. Another example of a xenobiotic is plastics.