Karst de la Craie

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 Norville spring and borehole: this spring is supplied by a principal karst conduit into which a perennial stream disappears. Its drainage basin, which is small in size, is principally used for cattle grazing. Transfer from the surface is quite rapid (less than 24 hours)
  • The Radicatel springs: these springs are highly productive (~50% of the potable water supply of the agglomeration of Le Havre) with rapid transfer through conduits. The associated drainage basin is extensive (106 km2) and very rural.
  • The borehole capture of Yport: The Yport borehole is installed directly within a karst conduit that measures about 3m in diameter, located about 40 meters under the ground surface. The BAC is 230 km2
  • The Bouville infiltration site: this less-than-0.5 km2 site is a test site set up for the investigation of heterogeneous infiltration via the perched aquifer of surface formations, which drains through dolines or sinkholes that are not visible on the surface topography. Hydrogeophysical investigation methods, particularly spontaneous potential (self potential) were originally developed at this site.
  • The Saint Martin le Noeud underground quarry is dug into weakly karstified fractured chalk, and provides access to the water-bearing layer and to percolating waters (leaving the unsaturated zone). Transfers from the surface are slow, between several months to several decades. This site provides a manhole on slow infiltration and in this sense makes it possible to isolate this process specifically from other infiltration modalities.

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.

These sites make it possible to expand our knowledge of:

  • the role of the cover on rapid or slow infiltration, notably by the study of relationships between hydrodynamics, hydrochemistry, and hydrogeophysics plus the characterization of recharge and various scales of time characteristics (Saint Martin le Noeud);
  • relationships between matrix porosities, fissures, and conduits (hydrodynamic behavior, diffusive transport, dispersive transport);
  • tidal impact on the hydrologic functioning of karst systems;
  • the role of karst (wells/springs) in sedimentary fluxes on drainage basins. Observation of the influence of long-term climatic oscillations on the chalk karst hydrosystem is possible as a result of historical piezometry.
  • contaminant (particularly agricultural) transfer and degradation in karstified chalk environments.

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