Long-term persistence of dieldrin, DDT, and heptachlor epoxide in earthworms

Earthworms can accumulate persistent soilborne insecticides and are an important source of contamination of terrestrial wildlife. We treated experimental plots once at $9.0\ {\rm kg}\cdot {\rm ha}^{-1}$ with dieldrin, DDT, or heptachlor, and measured changes in insecticide concentrations in earthworms over a 20-year period. We estimated "half-times," defined as the time for a concentration in earthworms to be reduced by half. Dieldrin had a half-time of 5.4 years. DDE, the metabolite of DDT most important to wildlife, increased until the third year and then decreased with a half-time of 5.7 years. Heptachlor epoxide, the metabolite of heptachlor most important to wildlife, increased until the second year and then decreased with a half-time of 4.3 years. The declining parts of the curves of all three compounds fit exponential decay equations reasonably well. The estimates of persistence are relevant to insecticides at low or moderate concentrations in relatively undisturbed soils in temperate climates.