Structure and hydrogeological functioning of an insular tropical humid andesitic volcanic watershed: A multi-disciplinary experimental approach

SummaryThe aim of the study is to establish an hydrogeological scheme of the structure and the functioning of andesitic volcanic systems in the insular and humid tropical context. A watershed has been instrumented at Féfé in Guadeloupe (French West Indies) in the Lesser Antilles to measure rainfall, runoff and piezometric fluctuations between 2003 and 2006. Given the heterogeneity of the volcanic deposits, a multi-disciplinary approach has been used: a geological and an hydrogeological survey, an hydrodynamic approach (hydraulic tests and signal processing of hydrological time series), and finally, an hydrogeochemical approach (major elements and some trace elements). Two main superposed aquifer respectively in aerial deposits, and in recent nuées ardentes (glowing ash) and lava flows, as well as the underlying aquiclude (the highly weathered older substratum) were identified. At a local scale, some hydrogeological heterogeneities internal to the aquifers were highlighted and related to a polyphased building-up of the geological formations. Hydraulic relationships between the multi-layered aquifers, such as the confinement of the deep aquifer upstream of the basin, were identified. Groundwater/stream exchanges were characterised, detailing various contributions of the shallow and the deep aquifers to stream and spring waters; significant river losses were also observed. At a global scale, within such relatively permeable volcanic formations, the aquifer recharge is about 85% of the annual effective rainfall for an average rainfall year. The hydrogeological scheme appears to be coherent with a geological structure of cut-and-fill paleovalleys frequently observed in andesitic areas. Consequently, groundwater drainage axis are mainly localised in recent nuées ardentes deposits, whereas the inversion of relief characteristic of volcanic areas explains the non-superposition of surface water and groundwater divides and both surface water losses and springs. Finally, this work shows the highly heterogeneous hydrogeological processes occurring in volcanic formations of andesitic-type. It also shows the relationships between the geological structure and the hydrogeological functioning of such basins, and proposes an affordable methodological approach to investigate such a hydrogeological context in depth.