Caractérisation géostructurale des émergences et typologie des nappes d’altitude en milieu volcanique insulaire. Ile de la Réunion

Abstract : Geologic investigations of four pilot study areas on the Reunion Island have permitted characterization of the hydrogeologic properties of volcanic aquifers associated with the Piton des Neiges volcano. The goal is to improve our general understanding of groundwater flow in volcanic terrains. The volcanic stratigraphy of Piton des Neiges indicates four main periods of activity from 2.1 million to 20,000 years before present. The two oldest stages (I and II) consist of flows of ocea-nitic composition while the subsequent stages. III and IV, are more differentiated. Horizons of pyroclastic and volcanic detrita! material are intercalated with flows. The hydrogeologic setting of the Piton des Neiges has been established in part based on comparison with studies of geologically comparable areas : - volcanic aquifers confined between basaltic dykes on the Hawaiian islands. - a system of “water bearing cells” on the Tenerife Island, subdivided into upper and lower vadose zones. - the existence of a piezometric surface steepens sharply towards the center of the island has been deduced from a detailed survey of water galleries in the Canary Islands. The results of Fernadopullé and Custodio in the Canary Islands are consistent with field observations presented for the Reunion Island. A total of 42 springs observed in the pilot study areas has been divided into three categories according to geologic and morphologic characteristics of water-bearing beds. Most of the observed springs(23 of 42) are classified in a superficial zone related to subsurface flow in aquifers of very limited extent. An intermediate zone is characterized by perched water tables ; flow is channelled through paleo-valleys or stretched on extensive zones of ash. Finally, a deep zone characterized by the presence of a “basal water table” is defined for springs (7 of 42) located in entrenched valleys of “amphitheater” morphology. Discharge rates are high and result from regional drainage of a thick section volcanic rock. Hydrometric measurements support distinctions made between the three categories of springs, and indicate large differences between the hydrologic regimes of springs in the deep and superficial zones. Banane spring in the deep zone has an average specific discharge of 0.045 m3/s with a 10% variability throughout the year. Dussac spring in the superficial zone has an average specific discharge of 0.015 m3/s with a 70% variability throughout the year. Geochemical characterization of springs is more complex. The overall mineral content is low with measured conductivities between 17 and 143 microsiemens. Springs in the deep zone give an average value of 86 microsiemens, but there is no significant difference with springs in the intermediate or superficial zones. In contrast, sodium versus chloride concentrations increase progressively from superficial to deeper springs. Results of six Na/CI analyses on each Banane and Dussac springs are consistent with this evolution.
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Jean-Lambert Join, Jean Coudray. Caractérisation géostructurale des émergences et typologie des nappes d’altitude en milieu volcanique insulaire. Ile de la Réunion. Geodinamica Acta, Taylor & Francis, 2015, 6 (4), pp.243-254. ⟨10.1080/09853111.1993.11105252⟩. ⟨hal-02100587⟩

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