Geophysical Imaging of Natural Free Convection in a Sabkha Aquifer near Abu Dhabi, UAE

Natural free convection is important for a wide range of disciplines including hydrogeology. Recent studies have examined brine movement below salt lakes, contaminant plumes, carbon sequestration, nuclear waste disposal, and hydrothermal deposits. Despite its importance and the increase of research in this area, free convection has not yet been conclusively detected in natural groundwater field settings. Here, we present recent geophysical measurements in a sabkha aquifer approximately 60 km west of Abu Dhabi, UAE. The sabkha material consists of uncemented and uniform fine sand with a nearly uniform porosity and hydraulic conductivity. Potential drivers for instabilities in this system include (1) sabkha waters of high total dissolved solids (TDS) and density overlying Miocene formation water with lower TDS, and (2) episodic downward infiltration of higher density water that forms after significant rainfall events due to dissolution of a halite crust. We characterized the system in March 2008, about 2 months after two unusually large rain events, using electrical resistivity (ER) and frequency-domain electromagnetic methods. Geophysical sounding data are consistent with a model of lower TDS water entering the sabkha from deeper (Miocene) formations. ER imaging data show a low resistivity zone just below the water table, from which distinct fingers protrude into a higher resistivity background. These geophysical images provide clear evidence of complex fingering likely associated with natural free convection.