Salinity and isotopic dynamics of the groundwater resources on the Bolivian Altiplano

The aquifer under investigation is situated on the central Bolivian Altiplano and covers 1000 km². Perforated wells ranging in depth from 20 m to 80 m produce water whose electrical conductivity increases from upstream (0.5 mS cm^-1) to downstream (5 mS cm^-1). Hydrogeological modelling and interpretation of isotopic data (mainly ¹⁸O with some ²H, ³H, ¹⁴C and ¹³C) has been conducted on data collected over two years from groundwater, rain, the Rio Desaguadero and a core from the unsaturated zone. The aquifer presents two systems. The western low salinity zone is a consequence of infiltration from mountain runoff. Flow is northwards towards the Desaguadero and also southeastwards parallel to the mountains. The eastern zone of higher salinity results from a combination of recharge by the Desaguadero, groundwater evaporation and dissolution of salty sediments from ancient Quaternary lakes which dried up during a drier hydrological regime between 8000 and 1500 years BP.