ARSENIC IN THE THERMAL SPRINGS OF THE CENTRAL BOLIVIAN ALTIPLANO

This study deals with the assessment of physicochemical characterization of thermal springs and their relationship to the distribution of arsenic (As) and other trace metals around the Poopó basin at the department of Oruro in the Central Bolivian Altiplano. The thermal springs were sampled as they are commonly used for consumption, irrigation and recreation purposes and for better understanding of the hydrochemistry at the geothermal activity sites. Sixteen water samples were taken from thermal springs that naturally emerge from earth surface. The pH, electrical conductivity, temperature, redox potential and alkalinity were determined in the field. Major anions were analyzed using ion chromatograph, while major cations and trace elements including arsenic concentrations were analyzed by ICP-OES.

The pH of the thermal water ranges from 6.3 to 8.3 and the redox potential ranges from +106 to +204 mV. Temperatures ranged from 40 to 75°C. A variety of water types are reflected for the water samples represented by Na-Cl-HCO₃-type (50%); Na-Cl-type (43.7%), and Na-HCO₃-type (6.2%). Dissolved As concentration ranges from 7.8 to 65.3 µg/L and averaged 23.2 µg/L. More than 81% of the samples exceeded the WHO guideline (10 µg/L). Arsenic speciation indicates that the predominant species is As(III) present in nine samples. Five samples had more As(V) and two of samples had both species in equilibrium. Arsenic in the thermal springs is attributed to the oxidation of sulfide minerals and to the dissolution from volcanic rocks as a source of natural contamination of the groundwater.

The distribution of heavy metals shows diversity of concentrations. Zinc range from below detection limit to 126.2 µg/L (average: 21.7 µg/L). Among the redox sensitive elements, Fe and Mn showed wide variability in the ranges of 2.0-825.6 µg/L (average 272.6 µg/L) and 21.3-314.6 µg/L (average 135.7 µg/L), respectively. These water samples showed concentration of Al (9.2-30.8 µg/L; average 16.2 µg/L) and Si ranging from 8.3 to 13.6 mg/L with an average of 10.6 mg/L. Among the trace elements, lithium has high concentration which is possibly related to the presence of non-metallic minerals and the geothermal activities in the area. Lithium concentration ranges from below detection limit (1.7 µg/L) to 15,400 µg/L (average 4.3 µg/L).