Hydrogeological conceptual model of andesitic watersheds revealed by high-resolution geophysics

Abstract

We conducted a multidisciplinary study at the watershed scale of an andesitic-type volcanic island in order to better 10 characterize the hydrogeological functioning of aquifers and to better evaluate groundwater resource. A helicopter-borne TDEM survey was conducted in 2013 over Martinique Island in order to investigate underground volcanic structures and lithology, characterized by high lateral and vertical variability, and resulting in a very high heterogeneity of their hydrogeological characteristics. Correlations were made on three adjacent watersheds between resistivity data along flight lines and geological and hydrogeological data from 51 boreholes and 24 springs, showing that the younger the formations, the 15 higher their resistivity. Correlation between resistivity, geology and transmissivity data of three aquifers is attested: the older the formation, the lower its resistivity, and the higher its transmissivity. Moreover, we demonstrate that the main geological structures lead to preferential flow circulations and that hydrogeological watershed can differ from topographical watershed.The consequence is that even if the topographical watershed is small, underground flow circulations can add significant amount of water to river watershed's water balance. This effect is amplified when lava domes and their roots are situated upstream, as 20 they present very high hydraulic conductivity leading to deep preferential groundwater flow circulations. We also reveal, unlike basaltic-type volcanic islands, that hydraulic conductivity increases with age in this andesitic-type volcanic island. This trend is interpreted as the consequence of tectonic fracturing associated to earthquakes in subduction zones associated to andesitic volcanic islands. Finally, our approach allows characterizing in detail the hydrogeological functioning and identifying the properties of the main aquifer and aquitard units, leading to the proposition of a hydrogeological conceptual model at the 25 watershed scale. This working scale seems particularly suitable due to the complexity of edifices, with heterogeneous geological formations presenting high lateral and vertical variability. Moreover, our study offers new guidelines for accurate correlations between resistivity, geology and hydraulic conductivity for volcanic islands. Finally, our results will also help stakeholders toward a better management of water resource.Hydrol. Earth Syst. Sci. Discuss., https://doi.