Currently, pesticides are massively used and end up in watercourses, mostly in agricultural areas, which are usually final receptacles of organic contamination. In agricultural catchments, there are a large number of ponds, which consist in a complex and dynamic environment. Indeed, today the role of such ponds is still poorly investigated. Ponds are composed of different compartments such as the water column, bottom sediments and vegetation. The Pestipond project aims to characterise the role of wetlands and more particularly natural or artificial ponds, in pesticides transfer and fate within agricultural catchments. More specifically, the objective of the work presented here was to understand the inherent processes in the different pond compartments, especially in sediments since they were indeed supposed to be highly involved in the storage and degradation of pesticides.

The Bassioué pond is located in an agricultural upper sub-catchment of the Auradé critical zone (Gers, France), with a wheat/sunflower crop rotation and a steep slope. It was investigated (i) to understand how and where pesticides are stored in the sediments, and (ii) to determine the physicochemical parameters and the environment characteristics, which influence the pesticides molecules behaviour. Two coring campaigns were conducted over two periods (autumn 2019 and summer 2020). Core samples of about thirty centimetres were taken following a regular quadrat of the pond in order to investigate the spatial variation and distribution of sediment characteristics and of the associated pesticides molecules. A set of pesticides molecules was quantified to evaluate the level of contamination, as well as sediment granulometry and physico-chemical parameters. The results highlighted that sediment texture varied between upstream and downstream of the pond, as did the spatial distribution of pesticides, which was partly controlled by their physicochemical properties. Hydrophilic pesticides had more affinity with the finest fractions of the sediments. This work provides new knowledge on the role of ponds in pesticide storage, dissipation and transfer downstream.