Groundwater recharge and circulation processes in an arid mountainous area of northern Chile (30°S): Hydrogeochemical and isotopic study of the Arrayán basin, Coquimbo region, Chile.

Abstract

Recent global hydrometeorological changes have increased in both duration and intensity due to anthropogenic impact. These changes are manifested, for example, in a generalized reduction of freshwater supply, threatening drinking water supply and socioeconomic development. In particular, in arid and semi-arid environments, precipitation and surface water resources are scarce and groundwater is a vital resource for sustaining life and fundamental for the integrity, development and conservation of ecosystems. Northern Chile (28-32°S) is characterized by transverse valleys and an arid climate, which has experienced an almost continuous sequence of relatively dry years since 2010, resulting in decreased flows and declining groundwater levels. Much of this area is characterized by plutonic and volcanosedimentary rocks. However, fractured hydrogeologic systems and secondary basins are poorly studied, and a lack of knowledge of local or regional groundwater flow paths exists in this region. Therefore, this study seeks to improve the scientific understanding of fractured hydrogeologic systems and their interaction with alluvial aquifers in secondary basins of the Coastal Cordillera (<2000 m a.s.l.) to improve long-term water management and contribute to ensure the sustainability of the resource. The study area is characterized by a mountainous morphology with high topographic gradients and narrow transverse valleys that give rise to numerous sub-basins dominated by the presence of plutonic and volcanosedimentary rocks. The hydrogeology of the studied area is characterized by hydrogeochemical analysis of 28 groundwater points and stable isotope analysis (δ18O and δ2H) at 35 points. The results reveal that hierarchical cluster analysis groups the samples according to the type of material storing groundwater, from bedrock or alluvial fill, while factor analysis evidence water-rock interaction processes in fractured aquifers areas and secondary evapotranspiration processes in alluvial aquifers. The calculated local meteoric water line shows the role of local precipitation in the recharge of the sub-basins. Recharge is distributed in alluvial and fractured environments, where it remains even though several years may pass without a recharge episode due to the large interannual variability of precipitation. Altogether, groundwater circulation is originated from local recharge, and the predominant processes are the dissolution of primary minerals in volcanosedimentary rocks and the precipitation of salts in alluvial aquifers.