Convection in groundwater below an evaporating salt lake: 1. Onset of instability

Convective groundwater motion can be generated in aquifers beneath and adjacent to saline lakes. Such motions strongly control the subsurface distribution of salts, the formation of saline minerals, and the distribution of contaminants that may be disposed of in the saline lake. In particular, schemes to use saline lakes as evaporation basins for irrigation waste waters must consider the groundwater dynamics associated with convection. The convection is driven by the evaporative concentration of salts at the land surface, leading to an unstable distribution of density. However, the evaporative groundwater discharge can dynamically stabilize this saline boundary layer and inhibit convection. In this work we investigate the nature and onset of convection for a range of boundary conditions typically found in saline lakes. Processes involved in the accumulation of salt at the surface of a groundwater discharge zone and in the gravitational instability of the near-surface groundwater are considered. Results of theoretical stability analysis are applied through laboratory and numerical experimentation to the more complex geometries typically found in the field. These comparisons indicate that for large saline lakes, the stability of the saline boundary layer can be parameterized by traditional Rayleigh criteria, in which the aquifer permeability and the evaporation rate from the lake bed are principal controlling factors. For typical saline lake environments, convection will dominate in sediments whose permeability exceeds approximately 10^-14 m². Below this threshold permeability, the boundary layer should be stabilized by the evaporative flux, resulting in the accumulation of salts and evaporites at the land surface. These results provide insight into the predictive behavior of groundwater dynamics and solute distributions in many natural systems.