Hydrochemical Characteristics of Mid-Low Sections of North Patagonia Rivers, Argentina

Abstract

The chemical composition of water in the mid-low sections of the three main rivers of Patagonia (Colorado, Negro and Chubut rivers) is an important proxy for the understanding of the water cycle in the region. River water samplings were done in summer and winter campaigns at Colorado (15 sites), Negro (18 sites) and Chubut (17 sites). Hydrochemical variables: pH, conductivity, salinity, and ion concentrations were measured. The data processing included regular hydrochemical diagrams, multivariate statistical analysis and saturation indexes calculation. The three rivers have two different sections: one inland section having the continental hydrochemical fingerprint and an estuarine section, with a markedly seawater mixing effect. Most water samples of the inland sites belong to the Mg2+-Ca2+-HCO3- type in the Negro and Chubut rivers and to the Ca2+-SO42- type in the Colorado River. In contrast, the prevailing hydrochemical facies was the Na-Cl type at estuarine sites. In general, rock weathering was the main hydrogeochemical process controlling chemistry composition of rivers, being the dissolution of gypsum, carbonate and silicate minerals the primary contributors. The inland section has a different composition for each river, which is related to differences in the rock-composition at the sources and chemical reactions during downstream flow. The Colorado River also showed the highest average values in salinity, conductivity and dissolved ions. Basin geology and brackish discharges from Curacó River during the high rainfall season contributed to explain the ionic concentration in Colorado River, in particular the excess of calcium and sulfate.