Aquifers and groundwater within active shield volcanoes. Evolution of conceptual models in the Piton de la Fournaise volcano
Résumé
The uncertainty regarding hydrogeological models that have been proposed for describing active volcanoes results from the difficulty of prospecting deep groundwater bodies. In the case of the Piton de la Fournaise volcano located in Reunion Island, recent geophysical exploration using deep electromagnetic (EM) prospecting tools has provided new geostructural and hydrogeological information. This paper introduces yet a new hydrogeological model, using a direct modeling approach, which then serves as a numerical tool for testing our most recent geological findings and for consolidating our understanding of the hydraulic behavior of the massif. For this purpose, the 3D Femwater finite element code has been implemented. The model is built to represent the global structure of the volcano defined by four main volcanic units as hydrogeological layers. While each layer is characterized by an average homogeneous hydraulic property, the boundary conditions correspond to the yearly groundwater recharge spatially distributed into homogeneous recharge zones, and constant head conditions correspond to the sea or rivers which form the edge of the modeled area. The numerical flow simulation provides piezometric heads reaching a height of 1800 m a.s.l. in the vicinity of the volcano summit. The computed piezometric surface is consistent with the shape of the saturated zone inferred by geophysical soundings, and the model simulated groundwater discharge is consistent with the observed main spring discharge identified within the deepest valleys or along the shoreline. This simulation gives hydraulic confirmation of a continuous aquifer conceptual model. High water levels in active volcanoes are thus not necessarily a result of perched or dike confined aquifers. These results indicate the presence of a central groundwater dome similar to those observed in the Canary or Azores islands, where volcanic terrains are much older. Our model implies potentially new kinds of interactions between groundwater and volcanic activity. In particular, we focus on the importance of water pressure distribution within the volcano. By itself, the three-dimensional groundwater model provides a guide for a better understanding of the structure and the eruptive dynamics of active volcanoes.