Sequestration of DDT and dieldrin in soil: Disappearance of acute toxicity but not the compounds

House fly (Musca domestica), fruit fly (Drosophila melanogaster), and German cockroach (Blattella germanica) were used to assay the acute toxicity of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and dieldrin aged in sterile soil. Significant reduction in mortality of the insects was observed in DDT- and dieldrin-amended soil samples aged for 30 d, and toxicity decreased further with additional aging. After 270 d, DDT and dieldrin were no longer toxic to house flies, and dieldrin was no longer toxic to fruit flies and cockroaches at 120 d. Nevertheless, 84.7% of the DDT and 92.1% of the dieldrin was recovered from the soil by vigorous extraction after 180 and 270 d, respectively. The results show that toxic chemicals residing in soil become less toxic with time, the extent of decline in bioavailability may differ among different species, and vigorous extraction may grossly overestimate toxicity.