Modeling the interaction between evaporation and chemical composition in a natural saline system

Evaporation controls the salinity of many natural and anthropic brine systems, but the reverse is also true. By controlling water activity, salinity affects evaporation rates. We present a method to compute the evolution of water activity in high salinity systems so as to evaluate evaporation rates. We place special emphasis on the assessment of invariant points, where activity is controlled by the set of precipitated minerals. The algorithm is tested on a natural Mg–SO4 rich brine evaporation experiment. In accordance with the experiments, the model predicts two intervals (invariant points) in which water activity, the concentration of all species and the amount of liquid water remain constant, because evaporating water comes from the dissolution of hydrated minerals. This suggests that mineral paragenesis might have a considerable influence on shallow brine lake evolution by fixing chemical composition for a significant portion of time. This conjecture was tested with a simplified model of a perennial saline playa lake. An analytical solution was developed to illustrate the evolution of the proposed system. According to the calculations, the system tends to a cyclical steady state for both lake level and chemical composition. The latter remains fixed at invariant points during long time intervals, where hydrated minerals act as the water source for evaporation. This situation is to be expected in epsomite lakes where two invariant points can be found in equilibrium with relative humidities of 57% and 50%.