The French agricultural sector uses just over 500 organic synthesis molecules included in the formulation of around 2,900 commercially available pesticide products, which have been approved for sale on the market. It is possible that the active ingredients in the pesticides and their metabolites could find their way into the various elements of the environment (air, soil, water, sediments, etc.), as well as into the food chain and thus into our food. These represent a lesser or greater danger to people and to ecosystems with immediate and/or long term effects. Given the risks inherent in pesticides, their presence in water courses and in ground water is being checked with a frequency that has been constantly increasing since the turn of the century. These tests highlight the widespread occurrence of pesticides in aquatic systems, mostly in very small amounts. The results of the analyses make it possible to check that standards are being complied with. These standards, defined at the European level per ingredient, are included in the quality objectives of the European Water

Framework Directive. The quality limit for each pesticide ingredient has been set by the amended Decree of 11th January 2007 at 0.10 μg/L (0.03 μg/L for aldrine, dieldrine, heptachlore and heptachloroe- poxyde) and at 0.50 μg/L for the total amount of quantified pesticides.

The team simulates the desorption of a pesticide from a clay surface in the presence of water. The thermodynamics of this desorption will make it possible to compare the simulated data with the experimental macroscopic data relating to the desorption process, of significant value in understanding the release phenomena in water tables.

At the Laboratoire de Chimie et Physique Quantiques de Toulouse (LCPQ), Fabienne Bessac & Sophie Hoyau, lecturer researchers at Toulouse INP-PURPAN and Université Paul Sabatier respectively, are conducting research aimed at improving our understanding of the interactions between the pesticide molecules and a type of clay, Montmorillonite. The pesticides being studied are atrazine (a herbicide banned since 2004 but still found in groundwater and surface water), metamitron (a herbicide), fenhexamide (a fungicide used most notably on vines), and subsequently glyphosate. A multi- stage approach is being used: From the isolated pesticide to the hydrated pesticide adsorbed in the clay and including the complex between the pesticide and the ions presents in the soil (Na+, Ca2+, etc.).

The theoretical chemical computations run in 2017 led to a publication on metamitron and fenhexamide complexes. The large volume of computing resources allocated to us in 2017 allowed us to obtain a free energy curve for the desorption of fenhexamide in the presence of water for Montmorillonite, as part of the thesis work by Bastien Belzunces submitted on 12th December 2017. The results to be published will act as a reference within the field. These computations made use of GENCI and CALMIP (CALcul en MIdi-Pyrénées) resources for a total of 2,400,000 hours.