It is widely recognized that DDT is long lived in the environment. For example, according to the Agency for Toxic Substances and Disease Registry "DDT lasts a very long time in soil; half the DDT in soil will breakdown in 2-15 years." So I was very surprised to read in Dixy Lee Ray's Trashing the Planet (page 70) that:
Contrary to common belief, DDT is not a persistent pesticide in the natural environment. Only in the unusual circumstances where soil is dark, dry, and devoid of microorganisms will DDT persist. Under normal environmental conditions, DDT loses its toxicity to insects in a few days, usually no more than two weeks.
So I checked Ray's source, Elizabeth Whelan's Toxic Terror. In a footnote on page 70 we read (note: parenthetical remarks left out):
DDT has been shown to remain active for a period on the order of one to two weeks. In the soil, without light, water, or soil microorganisms, DDT persists well. However, under normal environmental conditions, DDT loses its toxicity to pests often after a few days and usually within weeks.A paper by R. G. Nash and E. A. Woolsen is the basis of claims that it takes DDT seventeen years to lose 39 percent of its activity in soil. The experiment was performed on a small plot of soil to which DDT was applied at a concentration equaling forty pounds per acre. At this level soil microorganisms, which usually break down DDT at one to five pounds per acre, were killed. The plot was kept dry, in the dark, and was kept free of all vegetation. The authors stated that the figure of seventeen years could very well represent the maximum persistence of DDT and its metabolizes in the soil. The authors did a good job in their experimentation but the results should not be taken as indicative of the nature of DDT in the environment.
I then checked the Nash & Woolson paper, which appeared in the prestigious science journal Science. The paper does not support the claim that DDT breaks down rapidly in the environment, and the test conditions do not match Whelan's description. Here are some of the discrepancies:
1. "The experiment was performed on a small plot of soil"
Nash & Woolson used nine soil plots, three different soil types with three different rates of application. Three more plots were used as controls and received no DDT.
2. "DDT was applied at a rate equaling forty pounds per acre."
DDT was applied at rates of approximately 25, 100 and 400 lbs per acre.
3. " At this level soil microorganisms, which usually break down DDT at one to five pounds per acre, were killed."
Whelan did not document her claim that soil microorganisms break down DDT. The paper does note that that the high rates of application "may have eliminated much of the soil's zoological population", but no microbial analysis was done. The authors noted that DDT is metabolized to DDD, but DDD is still part of the DDT family and not a harmless substance..
3. "The plot was kept dry"
Nowhere in the paper is there any mention of the plots being kept dry. In fact, the authors describe extracting the insecticides from moist soils, and determining the moisture contents of the samples.
4. "in the dark"
Again, there is no mention of the plots being kept in the dark.
5. "was kept free of all vegetation"
Absolutely not. The authors state that "Soils were cropped at various times until 1962, in both series. Weeds were controlled by cutting, cultivation or black plastic film. Since 1962 weeds were cut when necessary and allowed to decompose on the soil surface." (Note: the DDT experiment ran from 1949 till 1966.)
Comments
Nash and Woolson make it very clear that their test conditions did not mimic normal agricultural practices. But the test conditions were not the severe and unusual ones described by Whelan. Ray distorted the test conditions even further, and made claims totally unsupported by the paper. Whelan and Ray both ignored the many additional papers on the persistence of DDT in the environment, several of which were cited in Nash & Woolson.
References
Nash, Ralph G. & Edwin A. Woolson, "Persistence of Chlorinated Hydrocarbon Insecticides in Soils", Science, Vol. 157, August 25, 1967, pages 924 to 927.
Ray, Dixy Lee & Lou Guzzo, Trashing the Planet: How Science Can Help Us Deal with Acid Rain, Depletion of the Ozone, and Nuclear Waste (Among Other Things), HarperPerennial, 1990.
Whelan, Elizabeth M., Toxic Terror, Jameson Books, 1985..
Written by Jim Norton
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