Half-Life of DDT
Name: John C. H.
We are studying exponential growth and decay models in a
precalculus course. One of my students has asked if there is such a
thing as a "half-life" of DDT. If so, where do I get the data to model
OOPS! Here is another iceberg inquiry -- the question is only reveals
about 10% of the answer(s). I searched the term "DDT" on www.google.com and
found many "hits". Estimates range from 28 days to 150 years, with some
consensus of about 15-20 years. Here is why the answer is so tricky.
1. If you feed DDT to some test species for some period of time and then
cease and monitor the level of DDT vs. time -- that gives one type of
half-life that depends on the species, its diet, and other factors. For some
birds I saw the figure of about 8 years.
2. If you measure the concentration of DDT in sun-exposed, oxygenated
surface water and have conditions so that there is no steady state -- you
get another half life. Under these conditions volatilization and absorption
onto the river/lake bed occurs. I saw the number 28 days for this route.
3. If you measure the level of DDT in a contaminated soil, to which DDT
adheres tenaciously, you get yet another half-life. This is where I saw the
number from 15 to 20 years.
4. If you test the soil in temperate climates where the average temperature
is milder, this decade or two stretches out to 3 or 4 decades.
5. In cool dry sand, I saw the number 150 years -- although that has to be
suspect because DDT was not widely available until 1945-1950 (after World
If this is for a pre-calc., you could introduce the concept of parallel
first order decay processes!
The half life of DDT is about 12 years.
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Update: June 2012