Abstract
Geochemical and isotopic tools are used to trace surface water groundwater interactions, which aremajor recharge processes for alluvial aquifers, especially in arid zones. In this context, geochemi calcharacteristics of surface and ground waters should be strongly linked. Nevertheless, geochemicalsignatures can be very complex in valleys, especially in volcanic contexts where different processesare involved such as rock weathering, mixing with geothermal fluids or dissolution of particularminerals 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 volcanicrocks including locally mineral-rich areas (El Indio Gold Belt) and is supplied by several glaciersincluding the Tapado (HCO3-Ca hydrochemical facies) and rock glaciers (SO4-Ca hydrochemicalfacies), with increasing mineralization from upper to lower parts of the bas in (EC from 150 to 630IS.cm-1). In zones including mining activities (El Indio mining) or mineral-rich terrains, increases inmineralization (up to 4000 IS cm), acidification (pH <4) and high concentrations of metallic elementsare observed in surface waters. The lower part of the basin is composed of plutonic rocks and detritalterrains 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 throughthe alluvial aquifer, hydrochemical and isotopic characteristics of surface waters and ground watersgenerally show good relationship, highlighting a good connectivity between these two compartments.Nevertheless, isotopic values of some tributaries can be different and contrasted compared to othersas a function of the origin of water (from glaciers, rain and or snow). Radon measurements haveshown some possible contributions from plutonic rocks. This mutitracer approach coupled wi thhydrodynamic information allow to better characterize surface water groundwater interactions, whichare complex to identify but are fundamental to optimize the integrated water resources management inarid contexts at middle to high altitudes.