Présentations Colloques

Poster
2.01
Session 2.01: Groundwater in semiarid regions – a long-term view on changes in aquifer balances. Hydrogeology in developing countries.
Batiot-guilhe (orateur)
Geochemical and isotopic characterization of surface river-alluvial groundwater relationships in a high altitude semi-arid basin, Elqui (Chile)
Geochemical and isotopic tools are used to trace surface water groundwater interactions, which are major recharge processes for alluvial aquifers, especially in arid zones. In this context, geochemical characteristics of surface and ground waters should be strongly linked. Nevertheless, geochemical signatures can be very complex in valleys, especially in volcanic contexts where different processes are involved such as rock weathering, mixing with geothermal fluids or dissolution of particular minerals producing specific geochemical facies. **The study area (upper Elqui valley and tributaries) is located in Chilean Andes (800 - 5500 m). The upper zone (> 3000 m) comprises various volcanic rocks including locally mineral-rich areas (El Indio Gold Belt) and is supplied by several glaciers including the Tapado (HCO3-Ca hydrochemical facies) and rock glaciers (SO4-Ca hydrochemical facies), with increasing mineralization from upper to lower parts of the basin (EC from 150 to 630 IS.cm-1). In zones including mining activities (El Indio mining) or mineral-rich terrains, increases in mineralization (up to 4000 IS cm), acidification (pH <4) and high concentrations of metallic elements are observed in surface waters. The lower part of the basin is composed of plutonic rocks and detrital terrains in which alluvial aquifers are developed. The hydrochemical facies of tributaries is HCO3-Ca, but the major river retains the SO4-Ca signature acquired in the upper part of the basin. All through the alluvial aquifer, hydrochemical and isotopic characteristics of surface waters and ground waters generally show good relationship, highlighting a good connectivity between these two compartments. Nevertheless, isotopic values of some tributaries can be different and contrasted compared to others as a function of the origin of water (from glaciers, rain and or snow). Radon measurements have shown some possible contributions from plutonic rocks. This mutitracer approach coupled with hydrodynamic information allow to better characterize surface water groundwater interactions, which are complex to identify but are fundamental to optimize the integrated water resources management in arid contexts at middle to high altitudes.
France