Geochemistry of waters and brines from the Salinas Grandes basin, Córdoba, Argentina. I. Geomorphology and hydrochemical characteristics

Abstract

This paper reports on hydrochemical features of diluted waters in the source areas and the brine end-members dominant in the playa of the Salinas Grandes Basin, Cordoba, Argentina. Special emphasis was placed on the study of the relationship between geomorphology and the resulting hydrochemical fractionation. Inflow is from springs and mountain streams which disappear before reaching the saline complex. The playa and intermittent saline lakes are mainly fed by groundwater flow and a few moderately saline and perennial springs. Conversely, ephemeral lakes are fed by atmospheric precipitation and groundwater, whereas small ponds are only fed by atmospheric precipitation. The absence of a clear linkage between geomorphological units and water types was evident in the source areas. Up to four types of water were recognized in a given geomorphological unit. From the sandflat downward towards the playa, the correspondence between geomorphological units and water types was clear, coinciding with a decreased hydrochemical heterogeneity. In this subenvironment, I have recognized two dominant types of water (SO 4 2− −Cl−−HCO 3 − −Na+ and Cl−−SO 4 2− −HCO 3 − −Na+), which can be considered the original members of the neutral brine in the playa (Cl−−SO 4 2− −Na+ where Ca2++Mg2+ do not surpass 5 meq per cent), and an intermediate type in the mudflat (Cl−−SO 4 2− −Na+ where the contribution of Ca2++Mg2+ reaches up to 15 meq per cent). It seems evident that in the zone between the source areas and the sandflat, hydrochemistry is governed by chemical weathering. In stream floodplains and in the distal alluvial plain, close to the saline complex, the increase in water types as well as the increase of HCO 3 − with respect to SO 4 2− , were explained by the mixture of aquifers controlled by the Salinas Grandes-Salinas de Ambargasta fracture. In the saline complex, the more concentrated end-members are the result of evaporation of the two more frequent water-types in the sandflat subenvironment, and salt dissolution of ancient evaporite deposit.