Geothermal influence on the hydrochemistry of surface streams in Patagonia neuquina

Abstract

The Domuyo System Natural Protected Area in the Patagonia neuquina presents a mountainous relief with steep valleys carved by both, glacial and fluvial processes, where the main water courses are the streams namedManchana Covunco, Aguas Calientes and Covunco, which are tributaries of the Varvarco River. These streams run through an area of intense geothermal manifestations composed by aqueous solutions of high temperature, which determine the chemistry of stream and springs. The surface water courses and springs located upstream the thermal springs are neutral to alkaline, of low temperature and electric conductivity, and of the Ca-HCO3/Ca-HCO3-SO4 types, respectively. Downstream the thermal springs, an increase in pH, temperature, and electrical conductivity is registered, as well as changes towards Na-Cl facies, the dominant composition of the thermal springs. Also, thermal springs increase the As content limiting the water potability. The contribution from thermal springs to streams varies between 24 and 47%, while in springs it is close to 10%. In addition to the mixing of water, carbonate, plagioclase, and pyroxene weathering processes are mainly evidenced in both, thermal springs and springs, while surface water is associated with carbonate and gypsum-anhydrite dissolution. The dissolved REE contents are 10?4?10?7 times lower than the Upper Continental Crust and show a decrease downstream the geothermal field. Under standing the processes that determine the hydrochemistry is of great relevance in this type of arid mountainous environments with scarce knowledge.