Re: How is pesticide removed from water (lakes,oceans,rivers)

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.