Pesticide Distribution in Pond Sediments from an Agricultural Catchment (Auradé, SW France)

Abstract

Currently, pesticides are massively used in agricultural areas end up in watercourses, as they are usually final receptacles of organic contamination. A large number of wetlands such as ponds occurred in agricultural catchments. They are dynamic and complex systems which contains different compartments like the water column, sediments and biota. The sediment compartment can store contaminants according to their physicochemical properties, but this process is continuously challenged because of settling and resuspension mechanisms. The role of the sediments regarding pesticides behavior and their fate is still poorly investigated. Our study aimed to fill this gap, particularly considering the Bassioué pond, which is located in the carbonated agricultural upper sub-catchment of the Auradé critical zone observatory (Gers, France), with a wheat/sunflower crop rotation and a steep slope enhancing erosion phenomenon. We focused on pesticide storage in the sediments which represent the main compartment due to the environmental conditions. Our current objective was to understand (i) how and where pesticides are stored in the sediments, and (ii) the relationship with the characteristics of sediments, supposed to be highly involved in the storage and degradation of pesticides. Two field campaigns were carried out to collect sediment samples within a regular quadrat from the inlet to the outlet of the pond in autumn 2019 and summer 2020 at different depth. A set of three pesticide was quantified as well as sediment texture. The results highlighted that sediment particle size distribution varied between upstream and downstream of the pond: from the finest to the coarsest; as did the spatial distribution of pesticides. This reveled that pesticides were partly controlled by their physicochemical properties: most hydrophilic pesticides had greater affinity with the finest fractions of the sediments. A difference in pesticide storage according to the depth has been observed, especially for boscalid (logKOW = 3) which was found in greater quantities in the deepest samples, with increasing coarse silt content. Finally, a seasonal effect was also observed on pesticide levels, as their presence was highly driven by firstly their uses in the catchment and secondly by the soil erosion occurring during intense spring flood events. This work provides new knowledge on the role of ponds in pesticide storage, dissipation and transfer downstream, which can·be used for agricultural landscape management.

Keywords

• wetland
• pesticides
• bottom sediments
• agricultural catchment
• storage
• spatial distribution