Reactive convection cells: A mechanism for transport and precipitation of salt in fractures under evaporative conditions

Vadose zone fractures are known to play a significant role in groundwater recharge in desert areas. This project will evaluate the hypothesis that evaporation from fracture surfaces in the upper vadose zone triggers capillary forces within the matrix that drive pore-water and solutes from the matrix toward the fracture faces. Evaporation results in salt crust formation on the fracture walls. Subsequent rain events move water through the fractures dissolving salt crusts and transporting them to the water table. Furthermore, under desert conditions, nighttime cold temperatures may result in the formation of thermal convection cells within fracture voids, dramatically enhancing evaporation rates. The project will:
1. Test the concept of accelerated night-time convection at a fractured-rock field study site.
2. Investigate salt crust formation rate in the laboratory for two rocks permeabilities to determine: (a) time rate of change and strength of the evaporation rate; (b) rate and extent of the salt crust formation; and (c) ratio of nighttime (convection driven) to daytime (diffusion driven) evaporation rates.
3. Develop a field scale numerical model to analyze the significance of this mechanism under a range of climatic, fracture and matrix conditions. Parameters will include fracture aperture, rock permeability and extent of thermal inversion.
This study will provide a better understanding of the mechanism for salt accumulation and transport. It will provide a valuable tool for exploring the importance of this mechanism on a global scale as it relates to salt loading of groundwater resources and bypass toxic solutes in already contaminated sites.