Arsenic and other trace elements in thermal springs and in cold waters from drinking water wells on the Bolivian Altiplano

Numerous hot springs and fumaroles occur along the Andes Mountains, in the Bolivian Altiplano, where people use thermal springs for recreational purposes as pools, baths and also for consumption as drinking water and irrigation once it is mixed with natural surface waters; most of these thermal springs emerge from earth surface and flow naturally into the rivers streams which drain further into the Poopó Lake. Physicochemical characteristics of the thermal water samples showed pH from 6.3 to 8.3 with an average of 7.0, redox potential from+106 to+204mV with an average of+172mV, temperatures from 40 to 75°C with an average of 56°C and high electrical conductivity ranging from 1.8 to 75mS/cm and averaged 13mS/cm. Predominant major ions are Na⁺ and Cl^- and the principal water types are 37.5% Na-Cl type and 37.5% Na-Cl-HCO₃ type. Arsenic concentrations ranged from 7.8 to 65.3μg/L and arsenic speciation indicate the predominance of As(III) species. Sediments collected from the outlets of thermal waters show high iron content, and ferric oxides and hydroxides are assumed to be principal mineral phases for arsenic attenuation by adsorption/co-precipitation processes. Arsenic concentrations in cold water samples from shallow aquifers are higher than those in thermal springs (range<5.6-233.2μg/L), it is likely that thermal water discharge is not the main source of high arsenic content in the shallow aquifer as they are very immature and may only have a small component corresponding to the deep geothermal reservoir. As people use both thermal waters and cold waters for consumption, there is a high risk for arsenic exposure in the area.