Présentations Colloques

Session 8.05: Karst aquifers
Carrière (orateur)
An integrated hydrogeophysical approach for karst water assessment and management in humanitarian context - Mahafaly Plateau in Madagascar
This research study has been applied to a humanitarian project implemented in the Mahafaly plateau (3600kmC), in southern Madagascar. Its objective was to allow a strategy for safe and sustainable water access for the most vulnerable population. **This operational research combined several prospecting tools and technologies (geology, hydrogeology, hydrogeophysics and hydrochemistry) to better understand the hydrogeology of this poorly documented area. The first phase consisted in a comprehensive characterization of the groundwater resource, including stock assessment, vulnerability to pollution and over-exploitation appraisal. The second phase consisted in the determination of the most favorable locations for the implementation of groundwater drawing infrastructures mainly through geophysics due to constant sand cover (≈+2m). The third on-going phase is to achieve an automatic piezometric and meteorological monitoring to ensure the sustainable management of the groundwater resources.**The study identified two main groundwater resources- a deep one (below 160m) located in karst conduits and a shallow one located in the near surface within recent sediment overlying the karst formations. Hydrochemical results highlighted the vulnerability of the deeper aquifer to saline water intrusion. Hence, the second phase focused on the shallow hydrogeological target. A piezometric survey (217 measurements) reveals a general flow in the west direction. A combination of geophysical measurements at moderate cost (262 TEMfast soundings, 2588 Slingram measurements, 35 electrical soundings) was used to investigate the complex and discontinuous geology of the recent sediments. Nine positive boreholes and wells were positioned. Despite numerous measurements, the study area remains insufficiently known to ensure a sustainable water management. In the global change context, it is primordial to prevent any over-exploitation. Therefore the project installed piezometric and meteorologic sensors and setup a team with local institutions to ensure data collection during ten years. This part of Mahafaly plateau is now a unique observatory for drought warning in one of the largest karst system of the southern hemisphere.**