Geophysical Characterization of Natural Free Convection in a Coastal Sabkha

Density variations in groundwater due to differences in solute concentrations or temperatures may result in instabilities when a dense fluid overlies a less dense fluid. Such conditions may be present below salt lakes, saline disposal basins, and contaminant plumes. Laboratory studies and numerical models have predicted that in situations where the convective flux is much more rapid than diffusion, free convection should occur. Although numerous field observations are consistent with theoretical work, until now there has been no convincing primary field evidence that shows the spatial patterns of fingering. Here, we present recent geophysical measurements that for the first time document complex fingering beneath a sabkha in the Emirate of Abu Dhabi. The sabkha material consists of uncemented and uniform fine sand with a porosity of ~38% and a uniform hydraulic conductivity at 1.0±0.2 m/day. A potential driver for instabilities in this system is evaporation of groundwater leading to an increased salinity of the water table and formation of halite at the sabkha surface. Redissolution of halite and recharge of higher density water during significant rain events then creates the necessary density inversion conditions for convection. We characterized the system in March 2008, about 2 months after a ~100 mm rain event, using electrical resistivity (ER) and time- and frequency-domain electromagnetic methods. Geophysical sounding data are consistent with a model of lower TDS water entering the sabkha from lower formations. ER imaging data collected in dipole-dipole configuration show a low resistivity zone at the top of the water table, from which distinct fingers protrude in a higher resistive background. The fingering occurs at scales predicted by theoretical analyses.