Modeling unsaturated flow and transport in the saprolite of fractured sedimentary rocks: Effects of periodic wetting and drying

Abstract

Whereas a number of unsaturated zone modeling studies have been conducted on dual‐porosity systems where the matrix has low permeability, few have been conducted on systems where the matrix has relatively high permeability. In humid climates, in situ weathering of bedrock can form saprolite. Saprolite usually has high matrix porosity (and variable permeability) that is a reservoir for solute storage and can have relict fractures that transport solute rapidly. On the basis of a field investigation where natural chemical tracers were monitored at high resolution during storm events, a numerical model was created that simulated variably saturated transport in the saprolite. A series of simulations were performed to explore solute transport during cycles of wetting and drying. Modeling results indicated that the advective flux of solutes from the fractures into the matrix during wetting was greater than from the matrix back into the fractures during drying, resulting in a net storage of solutes in the matrix. We hypothesize that the amount of net solute storage in the matrix may increase as the frequency of wetting/drying cycles increases, up to an optimum frequency. At frequencies higher than the optimum, the amount of solute storage in the matrix may decrease because the system behaves more like a fully saturated system where diffusion is the dominant transport process between fractures and matrix. These conclusions have significant implications for such processes as remedial strategies for contaminants in the unsaturated zone, the application of fertilizers, and quantification of mineral weathering and dissolution rates.