Local waterlogging often occurs on the steep slopes of clayey–calcareous soils in southwestern France, causing nutrients and pollutants transfer to the river bodies and reduced ecosystems services. These soils developed in the Miocene molassic hill formation and are generally impermeable with abundant traces of hydromorphy and heterogenous spatial distribution. This article aims to describe the hydrological functioning of these soils, based on a cross analysis of pedological, hydrological, and geophysical characterizations. Our experimental site is the catchment area located in Auradé (southwestern France). Here, we analyze the flows at the outlet of the studied watershed together with piezometric and climatic monitoring from September 2020 to September 2021. We show that the hydrological year is divided into three phases: first, a soil recharge phase with an effective rainfall of about 100 mm; second, a saturation phase, when 80% of the effective precipitation is drained mostly by runoff and hypodermic flows; third, a drying phase. Soil waterlogging events usually occur during the saturation phase. They are due to several forms of flow: surface runoff associated with return flow, hypodermic flow caused by the presence of soil layers with lower hydraulic conductivity in the subsurface (swelling clays and plowing sole) and groundwater flow with intermittent connection of the soil water table in the hillside to the alluvial groundwater table. We also conducted independent seismic refraction tomography analyses that validate localized waterlogging patterns along the catchment and open the way to spatializing areas with high waterlogging potential at the scale of the study plot.

Currently, pesticides massively used in agricultural areas end up in watercourses, since they are usually final receptacles of organic contamination. A large number of ponds occurred in agricultural catchments, but their role regarding pesticides behaviour, is still poorly investigated. The Pestipond project aims 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. 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. A regular quadrat of cores (#20cm) was sampled at autumn 2019 and summer 2020. A set of pesticide molecules was quantified as well as sediment texture, and organic carbon and nitrogen content. 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. More hydrophilic pesticides had more affinity with the finest fractions of the sediments. This study also highlights the difference in storage of these molecules according to depth. In particular, boscalid (logKOW=3) was found in greater quantities in the deepest samples, in relation with increasing coarse silt content. Finally, a seasonal effect is also observed on pesticide levels. This work provides new knowledge on the role of ponds in pesticide storage, dissipation and transfer downstream.