The functioning of karst aquifers in a sedimentary basin emphasized by catchment sinkholes analysis
Résumé
In parts of the Aquitaine sedimentary basin (France), groundwater resources management is hampered by the karstic nature of carbonate aquifers. This leads to temporal variability of flows and chemical signatures of springs and high groundwater vulnerability regarding pollution. For two years, a study has started in Dordogne Department in order to improve the knowledge of the carbonate aquifers structure and functioning. Part of the study focuses on the identification of infiltration processes: the role played by surface forms such as sinkholes can be critical because they may constitute preferential infiltration paths to the groundwater and impact the quantity, the quality and transit times of water from the surface. An inventory of sinkholes was conducted based on cartographic information (geological maps, satellite images, etc.) and literature (reports from consultants and thesis). This information was used to build a map of the distribution of sinkholes by aquifer. It revealed a heterogeneous distribution of sinkholes in the area and a contrasted density between different reservoirs. The confrontation of this map with hydrogeological data brings insights on the functioning of karst aquifers. It appears that Jurassic carbonate aquifers can be binary karst systems with part of the recharge coming from surface drainage on impermeable bedrock and Cretaceous carbonate aquifers are mostly unary karst systems solely fed by infiltration at the top of limestone. In addition, a specific work was initiated on the influence of sinkholes on the springs chemical signature. It was based on the analysis of springs with or without sinkholes in their catchment area. Initial results showed that higher contaminant concentrations (e.g., nitrate) are observed in springs fed by catchments with high density of sinkholes.