Origin of old saline groundwater in the deep coastal formations of the Atacama Desert region: Consideration of lithium, boron, strontium and uranium isotopes contents

Abstract

This paper presents a multi-isotopic study to investigate the origin of saline groundwater in the Cordillera de la Costa, in the arid Atacama Desert region in northern Chile. Understanding the origin of deep saline groundwater can be challenging using only hydrogeochemical and common isotope methods. New 87Sr/86Sr, δ11B, δ7Li, and 234U/238U values of dissolved salts have been added to previously known chemical and isotopic compositions of groundwater, inorganic dissolved carbon, and sulfate. In this extremely arid area, the chemical and isotopic compositions of coastal springs primarily reflect the concentration of salts from atmospheric wet and dry deposition, which accumulate in the topsoil and dissolve during the occasional heavy rainfall events. These events recharge the aquifer with sodium-chloride type, high salinity water, with rock weathering contributing minimally to the chemical composition of groundwater. However, high salinity water of the calcium chloride and sodium-calcium chloride types in the deep fractured aquifer have a significantly different chemical and isotopic composition. In this investigation, it has been concluded that the salinity of these last waters is due to remnants of ancient marine water, mineral alteration and interaction with predominantly andesitic volcanic rocks in the Cordillera de la Costa. The deep saline groundwater has δ11B values between 17.3‰ and 20.7‰, and 87Sr/86Sr values below 0.706, suggesting mixing with recharge groundwater. The δ7Li composition of the deep groundwater varies between 23.5‰ and 31.5‰, pointing to a contribution of salts of marine origin and mineral alteration. However, the low Li concentrations relative to B in recharge water and in deep groundwater shows a preferential Li removal from pore water. Uranium and uranium isotopes data are limited to shallow groundwater due to the very low U concentrations in deep groundwater following reduction of U, and point to enhanced water–rock interaction in a rock-dominated environment. The dissolution of old buried saline deposits at shallow depth as a source of salinity is ruled out.