COMMON fungicides could be to blame for the puzzlingly high levels of DDT still found in some soils, even in regions where the potent insecticide was banned decades ago.
DDT, introduced in the 1940s, was widely banned at the beginning of the 1970s after research revealed a host of damaging effects on animals. It disrupts hormones, dents fertility rates and causes drastic thinning of birds鈥� eggshells. Controversially, its use is still permitted in some malaria-ridden countries.
When DDT gets into the ground, it is usually broken down by soil bacteria into the by-products DDE and DDD. Research on farmland has shown that the breakdown is generally slow but effective: 20 years after the last spraying with DDT, the soil commonly contains just 1/20th as much DDT as DDE. But in some areas levels of DDT are still surprisingly high, even when the insecticide has not been applied in more than 30 years.
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Sally Gaw and her team at Waikato University in Hamilton, New Zealand, think they have discovered why. When they analysed soil from 13 orchards around Auckland they found nine that tested positive for DDT, of which six had much higher levels than expected. New Zealand has no guidelines for DDT levels, but the most contaminated of Gaw鈥檚 samples contained 24.4 milligrams of DDT per kilogram of soil, four times the maximum permitted by the US Environmental Protection Agency.
Further testing of samples of the contaminated soil revealed a high copper content, most likely from common fungicides which contain copper as their active ingredient. The correlation between copper and high DDT levels was striking, says Gaw.
鈥淚 was very surprised when I drew that first straight line,鈥� Gaw says. 鈥淩esearchers in Canada and Australia have reported elevated DDT in soils, but because they didn鈥檛 look at copper, they didn鈥檛 pick up the link.鈥�
Previous studies have shown that copper can kill some soil microbes. Now it seems that communities of bacteria that combine to break down DDT could be among their victims, says Gaw. The DDT levels Gaw鈥檚 team report are high enough to be a problem for animals. Rai Kookana, an expert in soil contamination at CSIRO Land and Water in Adelaide, part of Australia鈥檚 largest national research organisation, says they are 鈥渁 cause for concern from the point of view of impact on wildlife鈥�.
Copper-containing fungicides have been banned in the European Union since March 2002 because of their harmful effects on soil bacteria, but they are still widely 0used in the US, Australia and New Zealand.