Environment Safety Reducing the Risk of Insecticide-Contaminated Soil with Biochar

Researchers have come up with a new method to reduce the spread of the poison and environmental risks associated with insecticide-contaminated soil. In this new method, they bind the insecticide-contaminated soil with biochar.

Gothenburg/Sweden – Thousands of places in Sweden are still contaminated by the insecticide DDT. For three years, researchers at Chalmers have been testing a new method that reduces the spread of the poison and environmental risks by binding it with biochar. When they mixed biochar into contaminated soil at a former forest nursery, they managed to halve the amount of DDT taken up by earthworms in the soil. The method may make it possible to grow certain crops on land that has so far been considered unusable due to the environmental risks.

The legacy of previous generations' use of the insecticide DDT still affects us. During the 1950s and 1960s, the substance was used to control pests, and despite being banned for over fifty years, there are now over 700 DDT-contaminated former forest nurseries in Sweden – as well as thousands of commercial gardens with similar problems.

The toxin has been linked to a range of negative health effects in humans and animals, and breaks down very slowly. It poses an environmental risk because it can be taken up by soil-dwelling organisms such as earthworms. When these are in turn eaten by birds and other animals, DDT begins to accumulate along the food chain, meaning that predators at the top of the chain are affected by the highest levels of the toxin.

Researchers at Chalmers have now tested a new method to reduce the environmental risks of the poison, on a DDT-contaminated former forest nursery of 23 hectares in Skåne.

“In our field experiments, we have mixed the soil with biochar and grown different plants, and it has been shown that the biochar binds DDT effectively so that it cannot be absorbed by soil organisms,” says Paul Drenning, postdoctoral researcher at the Department of Architecture and Civil Engineering and first author of the study.

Cheap and environmentally friendly solution on site

Biochar is an environmentally friendly product similar to charcoal, and is cheap to produce. It binds pollutants and can improve soil health when added to soil. This measure can also be done for climate reasons, as it involves long-term storage of carbon in the soil.

In measurements, the researchers were able to see that the amount of DDT that was passed on to earthworms in the soil was reduced by an average of 50 percent where the soil had been mixed with biochar. This suggests that DDT's bioavailability had been reduced – meaning that the soil becomes less toxic, and causes a lower risk of spread via bioaccumulation in the animal food chain or leaching into water.

These reduced environmental risks could, in turn, lead to landowners being able to start farming again on land that is currently unused, pending decisions on how to manage the contaminated soil.

“Treating contaminated soil in such large volumes is costly and complicated. A common solution is to dig up and then transport the soil to a hazardous waste landfill, but this means that fine natural soils are destroyed and is not a feasible solution for large areas,” says Paul Drenning.

“Treating with biochar on site could therefore make the soil useful instead of leaving it unused or emptying it, and at a significantly lower cost for both the landowner and the environment.”

Long-term effect expected

Examples of crops that could be grown in the test area are seeds for pine and spruce, hay for animal feed or energy forests such as salix (willow, willow and willow). The treatment with biochar also means that plants absorb less DDT from the soil, but they absorb very little even without treatment.

“The reason why the contaminated land is left unused today is not that there are health risks with the crops, but that the landowner is obliged to address the environmental risks with DDT. While awaiting an investigation and decision on this, the land has been unused,” says Jenny Norrman, assistant professor at Architecture and Civil Engineering and leader of the research project.

Biochar breaks down very slowly in the soil, and the researchers expect the effects of the treatment to last for a long time – perhaps decades. They will continue to take samples at the site for several years to follow the progress. In parallel, they will explore how to scale up the experiment and mix biochar into the soil on a large scale, without having to dig up large volumes.

Great potential for the method

Using biochar for post-treatment of contaminated soil is unusual today. As far as the researchers know, the method has not been tested previously in forest nurseries in Sweden, or internationally in the same type of soil and climate.

“There is great interest in using biochar in various situations, and not only to handle DDT but also to be able to bind metals, polyaromatic hydrocarbons and other pollutants. Therefore, it is positive that we have been able to see good effects in our trials,” says Paul Drenning.

Soil is a valuable resource with a very slow rate of regeneration – a single centimetre of soil can take hundreds of years to form. Within the EU, it is estimated that 60-70 percent of soils have deteriorated health, and soil pollution is an important cause. Active work is now underway within the EU to be able to control these pollutants better. The EU Commission's upcoming soil directive contains new, stricter rules for sustainable land management and the remediation of contaminated areas. For Sweden, the directives require new working methods to be able to take soil health into account.

In addition to the treatment's effect on DDT, the Chalmers researchers have investigated several other aspects of soil health, such as effects on soil functions such as nutrient turnover, water regulation and carbon storage in the soil. The study is a demonstration of a general methodology that the researchers have developed for soil health evaluation. It is designed to be used in society, for example by landowners who need to decide on different remediation options for contaminated land.

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