Spatial and temporal variability of the functioning of a Dogger karst aquifer

Célestine Delbart – 2013

The karst aquifer are among the aquifer the most difficult to characterize. They are known to be characterized by complex hydrological responses to rainfall event due to the high heterogeneity of porosity, from matrix porosity to conduit porosity. The porosity of conduit induces a high transfer velocity in the aquifer which is the cause of the karst aquifer vulnerability. The study objective is to define the modality of water transfer and solute in a karst aquifer of Dogger in Burgundy. Two complementary methods are used : (i) the analyse of piezometric and debitmetric time series, (ii), the geochemical and isotopical characterization of groundwater based on the using of large panel of tracers, notably some dating tracers. These two methods are applied at different temporal scales : a high frequency monitoring (1 hour time step) during discharge event to study fast water transfer and a long time monitoring to characterize slow water transfer and study the seasonal variability of water transfer processes. This study allow to prove a large distribution of residence time of groundwater, from several hours during recharge event underlined by short response time rainfall-piezometric level (less to 70 hours) to several years for the baseflow showed by the using of dating tracers (39 Kr, CFC-12 et SF6). In response of rainfall event, several water bodies can be described : (i) a water body with slow transfer velocity ; it is the baseflow; showed by dating tracers ; (ii) a direct recharge, whose the residence time in the karst aquifer varies depending on the porosity, from several hours to several days, underlined by a continous monitoring of electrical conductivity, of major element and tritium ; and (iii), stored water in unsaturated zone, remobilized during recharge event, showed by the variation of geochemical composition in tritium and Ca2+ in groundwater during recharge event. Despite of the high heterogeneity of water transfer processes, the fast water transfer is spatially organized. The response time of piezometric level to rainfall event and the residence time increase toward the aval of hydrogeological hill. The temporal study of impulse response by the method of the sliding-window cross-correlation shows that the response time varies seasonally, it is shorter during the summer. The variability of response time is significantly, until a hundred hours. This temporal variability could be explained in part by a variability of the rainfall intensity which induces a variability of water flow processes in the epikarst. During intensive rainfall, the saturation of epikarst is higher which induces lateral transfers and allows transferring water toward large conduits.