The origin of solutes in groundwater in a hyper-arid environment: A chemical and multi-isotope approach in the Atacama Desert, Chile

Abstract The major ion and the multi-isotopic composition ( 87 Sr/ 86 Sr, δ 11 B, δ 34 S(SO 4 ) and δ 18 O(SO 4 )) of groundwater from the Central Depression in northern Chile have been investigated to identify the origin of groundwater solutes in the hyper-arid core of the Atacama Desert. The study area is between the Cordillera de Domeyko and the Central Depression, at latitudes 24–25°S, and is characterized by near-zero air moisture conditions, rare precipitation and very limited runoff. Groundwater composition varies from Ca-HCO 3 to Ca, Na-SO 4 type below elevations of 3400 m a.s.l. The rCl/rBr ratio of meteoric waters and groundwater overlap, but significantly increase in the aquifer as salinity goes up due to evapoconcentration far from the Domeyko Cordillera. The wind-displaced dust originating in the Central Depression ( 87 Sr/ 86 Sr: 0.706558–0.710645; δ 34 S(SO 4 ): 0 to +4‰) affects the precipitation composition in the highest parts of the Domeyko Cordillera ( 87 Sr/ 86 Sr: 0.706746–0.709511; δ 34 S(SO 4 ): +1 to +6‰), whose δ 34 S(SO 4 ) and δ 11 B values are greatly different from marine aerosols, discarding its contribution to dust at this distance inland. Sr and S isotopic values in groundwater indicate a strong relation with three main geological units: i) Paleozoic rocks contribute high radiogenic strontium isotope ratios to groundwater (0.707011–0.714862), while sulphate isotopic composition is probably acquired from atmospheric dust (>− 1.4‰), ii) Jurassic marine limestones contribute low-radiogenic strontium isotopic ratios to groundwater ( 87 Sr/ 86 Sr: δ 34 S(SO 4 ): +0.1 to +7.7). These three processes reflect water-rock interactions. The δ 11 B of groundwater generally up to +13‰, does not increase along the regional groundwater flow path, discarding fractionation by interaction with clays. These results improve the understanding of the groundwater evolution in hyper-arid systems through a new conceptual model.