Conceptual hydrogeological model of Alpine Corsica metamorphic formations from a pluridisciplinary, multi-scale approach
Résumé
Alpine Corsica is located in the northeastern part of the Corsica Island (France). It consists of a series of metamorphosed units including (1) the meta-ophiolite unit which mainly consists in metabasalt, metagabbro and serpentinite and (2) the “Schistes Lustrés” complex formed of metasediments such as calcschists or marbles.
The Alpine Corsica groundwater body is a key water source for drinking supply that represents nearly 25% of the groundwater withdrawal in Corsica. However, the geological and hydrogeological properties of these formations are poorly known and differ from those of granite. A conceptual hydrogeological model of these hard-rock aquifers based on the results of a multidisciplinary approach performed at several spatial scales, from the lithological contact (meters) to the catchment (kilometers), was developed in order to improve the knowledge of these complex aquifers.
Lithology and tectonic determine the groundwater circulations inside the Alpine Corsica massif. A statistical analysis based on air-lift flow measurements performed during the drilling of sixty boreholes has been conducted. It provides a qualitative characterization of the vertical distribution of permeable zones which are, statistically, mainly located within the first 40 m from the ground surface but may reaches up to 70 – 80 m deep in schists and marbles. Distribution and location of springs, and the measurements of differential stream-flow during base-flow periods allowed identifying aquifers and estimating their hydrodynamic properties. Hydraulic conductivity of the meta-ophiolite unit is found homogeneous and estimated between 1.10-8 and 2.10-8 m/s (minimum estimates of K). For this unit the hydrogeological model of these formations is similar to the “classical” granite model. On the contrary, the hydraulic conductivities of schists and marbles show a wide variability -from 1,4.10-9 to 4,2.10-8 m/s, which reflects the heterogeneous and compartmentalized characters of these aquifer systems because of local dissolution of marbles.