An example of POP persistence from an era of peak-POP-use

Nash, R. G., & Woolson, E. A. (1967). Persistence of chlorinated hydrocarbon insecticides in soils. Science, 157(3791), 924–927. https://doi.org/10.1126/science.157.3791.924

Credit to Nic Uren for providing me with this article.

Why read this article?

This was a controlled long-term field trial executed by the Crops Research Division of the U.S. Department of Agriculture (USDA) which covered over 16 years from 1949 to 1966 to determine decomposition rates of various persistent organic pollutants (POPs). Hence, this article was an opportunity to compare decomposition rates of different POPs which were applied at different rates.

It was claimed that the decomposition rates that were observed would be as low as they can be (i.e. be the upper limit of persistence), because efforts were apparently made to control and minimize losses of the pesticide from the soil (mostly due to minmum soil disturbances).

What were the main conclusions?

The half-lives of eight different chlorinated POPs (not including DDT) applied to one soil called Congaree loam were 2 - 11 years.

DDT was separately applied to three other soils (Sassafras loam, Chester loam and Evesboro loamy sand) and had the greatest mean persistence with half-lives from 2.5 - 35 years. In two of those soils the application rate of 200ppm appeared to reduce biological degradation, i.e. this concentration was increasingly toxic to soil biology.

What were its main limitations?

From the results in this paper it is impossible to make predictions for other soils and locations without knowing additional soil characteristics.

No other soil measurements were provided (e.g. soil carbon content, mineralogy, pH, EC etc).

No climate data were provided (mean annual max/min temperature, precipitation).

No information was available on the soil characteristics other than their names and texture (Congaree sandy loam, Sassafras loam, Chester loam, Evesboro loamy sand but the soil profiles and locations are described in an official series which is available online - see links.

It also used two different methods to assay the insecticide concentrations throughout that period (1949 - 1966) because methods have evolved (Although since the use of the GC/ECD which were used in 1966 in this paper - it seems that methods have stayed the same pretty much…).

What does this mean for POPs?

Different types of chlorinated POPs have different decomposition rates.

What do I think about it?

I found it difficult to make much use of the data provided. It certainly provided a reference point of half-lives for the climate around Beltsville, Maryland and showed that different chemistries of chlorinated pollutants have different decomposition rates.

From what I could determine, the climate in Beltsville, Maryland is a humid subtropical climate (Cfa according to the Köppen climate classification) which means that persistence would be relatively lower compared to drier mediterranean climates. I expect that risk of persistence is greater in less humid climates.

However, I would love to test those soils today and see how much POP residue is left in these soils. I hypothesise that they are still present today!