Climate change and recycling over thousands of years of chloride in lakes and the deep unsaturated zone: an example from the Bolivian Altiplano

Modelling chloride (Cl) transport may be useful for understanding hydrological responses climate modifications over periods of several thousands of years. In arid zones, Cl profiles of the very deep unsaturated zone (reaching several hundred of metres) are used to reconstruct the evolution of precipitation. In arid zones where the unsaturated zone does not exceed about 20 m depth, and outside the spatially very limited infiltration zones, the net flow rises from the aquifer to the soil surface. Evaporation occurs close to the soil surface and chloride accumulates in the unsaturated zone. The quantity of Cl that is observed today in the unsaturated zone can be related to different periods of accumulation according to whether the depth of the phreatic level is assumed unchanged or not. In endhoreic catchments (with no outflow to the ocean), the lower extremity of the hydraulic system is generally a salty lake or even a salt crust. The quantity of chloride at this point is always noticeably less than the value corresponding to the net inflow of chloride over the period of its existence. Recent work shows that the principal mechanism responsible for this situation is flow to groundwater during periods of high lake level. The schematic functioning of these systems is: (a) infiltration of freshwater close to hinge line of the topography, (b) evaporation and accumulation of salt in the lower zone called the playa, and (c) free convection under the playa. Several modelling studies have focused on characterization of the free convection and on the position of the limit between freshwater and brines. The example of the aquifer of the central Bolivian Altiplano illustrates a case where this limit progressively moves towards the salt crust when the lake level falls. in an area of 3500 km situated in the upstream part of the catchment that includes the great salt crusts of Uyuni, the current groundwater chloride concentration varies there from 0.5 meq l(-1) upstream, to 150 meq l(-1) downstream. The transport of Cl in the aquifer, associated with its accumulation in the unsaturated zone, has been modelled over 11 000 years, In order to compute the evaporative outflow from the aquifer, a simple relationship between this flow and the depth of the water level below the soil surface has been established for all arid zones on the basis of isotopic profiles. The initial conditions of the transport simulation correspond to the moment of the withdrawal of the salty palaeo-lake Tauca that previously covered the area. Simulation takes into account the return of Cl accumulated in the unsaturated zone to the underlying aquifer, during the humid episodes around 2000 BP. Results of the modelling are compatible with the observed spatial evolution of the Cl concentration in the underground water and of the quantities of Cl accumulated in the unsaturated zone.