Distribution, salinity and pH dependence of elements in surface waters of the catchment areas of the Salars of Coipasa and Uyuni, Bolivian Altiplano

Abstract

The variation in concentrations of a wide variety of hydrochemical parameters in surface watercourses of the Bolivian Altiplano is statistically characterised. A variation of four to five orders of magnitude is characteristic for several parameters (Al, As, B, Cd, Cl −, Cu, Fe, Li, Mn, Na, S, and SO 4 2−), reflecting (i) evapoconcentration between mountainous watersheds and saline desert evaporative sinks, and (ii) geologically varied sources such as volcanic/thermal springs, fumarolic sulphur deposits and widespread sulphide mineralization. A tendency to increasing salinity from catchment headwaters to the salars is observed. pH values are generally rather high (median 8.3), and exhibit a similar, but weaker increasing trend. Arsenic concentrations are high (median=34 μg/l) and are probably largely derived from volcanogenic sources (fumarolic native sulphur deposits and thermal springs) on the Andean Cordillera Occidental. Arsenic exhibits positive correlations with pH and salinity, indicating its mobility as an oxy-anion under the generally high pH conditions and its susceptibility to evapoconcentration. Arsenic appears to be only weakly attenuated relative to chloride, probably by adsorption onto iron hydroxide precipitates or flocs.