DDT’s legacy lasts for many decades
Mucky agricultural soils containing residues of DDT, a banned insecticide, will contaminate the atmosphere for more than a generation, much longer than previously believed.
More than 40 years after Rachel Carson’s Silent Spring indicted DDT with harming eagles, and 30 years after the insecticide was banned in North America, historic residues in soil remain the largest source of contamination to the air worldwide. For the first time, research posted today on ES&T’s Research ASAP website (DOI: 10.1021/es060216m) measures the evaporation of DDT from farmland where it was last applied more than 3 decades ago. The study shows that DDT, a persistent organic pollutant, will malinger in some highly organic soils for more than a generation.
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“We’re doing environmental archaeology—we’re digging up ‘old bones’ of pesticides last used in the 1960s and 1970s and finding out what impact they’re having on atmospheric levels today,” says Terry Bidleman, an environmental chemist at Environment Canada and a coauthor of the study. He and his colleagues analyzed a mucky field in southern Ontario that is high in organic matter and has been continuously farmed for vegetables such as carrots and cabbage. Although tilling the soil helps DDT to dissipate, the levels remaining in the soil are sufficiently high to pose a hazard to the animal food chain there, including earthworms and birds, says Jules Blais, an environmental toxicologist at the University of Ottawa (Canada).
The study is the first to use micrometeorology, the measurement of wind speed and temperature gradients within the first 2 meters above the surface, to track the evaporation of long-banned pesticides. The researchers discovered that the soil contains 19 parts per million (ppm) DDT, exceeding the Canadian Council of Ministers of the Environment’s criterion [156KB PDF] of 0.7 ppm for protecting the environment and human health. Air concentrations of DDT were 60–300 times higher than background levels. The scientists recorded 5.7 nanograms of DDT per cubic meter (ng/m3) at the soil surface and 1.3 ng/m3 at 200 centimeters above the ground.
The researchers expected to find high soil levels of DDT in this field because large amounts had been applied before 1972 and the soil’s high organic content is a magnet for fat-loving DDT, says Perihan Kurt-Karakus, an environmental scientist at Lancaster University (U.K.) and lead author of the study. However, the scientists were stunned when they estimated that by evaporation alone it would take 200 years for half the DDT in the soil to dissipate, Bidleman says. Fortunately, other mechanisms, such as breakdown by microbes, bring the actual dissipation rates of DDT in soil down to the order of decades, not centuries, he says.
In contrast, two surveys of southern U.S. agricultural soils that were done within the last 10 years found that only 1 out of 100 sampled soils exceeded 0.7 mg/kg. Southern U.S. farm soils, which are low in organic matter, are calculated to have a DDT half-life of 5–20 years, Bidleman says. “The concentration of DDT in soil can vary 1000-fold depending on the past application rate, organic matter content, and cultivation practices,” he says. The researchers would like to repeat the study on other soil types, then combine the data with GIS (geographic information system) maps of land use to predict soil residues and evaporation of DDT over large geographic areas.
The Organisation for Economic Co-operation and Development is currently adopting models to assess the long-range transport potential of chemicals, Blais says. “We can draw from our DDT experience to learn and ultimately place this information in the models and use DDT to [verify the accuracy of] the models,” he adds.
“This study and others like it indicate that we will be exposed to a sea of chemicals in the atmosphere for quite a while to come,” says Mary Jane Incorvia Mattina, an analytical chemist with the Connecticut Agricultural Experiment Station. In most cases, environmental exposures to individual contaminants involves relatively small concentrations, but the sum total of exposure to banned pesticides, such as DDT, chlordane, and toxaphene, on top of the smorgasbord of new chemicals on the market, may be impinging on our overall health, she says.
