Re: How long does untreated oil take to decompose

Hi Tara, This is a very interesting question. You have probably seen pictures of the worst oil spills such as the Exxon Valdez spill a few years ago. The immediate effects on wildlife can be very severe, but it's often observed that there are fewer long-term effects than you might expect and affected areas may recover well after about two years.

To answer your question, the answer as often in science is "it depends". When oil is spilled in water the first way in which it may be broken up is through physical processes. This means by the action of the waves and by evaporation. If it is a light,refined oil, it may be broken up into fine droplets which can evaporate quite easily and this can be quite effective at removing the oil. But a heavier more "tar-like" crude oil may not break up and evaporate so easily. Physical processes act quickly and their effects are noticeable in a few days.

The next processes that act upon the oil are chemical. Oil reacts with oxygen and chemical reactions in the oil are accelerated by sunlight. Again these depend on the nature of the oil; a thick crude oil tends to produce large molecules that stick together and may sink to the bottom of a lake or ocean. The chemistry would therefore depend on the type of oil, the temperature and the sunlight, but would again be noticeable within days or a few weeks.

Finally, there are biological processes that can break down oil, which I suspect is what you were thinking of when you said "decompose". Many bacteria are capable of producing enzymes that can break down the chemicals in oil. However this is a slow process-populations of bacteria that are doing this are usually not observed until several months after a spill. Again, the large complicated molecules in thick crude oils are harder to break down than the smaller, simpler molecules because there are less enzymes that can do these reactions. Also for bacteria to work effectively they have to grow and multiply, so it needs to be warm and they need a supply of nutrients (especially nitrogen and phosphorus) to do their work. In some oil spills people have tried adding fertiliser to the water to encourage the bacteria. Of course the Great Lakes are quite badly polluted with excess nutrients already, so the bacteria might be well fed! But it might also be rather cold for them.

So you can see there are many factors involved in how quickly oil disappears-the waves, sunlight, temperature, size of the spill, type of oil, type of bacteria present, nutrients and so on. There are some beaches around the coasts of Europe where oil from spills 20 or 30 years ago can still be found under the sand, in other places it is all but gone 2 years after the spill. I would guess at a few years for oil to degrade in a place like the Great Lakes.
Neil