Organochlorine pesticides are called 'persistent' for a reason.

Apr 22, 2010

Clarke, BO, NA Porter, PJ Marriott and JR Blackbeard. 2010. Investigating the levels and trends of organochlorine pesticides and polychlorinated biphenyl in sewage sludge. Environment International http://dx.doi.org/10.1016/j.envint.2010.01.004.

Synopsis by Giffe Johnson

For the first time, Australian researchers find that organochlorine pesticides remain in sewage sludge for more than 10 years.

Despite having phased out the use of organochlorine pesticides in the 1990s, the chemicals continue to be detected in sewage sludge in Australia, according to a study that is the first to examine this issue on that continent.

The findings are important because they show that the chemicals can linger long after they have been banned. As in many countries, the sewage sludge – the solid part of processed sewage – is often recycled and used to fertilize crops. This suggests the pesticides are still  present in agricultural areas of Australia.

The low levels now found in the sludge are in most cases below government regulatory standards and international policy agreements.

Organochlorine pesticides are a class of chemicals that were used to control insect pests since the 1940s. They were mostly phased out of use in the last part of last century due to their longevity, a trait that made them effective for long term pest control, but also increased concerns of potential health outcomes such as cancer in humans and ecosystem disruption.  The chemicals are still detected in people even though have been phased out in many parts of the world. Some countries still use them – mainly DDT – to control mosquitoes that carry malaria.

In this study, the investigators gathered published data about the amounts and sources of the chemicals found in sewage sludge around the world, including the United States, Canada, China and several European countries. They also obtained similar data gathered from five Australian waste water treatment plants between 1995 and 2006. They analyzed and compared the amounts and sources of several organochlorine pesticides – aldrin, chlordane, dieldrin, heptachlor, hexachlorbenzene and DDT – in the sewage sludge over time and among countries.

Researchers first detected the suite of chemicals in sludge in the 1980s and have monitored them since. In some cases, it has taken as long as 15 years after the chemicals were phased out for levels to drop below detection.

Of the organochlorine pesticides monitored, DDE – the breakdown metabolite of the insecticide DDT – and dieldrin are still found at detectable levels in some countries, suggesting it takes even longer for these two compounds to break apart. In Australia, low concentrations of dieldrin and chlordane were detected in sewage sludge for up to 10 years after the pesticides were no longer being used.

These are important finding for countries continuing to use, or that have only recently discontinued use, of organochlorine pesticides. The authors indicate that sewage sludge may not be the only recyclable media that may still contain these persistent pesticides.

While the chronic health effects from low level and long term exposure are poorly characterized, the presence of these persistent chemicals in sewage-based fertilizers will be an environmental factor to consider in future research on the health of people who live in agricultural communities.