Institution: | aUMR Sisyphe—Université Pierre et Marie Curie, case 123, 4 place jussieu, 75252 Paris Cedex 05, France bUR Géovast—Institut de Recherche pour le Développement, 32 avenue Henri Varagnat, 93143 Bondy, France cLaboratoire de Géomorphologie et Environnement Littoral, EPHE 15 Boulevard de la mer, 35800 Dinard, France dGEOLEP—Ecole Polytechnique Fédérale de Lausanne, 1015-Lausanne, Switzerland |
Abstract: | This study assesses the causes of the high spatial variability of the mineral content of groundwater in crystalline bedrock of Southern Madagascar. Although many kilometres from the coast and at a mean altitude of 400 m a.s.l, wells drilled in this area produce water with electrical conductivities in the range of 300–30,000 μS cm−1 with a high spatial variability. Chemical and isotopic data are used to identify the processes involved in the groundwater mineralization. It is shown that the chemical composition of the groundwater in this region has its origin in (i) normal silicate and carbonate weathering reactions and (ii) input of marine salts, probably via rainfall recharge, modified by evapo-concentrative processes probably including precipitation and re-dissolution of secondary evaporites in the unsaturated zone. To obtain a better understanding of the spatial salinity distribution, well parameters such as yields, weathered zone thickness, weathered materials and morphological positions (upper slope, mid-slope, lower slope or valley bottom) are scrutinized. A correlation was found between high salinity and low flow, shallow groundwater environments (flat hill tops, valley bottoms, weakly developed and clayey weathered zones) and between low salinity and high flow environments (granular, well-developed weathered zones and situation on valley slopes). |