Geochemistry of waters and brines from the Salinas Grandes basin, Córdoba, Argentina. II. Gypsum dissolution-calcite precipitation, and brine evolution

Abstract

Rivers and streams originating in the surrounding mountainous area are the major sources of salt in the Salinas Grandes basin (Cordoba, Argentina). These rivers infiltrate when they reach the sandflat or in the fringes of the mudflat, feeding springs which often form shallow lakes. Presently, the lakes are distant from the playa edge, thus allowing inflow waters to dissolve ancient (Pleistocene?) evaporite beds. In the sandflat environment, two dominant types of water have been recognized (SO42−-Cl−-HCO3−-Na+, and Cl−-SO42−-HCO32−-Na+), both considered as original members of the brine in the saline complex. Two main sources of solutes were distinguished, one related to the waters supplied by the southern sector and another to waters of the eastern sector. As a result of the chemical evolution in the playa environment, all brines belong to the neutral type (Cl−-SO42−-Na+). Following Hardie and Eugster's (1970) model, waters from the southern sector should evolve towards an alkaline brine (Cl−-SO42−-HCO3−-Na+), whereas those from to the eastern sector should evolve towards a neutral one (Cl−-SO42−-Na+). A computer simulation was carried out to model the chemical evolution of source waters. The results obtained by this methodology showed the same dichotomy (alkaline vs. neutral) established by Hardie and Eugster's (1970) model. The deficit in alkalinity could not be explained by any of the mechanisms published until now. Gypsum dissolution is the most likely mechanism which accounts for the chemical evolution of the waters investigated. When such a process is included in the computations, the Ca2+ supplied by gypsum beds generates an increase in the ion activity product (aCa+2·aCO32−) and produces a significant change in the 2Ca+2/(2CO32−+HCO3−) ratio, switching from values less than 1 to values greater than 1. This process determines the precipitation of calcite, and leads to a decrease in alkalinity, which in turn would explain the existence of a neutral brine in the saline complex. An intermediate salinity brine was detected in the mudflat, which, according to the model (Hardie and Eugster, 1970), should evolve towards a SO42−-free neutral brine (Cl−-Na+-Ca2+). The absence of this type of brine may be explained through mixing processes.