Subsurface transfer of chloride after a lake retreat in the central Andes

The area under study covers 3500 km(2) in the upstream part of the closed c atchment basin of the salt crust of Uyuni. This crust is a remnant of the s aline Lake Tauca, which covered the area about 15,000 years ago. In the dow nstream part of the aquifer, the CI concentration of ground water and Cl co ntent in the unsaturated zone exceed 20 meq/L and 18 kg/m(2), respectively. With the present hydrological conditions under semiarid conditions, the gr ound water residence time in the study area exceeds 3000 years. Transient s imulations over 11,000 years were made using initial conditions as the retr eat of Lake Tauca and taking into account a low recharge during the and mid -Holocene period. The modeling simulates ground water flow, Cl transport, a nd ground water residence time. It includes the evaporation from the aquife r that leads to the accumulation of chloride in the unsaturated zone. Resul ts of the modeling are consistent with the observations if it is assumed th at the Cl previously accumulated in the unsaturated zone was flushed back i nto the aquifer around 2000 years B.P., contemporaneously with the end of t he and period.