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Persistence of pesticide residues in soils: the benefits of national monitoring

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Unlike in aquatic environments and the atmosphere, pesticide contamination of soils is not monitored on a national scale. However, recent work by INRAE researchers, in collaboration with the University of Bordeaux, shows that a large number of substances persist in the soil in the form of residues.

Photo : A large number of substances, in significant quantities, persist in the soil in the form of residues © pixabay.com
A large number of substances, in significant quantities, persist in the soil in the form of residues © pixabay.com

Environmental contamination by pesticide residues has been monitored for many years in aquatic environments and in the atmosphere. This is not yet the case for soils.

However, a few recent European studies, notably involving teams from INRAE and Ifremer, point to the presence of many substances in soils and the resulting risks for biodiversity.

To build on these initial results, INRAE researchers worked with scientists from the University of Bordeaux to assess pesticide contamination in nearly fifty soils sampled throughout mainland France.

Using the Réseau de Mesures de la Qualité des Sols (RMQS - in French), they were able to measure the persistence of these substances and the associated risks for soil biodiversity.

Result: 98% of the sites studied contained at least one substance. A total of 67 different molecules were found, mainly fungicides and herbicides.

Plots of arable land are the most contaminated, with up to 33 different substances found at a single site, and an average of 15 molecules in the soil. More unexpectedly, in soils under forests, permanent grasslands, wasteland or used for organic farming for several years, more than 32 different pesticides were detected, at mainly lower concentrations than for field crop sites.

The most frequently detected molecules were glyphosate and AMPA, its main metabolite, present in 70% and 83% of the soils sampled. Triazole fungicides (epoxiconazole) and succinate dehydrogenase inhibitors (SDHI) were also found in over 40% of sites, as were pyrethroid insecticides such as tefluthrin [1]. While most of the substances tested are used only in conventional farming, some molecules, such as pyrethroids, can be used in organic farming.

If herbicides contribute the most to the total concentrations of pesticides found in soils, the major risk estimated for earthworms is linked to insecticides and fungicides. The risks of chronic toxicity for these earthworms are moderate to high for all cultivated plots.

These large-scale studies demonstrate the unexpected persistence of pesticide molecules in the environment, well beyond their theoretical degradation time and at higher concentrations than expected [2]. These results underline the need for increased soil monitoring, which could be based on the French national RMQS network that has been in place for over 20 years.

[1] The theoretical degradation times of 90% of the initial concentrations of the molecules in the soil (DT90) are as follows: Glyphosate 170 days; AMPA 1,000 days; Epoxiconazole 2,960 days; tefluthrin 160 days.

[2] The expected concentrations were calculated on the basis of information provided by the farmers on the phytosanitary treatments carried out on the plots (date of application and doses applied), using theoretical degradation times.

How are pesticide residues measured in soil?

Forty-seven RMQS sites were sampled by the programme's regional partners throughout France between 2019 and 2021, at a depth of 0 - 20 cm based on the protocol implemented within the network (Jolivet et al., 2018). These are mainly cultivated soils (field crops, vineyards and orchards) but also supposedly untreated soils (grasslands, forests, wasteland). 111 substances, prioritised by the French Agency for Food, Environmental and Occupational Health & Safety (Anses) on the basis of their use and behaviour in the environment, were looked for in soil samples by the LPTC laboratory at the University of Bordeaux and the Laboratoire d'Analyse des Sols d'Arras at the INRAE Hauts-de-France centre. This project, called Phytosol, was funded by Anses via a research agreement (2018-CRD-17_PPV18).

Bibliographic references

Claire Froger, Claudy Jolivet, Hélène Budzinski, Manon Pierdet, Giovanni Caria, Nicolas P. A. Saby, Dominique Arrouays, and Antonio Bispo Environmental Science & Technology Article ASAP DOI: 10.1021/acs.est.2c09591

Scientific contact

  • Antonio Bispo

    AGROECOSYSTEM scientific department - INRAE Val-de-Loire centre

    antonio.bispo%40inrae.fr