Groundwater Prospection in Grande Comore Island—Joint Contribution of Geophysical Methods, Hydrogeological Time-Series Analysis and Groundwater Modelling

Abstract : In Grande Comore island, groundwater is the only safe water resource for drinking but only one third of the population have access to this resource. All existing wells are drilled in the volcanic aquifers of the coastal zone. Among them, about one third provide groundwater of acceptable salinity (less than 1 g/L), one third provide water of salinity comprised between 1 and 3 g/L, and the remaining third is generally disused due to salinities higher than 3 g/L. To date, inland groundwater resources have been largely overlooked. The development of groundwater in Grande Comore requires an improved understanding of the complex young volcanic aquifers, quantitatively and qualitatively. This work applies an integrated hydrogeological methodology aiming at improving the conceptual understanding of Grande Comore volcanic aquifers in both coastal and inland areas, and proposing a robust approach for prospecting and managing the groundwater resources. This methodology comprised (1) a review of the current hydrogeological knowledge regarding the structure, properties and conceptualisation of the volcanic aquifers, (2) the spatial characterisation of both aquifer structures and seawater intrusion in coastal areas, through the implementation of geophysical surveys comprising electrical resistivity tomography (ERT) and time-domain electromagnetic soundings (TDEM), (3) the characterisation of coastal groundwater dynamics through the acquisition of high temporal resolution heads and salinity measurements in three wells from contrasted hydrogeological settings, (4) the quantification of the impact of volcanic heterogeneity on coastal groundwater salinity through numerical groundwater modelling and (5) the exploration of the poorly known inland aquifers through the interpretation of ERT and TDEM investigations together with a discussion of the hydrogeological analogy with the better known volcanic island of La Réunion. Results reveal a strong potential for both further developing coastal aquifers and initiating the prospection of inland aquifers, and provide methods for improving the management of existing groundwater infrastructures.