Density-driven groundwater flow in closed desert basins: Field investigations and numerical experiments

With a numerical model, Duffy and Al-Hassan demonstrated that free convection of brine could occur under the playa lake in a closed basin. Since then, continued effort has been made to further our understanding of the role of free convection in closed basin hydrology. In this paper we present the results of 5 years of field investigation in Pilot Valley, Nevada, of a literature search for field studies in closed basins around the world, and of numerical simulations to examine the effect of heterogeneity on free convection and the effect of climate changes. Observations along the divide-playa transect revealed evidence of forced convective flow and the existence of a hinge line. The salinity profile and the inferred flux suggest that diffusion is the dominant solute transport mechanism under the field sites, but the stable isotopes (O, H) suggest that free convection of playa brine also occurs. We propose a model that explains these observations. First, free convection could occur farther out on the playa where the density of surface brines is higher. Second, once free convective flow penetrates the surface sediments, it could migrate laterally in the more permeable layers of alluvium. Third, free convection could occur during periods of time when brines pond on the surface for extended periods. The search for field studies of groundwater processes in other closed basins leads to a few cases where free convection has occurred on a basin scale. Based on this survey, we discuss the geologic and climatic conditions that favor free convection. Numerical simulations are carried out to examine the effect of basin-scale heterogeneity in hydraulic conductivity. The results indicate that heterogeneity changes the pattern of flow by creating multiple convection cells. Finally we address how a closed basin may respond to climate change. Climate changes alter the salinity of surface brines and the freshwater recharge, both acting as 'boundary conditions' for subsurface processes. Using dimensional analysis and numerical simulations, we evaluate the effect of a warming and drying trend on free convection, and how the geologic features of a closed basin affect the response.