2010, Vol 26

The Salar de Uyuni, a salt pan located in the Bolivian Andes, was subject to hydrogeological and hydrochemical investigations between June and September 2009. During a drilling and sampling campaign halite cores were gained from different bore holes with depths between 2 and 12 m at two locations in the north eastern and the central part of the Salar (named SLT-01 and SLT-03, respectively). Brine samples, taken in different depths from the installed wells, were subject to field and laboratory analyses (IC, ICP-MS) and the determination of physical parameters (viscosity, density). Geochemical analyses of the sediment cores from SLT-01 show the large excess of halite with minor amounts of gypsum. The underlying mud layer is characterized by a high content of water insoluble minerals. Computer tomograph analyses of selected core samples revealed significant differences between the surface-near salt crust and deeper halite layers by means of pore volume, connectivity and structure of pores. The high permeability indicated by the calculated effective pore volume of almost 20%, was roughly confirmed by the results of a pumping test performed in SLT-03, which showed an intrinsic permeability of 1.95 · 10 m2 of the uppermost halite aquifer. Brines from the two drilling locations exhibit high differences in chemical composition, but variations with depth could not be observed. The major part of ions and minor elements is clearly enriched in SLT-03, which is located in a lower distance to the inflow delta of the Rio Grande in the south, the main tributary of the Uyuni salt flat. Concentrations of calcium and sodium show a converse trend indicating the removal from solution by the precipitation of gypsum and halite. Lithium, the element of highest economical interest, follows the trend of enrichment in southern direction and has concentrations in the brine of 0.3 g/L in SLT-01 and 1 g/L in SLT-03, which is in good accordance with results of former investigations in the area.