Geochemistry of mountain rivers in the Sierras Pampeanas: II. Los Reartes River, Sierra de Comenchingones, Province of Córdoba

Abstract

We analyzed the hydrogeochemistry of Río Los Reartes, a typical high-altitude, mountainous drainage basin in the Sierras Pampeanas of Córdoba, Argentina. The dominant rocks in the area are granites and gneisses of the crystalline basement. Maximum and minimum altitudes are 2,400 m, and 670 m above sea level, respectively. The slopes decrease from 20% in the headwaters down to 6% at the outfall. Climate is high-altitude semiarid with most of the rainfall concentrated during the southern summer. The water pH oscillates between 6.9 and 9.1 and most total dissolved solids (TDS) determinations are in the 31 - 114 mg/l range. The preponderant chemical composition is characterised by HCO3- > Cl-+ SO42-, and by Na++ Ca2+ > K++ Mg2+. TDS, pH, with an alkalinity increase downstream, evolving from HCO3-- Na+- K+- type to HCO3-- Ca2+- type. Dissolved trace elements reach concentrations of 10-3 to 10-7 times lower than those in the upper continental crust. The more soluble elements (Ca, Na, K, Mg, Ba, Sr) exhibit the highest concentrations, whereas the less mobile are the least concentrated. As a result of pH and adsorption processes, rare earth elements (REE) show extremely low concentrations in the dissolved phases (ΣREE between 30 and 319 ng/l). The irregular distribution of rainfall exerts a seasonal control over REE concentrations; moreover, the chemistry of these waters reflects the joint control of rocks and climate. The prevailing semiarid conditions determine an incipient alteration in the silicates and the nature of the dominant rocks in the drainage govern a common chemical signature in the waters of the region.