Modeling Salt Redistribution in Fractured Porous Media Caused by Convection Driven Evaporation Within the Fracture

A set of numerical experiments demonstrate a dramatic increase in salt precipitation on fracture surfaces caused by convection driven porewater evaporation within the fractures. A model of soil evaporation was created using the EWASG module of TOUGH2, a three dimensional, multiphase, multicomponent, finite difference porous media simulator. This model was tested against laboratory and historic field data and used to investigate the potential effect of fracture-air convection on evaporation and matrix salt redistribution. System evaporation was found to increase 21% over a 100 day dry spell due to the presence of a large soil fracture. This increase was lower than predicted from historic research, which found a nearly 100% increase in evaporation(Ritchie and Adams, 1974). Salt redistribution due to evaporation driven pore solution flux was found to increase near-fracture- salt content by over 350%. A majority of the increase in near-fracture salt,73%, appeared as solid phase precipitated salt. Field and laboratory evidence indicate that concentrated salt can occur as precipitated salt along the fracture wall. In low permeability porous media, fractures are the major source of bypass flow to the subsurface. Evaporation and salt precipitation along fracture surfaces is proposed to be a potentially major source of salt flux to the water table, due to the high permeability of the fractures, the low permeability of the matrix, and the potential for preferential flushing of the fracture during rain events. While net evaporation was not significantly affected by the presence of the fracture, the simulated effect on salt transport can have significant application to the fields of waste management, agriculture and environmental remediation.