The Larzac karst is located in the Grands Causses region, in the southern Massif Central. The aquifer in question is located in limestone and dolomite formations of the Middle and Upper Jurassic, some 400 m thick. These formations overlie the marly Upper Lias series, 200 m in thickness (Bruxelles, 2001a). This sedimentary horizon acts as an impermeable barrier and defines the saturated limit of the karstic system. The monoclinal regional structure is cut by E-W thrusts of Pyrenean age, which uplift the impermeable Lias marls towards the north. The Neogene and Plio-Quaternary uplift led to down-cutting in the plateau and the creation of canyons (Ambert and Ambert, 1995), thereby facilitating karstification, given the hydraulic head gradient between the recharge (plateau) and discharge (canyon) zones. Recharge from rainfall infiltrates on the surface of the plateau (750 m), while discharge takes place at the Durzon Spring (530 m). The average discharge measured at this Vauclusian-type spring is 1.4 m3/s, but it varies from 0.8 to 18 m3/s. Tracing experiments, geomorphologic observations, and mass balances have produced estimates of 100 km2 for the recharge area (Ricard and Bakalowicz, 1996). During certain exceptional rainstorms temporary lakes form in the southern recharge zone (Bruxelles, 2001a), suggesting that the unsaturated zone (epikarst and infiltration area) cannot have the same hydraulic properties on its south and north sides (Bruxelles, 2001b; Ricard and Bakalowicz, 1996).
The principal scientific objectives of the experiments and monitoring carried out are as follows:
The overall goal of the project is to gain a better understanding of the processes which control the spatial and temporal variations in the supply of water in karstic environments, by in situ observation of the hydrologic, geophysical, and geodesic parameters associated with water flows within the karst. The following points in particular will be investigated:
- The respective roles played by the unsaturated zone (epikarst and infiltration zone) and the submerged zone. We propose to make a particular study of the epikarst, for which research has demonstrated a significant contribution over several seasonal and interannual cycles.
- Water infiltrates the unsaturated zone in a multi-modal fashion, including slow diffusion in the porous media, and more or less rapid travel in microfissures and cracks. We would like to characterize the respective contributions of the various modes of transport in terms of characteristic variations in water mass and travel time.
- A multidisciplinary approach to the epikarst and the infiltration zone based on structural, petrophysical, and hydraulic studies of these units. The project employs the additional tools of geodesy (gravimetry and tiltmetry), and geophysics (electrical, seismic, and proton resonance techniques) both on surface and in boreholes, and also hydrogeology. It also applies in situ geodesic and hydrogeologic observation tools.
- Investigations of the highly heterogeneous karstic environment raise the problem of the changing scale of our observations. We are currently installing a “local” experimental site where integrative methods of investigation, such as gravimetry, will be compared against geophysical observations made on surface and in boreholes.
- On a larger scale, the Larzac karst constitutes a system whose limiting hydraulic conditions are well known. We will offer an overall circulation model combining the various modes of transport and the observations made at different scales, particularly the spatio-temporal variations of gravity at the regional scale.