Chalk is a porous aquifer (matrix porosity up to 40%), fissured and karstic (the extent of karstification varies regionally), and the site of both regional flows and rapid karst circulation. Surface flows infiltrate preferentially through dolines (locally called bétoires) or sinkholes within surface cover formations, which constitute a temporary perched aquifer that plays an essential role in aquifer recharge. At some sites, the proximity of the estuary or the sea causes tidal oscillation to affect hydraulic functioning to a high degree.
The systems are binary karsts developed in the chalk under the cover of surface formations. The intensity of karstification varies spatially within the Paris Basin. The chalk karst observatory consists of 5 sites with high degrees of karstification from the zones of introduction to outlet zones: the first four are monitored by the M2C laboratory of the Université of Rouen Normandie and the 5th by the METIS laboratory of the Sorbonne Universités (formerly UPMC-Paris6).
The systems are binary karsts developed in the chalk under the cover of surface formations. The intensity of karstification varies spatially within the Paris Basin.
These sites may correspond to natural karst springs (Norville, Radicatel), to borehole captures (Norville, Yport), or to the unsaturated zone of the chalk: the boundary of the saturated/unsaturated zone (the Saint Martin le Nud underground quarry) or the recharge zone with flow to the perched aquifer in cover formations (Bouville). These sites together thus make it possible to characterize almost completely the various hydrologic processes at work in the karstified chalk aquifer of the Paris Basin.
The Radicatel and Yport sites provide almost all of the potable water to the agglomeration of Le Havre (CODAH), which ensures the maintenance and monitoring of the sites. An agreement has been set up between the M2C laboratory and CODAH. The study of other sites is done with the support of the Normandy region and Ile de France region and the federations FR CNRS 3730 SCALE and IPSL, among others. Chalk karst observation activities also benefit from the support of the piezometric network in regions maintained by BRGM.
The data collected make it possible to characterize the various components of hydrodynamics based on the structure of the karst systems through approaches that combine hydrodynamics, hydrologic signal treatment, hydrogeophysics, and physico-chemical monitoring (conductivity/temperature) because the aquifer has triple porosity and the particularity of cover formations that constitute a perched, non-karstifiable aquifer