Evaporation from Fractures Exposed at the Land Surface: Impact of Gas-Phase Convection on Salt Accumulation

A mechanism is investigated by which surface-exposed fractures could be a source of aquifer salinization in low-permeability fractured formations under arid conditions. It is hypothesized that evaporation of pore water within surfaceexposed fractures is enhanced by convective air circulation within those fractures that vents moisture to the atmosphere. This evaporation also simultaneously enhances lateral movement of pore water from the adjacent matrix towards the fracture surface, permitting dissolved solutes to precipitate on the surface and form a crust. The salt crust can then dissolve during infiltration events and be flushed downward to the aquifer. Theoretical analysis shows that convective venting is expected during cool nights when atmospheric air is denser than the fracture air. Laboratory experiments support the hypothesis of rapid salt-crust formation in the presence of convectively moving air across a fracture face. A numerical model is developed and used to quantify the buildup of salt on a fracture face.