Présentations Colloques

Oral Presentation
Session 8.02: Groundwater Development and Protection in Coastal and Volcanic Environments with Complex Geological Structures
Marc Dumont
Multi-scale characterization of saltwater intrusion in heterogeneous volcanic aquifers with airborne electromagentics
In heterogeneous coastal environments, extension and geometry of saltwater intrusion are controlled by numerous parameters at different scales. Their analysis requires both local accurate data and extensive coverage. Given the significant electrical conductivity of the saltwater, Airborne ElectroMagnetic (AEM) method is particularly adapted for imaging the contrast between freshwater covering saltwater.**A multi-scale hydrogeophysical approach has been achieved at La Reunion Island in order to study controlling factors of saltwater intrusion in heterogeneous context. Its aim is to provide a high resolution mapping of saltwater intrusion at regional scale based on AEM results. Composed by two basaltic shield volcanoes, Reunion Island is characterized by a complex geological setting. The French Geological Survey, BRGM, conducted an AEM survey over the entire island (2500 km2) and an offshore ring. First, the regional extensions of saltwater intrusion have been defined through a statistical analysis of AEM soundings. Secondly, at local scale, AEM soundings was confronted with water electrical conductivity logs and interpreted considering the geological contexts and the regional recharge.**The AEM capacity to determinate water salinization has been estimated to validate geometries and lateral extensions in various configurations. These results have been used to produce a saltwater intrusion 3D map over the entire island. This approach allows comparing parameters that govern saltwater intrusion at different scales (geological settings, recharge rates, drainage networks and coastal geomorphologies…). Thus, the depth and the geometry of saltwater intrusion with a fine mixing zone and low slope have been related with different geology settings and recharge rates. On the opposite, different behaviors have been detected in the same geological settings. For example, this approach characterize in alluvial deposits from the same volcano, stratified saltwater intrusion, saltwater intrusion within a thick mixing zone or no intrusion. **This approach can be used to set up hydrogeological modelling parameters and boundaries or to elaborate sustainable coastal aquifer managements.**