Origin of the high variability of water mineral content in the bedrock aquifers of Southern Madagascar
2005
Rabemanana, Véro | Violette, Sophie | de Marsily, Ghislain | Robain, Henri | Deffontaines, Benoît | Andrieux, Pierre | Bensimon, M. | Parriaux, A. | Structure et fonctionnement des systèmes hydriques continentaux (SISYPHE) ; Université Pierre et Marie Curie - Paris 6 (UPMC)-École Pratique des Hautes Études (EPHE) ; Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Mines Paris - PSL (École nationale supérieure des mines de Paris) ; Université Paris sciences et lettres (PSL)-Centre National de la Recherche Scientifique (CNRS) | Solutions (UR 176) | Biogéochimie et écologie des milieux continentaux (Bioemco) ; École normale supérieure - Paris (ENS-PSL) ; Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Institut de Recherche pour le Développement (IRD)-Institut National de la Recherche Agronomique (INRA)-Université Pierre et Marie Curie - Paris 6 (UPMC)-AgroParisTech-Centre National de la Recherche Scientifique (CNRS) | Laboratoire de géomorphologie et environnement littoral ; Centre National de la Recherche Scientifique (CNRS)
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Показать больше [+] Меньше [-]Английский. 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 mu S 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).
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