Présentations Colloques

In the context of global change, understanding the long-term evolution of groundwater resources in irrigated regions is essential for water and food security in the face of multiple pressures on the aquifers (increases in water demand, urbanisation and climate change). This project explores effects of the pumping and irrigation cycles on both groundwater quality and quantity in a region in France exposed to a strong growth in groundwater irrigation over the past 20 years. The scientific challenges in irrigation regions include the need for a deeper understanding of the temporal and spatial variability in recharge from rainfall and irrigation waters. Secondly, defining the origins and mixing of waters from multiple sources (rain, surface water, irrigating waters, shallow and deep groundwater) that eventually contribute to the groundwater resource is essential for sustainable use. Furthermore, a comprehensive understanding on the links between the hydrodynamic variability and agrochemical contaminant transfers in heterogeneous hydrogeological systems is required. In this project, dating tracers (14C, CFCs, SF6 and 3H) are coupled with environmental tracers (delta18O, delta2H, delta13C, and major and minor ions), water quality parameters (nutrients and pesticides), and physical and chemical models to investigate shallow groundwater in an unconfined sand and clay aquifer in the Allier Department, central France. Due to the limited historical hydrogeological data for this aquifer, the use of dating and environmental tracers is essential to understand hydrosystem dynamics. Results highlight a groundwater resource that rapidly recovers from seasonal declines in hydraulic heads of up to 27.5 m due to pumping for irrigation. This recovery is associated with variable chemical responses depending on the tracers+ highlighting the influence of water from diverse sources. The evolution of nutrient (e.g. NO3 10-24 mg L) and herbicide (e.g. atrazine 0.05 to 0.17 micro g L) concentrations during variations in groundwater drawdown indicates a shallow system vulnerable to rapid vertical transfers of contaminants.< p>