Salty Fingers

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Science  07 Aug 2009:
Vol. 325, Issue 5941, pp. 656
DOI: 10.1126/science.325_656a

Convection of groundwater through porous rock plays a critical role in many geological processes in the crust. It occurs whenever groundwater density decreases with depth (on account of either a temperature or salinity gradient relative to overlying water layers) and the rock permeability and pore connectivity are sufficient to allow flow. Convection at ocean ridges is responsible for altering the composition of ocean crust globally; locally, convection is the basis for most geothermal energy systems. Despite the wide-ranging implications of the phenomenon, clear-cut visualization of convection in the field has proven difficult. Laboratory experiments have shown that stratified systems, in which salty water overlies fresh water, should produce interfingering of the layers at a wavelength of about twice the thickness of the system. In this context, Van Dam et al. studied a sabkha aquifer near Abu Dhabi, United Arab Emirates, where evaporation produces salty groundwater perched above fresh ancient water at depth. Electrical resistivity measurements, sensitive to the salt content of the groundwater, revealed prominent fingers of descending salt water that were generally consistent with predictions.

Geophys. Res. Lett. 36, L11403 (2009).

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