Hydrogeological Configuration of Grande Comore Island: Use of 3D Geological Modelling and Piezometry

Hydrogeological modeling is an interesting and widely-used approach to improving our understanding of groundwater, both to test existing hypotheses on the behavior of hydrosystems and to predict their responses to various natural or man-made problems. Today, software such as Leapfrog Geo offers the possibility of building a 3D geological model with a more accurate representation of the subsurface. Statistical tools such as ordinary kriging can be used to simulate the spatial distribution of groundwater. These modeling approaches were combined to improve knowledge of the groundwater flow context within three massifs on the island of Grande Comore. The delineation of the 3D geometry of litho-stratigraphic units has enabled a more detailed conceptualization of groundwater flows in a complex volcanic environment. Piezometric interpolations were used to validate aquifer geometry. It has been demonstrated that an implicit geological model coupled with piezometry can provide very interesting information on the hydrogeological configuration of a volcanic massif. In the Karthala and Badjini massifs, the respective confined and semi-confined configurations of the aquifers are observed, with thicknesses that progressively decrease with distance from the coast. In the Grille massif, on the other hand, the aquifer configuration is unconfined, with thickness increasing with distance from the coast. In all three massifs, the flow of water in the underground hydrosystems is from the central part towards the coast, naturally following the geological configuration of the ground. It should be noted that the absence of data in the central parts of the massifs still leaves uncertainties about the geometry in these parts of the aquifers. However, the models that have been established provide valid hypotheses for characterizing the hydrogeological configuration at the scale of each massif.