|MadSci Network: Environment & Ecology|
Good question with a complex answer unfortunately. Someone once pointed out to me that nothing on earth is new. The gold your mother has in her ring may once have belonged to a famous Egyptian king, the oxygen you are breathing may once have been in water drunk by Elvis and the paper you print this answer on may contain carbon that was once in the hair of Alexander the Great. Natural systems are set up so they can reuse resources time and time again sometimes (as in the oxygen and carbon) in totally different chemical forms. The problem is that the change may take a very long time and/or in the case of pesticides cause harm to organisms that it comes in contact with. The answer to how are pesticides removed from water lays in several processes that change the form of the chemicals involved. It is an accepted law of science that all things over time reduce to less structured forms. This is called entropy. In the case of pesticides (or a leaf from a tree, an animal corpse or a log on the forest floor) this process is mostly undertaken by micro-organisms and is called bio (Greek for “life”) degrade (to break down) – you have probably heard of “environmentally friendly”, biodegradable products. This process can either happen by micro-organisms in the presence of oxygen (aerobic) or without oxygen (anaerobic). For good word definitions and lots of science info check out http://www.factmonster.com. I have used a now infamous pesticide DDT, or dichloro-diphenyl- trichloroethane, to walk you through the process of removal of pesticides from a river. DDT was invented in 1873 and discovered for its pesticide potential in 1939, its inventor, Paul Muller, received the Noble Prize in 1948. However, by the late 1940s problems relating to the affects of DDT were being discovered and articles doubting its safety being published. In 1973 it was banned in the US after nearly wiping out the Bald Headed Eagle and other top predators. DDT has fairly complex and unfortunately very stable structure which means that natural processes have a difficult job of breaking it apart into its basic building blocks (atoms such as hydrogen, carbon and chlorine). DDT can therefore have a half life of up to eight years. So if you have 10 mg of DDT per kg of soil then in 8 years you will have 5mg/kg. Unfortunately it will take another 8 years to reduce this to 2.5mg/kg etc. Information sourced at Chemistry@University of Oxford. In aerobic (such as in flowing streams rich in oxygen) DDT is degraded by an enzyme (a protein that the micro-organism uses to break down its “food”) called DDT-dehydrochlorinase which, as its name would suggest, removes a hydrogen and a chlorine atom from the DDT’s structure. This is the beginning of the end for the DDT molecule. Under anaerobic conditions (such as a oxygen-poor lake that is clogged up with rotting plant life and weeds) a different micro-organism breaks down DDT. Its “eating” results in a chlorine atom being removed which is automatically replaced by a nearby hydrogen atom. Once again the chemical is changed to a less strong structured molecule and this is the beginning of the end of DDT. Other water based degradation processes include photodegradation where the energy from the sun is capable of energising the atoms that make up DDT and can cause enough movement to split the molecule. This is similar to the heat energy you add to water when you turn the kettle on. Eventually individual water molecules become so excited that they start to separate from the mass of water and float off as steam. DDT can also be absorbed by particulates in the water (things like dust and dirt and other things that make water seem murky) and/or sink to the floor of the lake and be held in the sediment. While this would result in any water tests for DDT being reduced you must remember that it is still there in the lake and could resurface at a later time. One way it may resurface is when it is metabolised (eaten up) by bottom dwelling insects, micro-organisms, worms etc and stored in their fatty tissues. Once in one organism it is not long before another comes along and eats it and then it is exposed. So in a strict sense, in answering the question you have asked another way of “removing” pesticides from the lake is in the tissues of those being slowly poisoned by it. If the animal leaves the lake (like a frog) or does not live in the lake (like a cat or a bird) then the DDT is lost from the lake. If the predator lives in the lake (like a predator fish) then the DDT is no longer in the actual water but will be later recycled into the fatty tissues of the predator’s predator or in the organisms that eat it when it dies. Therefore the cycle goes on and on. To learn more on this natural recycling system look up “nutrient cycle”. To learn more on the storage of toxins in animals tissues look up “biomagnification” – this is where concentrations of toxins build up in a predator as it eats lots of smaller animals each with very small quantities of toxins in their tissues. Under a combination of all of these processes surface water efficiently pulls DDT’s half life down to only 56 days. Info sourced at http://wvlc.uwaterloo.ca. These days there is a growing environmental awareness (particularly in the western world) about long-term environmental problems such as DDT or CFCs. Therefore there is a greater focus on non-toxic, biodegradable chemical products. Cross your fingers that modern pesticides do not become infamous in 25 yrs time.
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