ROLE OF FRACTURE-MATRIX INTERACTION IN CONTROLLING RECHARGE THROUGH VARIABLY SATURATED AQUITARDS

Fractures and other large macropores have a significant influence on groundwater flow through low permeability aquitard materials. Groundwater recharge through these fractured materials is extremely complex, due in part to the highly variable saturation conditions and the degree of fracture-matrix interaction within the vadose zone. In this study, the dynamics of groundwater recharge in variably saturated discretely fractured porous media were investigated using numerical simulations. Several hypothetical groundwater recharge scenarios were simulated for a simple porous matrix containing a single vertical rough-walled fracture. Aperture variability within the fracture plane was generated stochastically and assumed to follow a spatially correlated log-normal distribution. The influence of fracture and matrix properties as well as antecedent moisture conditions on vertical water and tracer fluxes was assessed. It was shown that moisture exchange between the fracture and the matrix had a significant influence on the rate and depth of penetration of infiltrating water. Groundwater recharge was highly sensitive to matrix properties and considerably less sensitive to fracture properties, especially aperture variability. Vertical tracer migration was controlled by the fracture flux. The results have implications for estimation of groundwater recharge and the rate at which contaminants, particularly colloids, are transported through fractured aquitards. The simulations are being used to aid in the design of a laboratory experiment to further investigate the nature of the fracture-matrix interaction.