Recharge processes and groundwater age along the hyperarid western edge of the Andes (Atacama Desert): fossil groundwater recirculation

The assessment of groundwater recharge processes is mandatory for the sustainable use of aquifers, especially in arid environments. In the literature, natural recharge processes (i.e. direct or indirect addition of water to the saturated zone of an aquifer from the ground-surface) are exclusively related to the contemporary (or coexisting) hydroclimatic context. Along the hyperarid western edge of the Andes, in the Atacama Desert (Northern Chile), recent findings in the Andean Piedmont have revealed a hydraulic correlation between aquifer recharge areas and perennial river losses areas. In this hyperarid area, where mean annual precipitation below 2000 m a.s.l are less than 10 mm.yr-1, the perennial base flow originates from springs in the bedrock of the Precordillera. In studies performed during the 1980s and 1990s, carbon and water isotope analyses of base-flows revealed that groundwater stored in the bedrock of the Andes is old and related to another hydroclimatic context than the current one. This work addresses the role of fossil groundwater recirculation in the renewal of groundwater of the regional scale Pampa del Tamarugal Aquifer. This study analyses 14C, ä13C, ä18O and ä2H isotopes of surface and groundwater in the Quebrada de Tarapaca catchment (19.8°S). In the Precordillera and the regional aquifer, groundwater recharge occurred during the late Pleistocene wet phases related to the Bolivian Altiplano paleolakes (Tauca and Coipasa). Depleted ä18O and ä2H isotopes, disconnected from modern precipitation, agree with the cooler and wetter climate of the late-Pleistocene. In the bedrock of Precordillera, the observed constancy of water isotopes for 50 years highlights both the disconnection of groundwater from modern precipitation and the fossil nature of groundwater. In the area of perennial river infiltration, ~80 % of groundwater isotope content match with the water isotope content of these perennial rivers base-flow. Numerical modeling of vadose zone processes shows that and 90% of this base flow recharge the aquifer (~ 180 l.s-1 for Quebrada de Tarapaca) while only 10 % goes to evaporation. Fossil groundwater recirculation appears to be a significant process in the renewal of groundwater of the Pampa del Tamarugal Aquifer. It implies to reconsider the real proportion of modern precipitation in the current recharge of the Atacama Desert aquifers, but also to reconsider the basis concept of recharge usually assumed for such aquifers.