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A mechanistic model for water seepage through thick unsaturated zones in fractured rocks of low matrix permeability

The theme developed in this paper is that water seepage in thick unsaturated zones in fractured rocks of low matrix permeability cannot be understood by considering spatial and temporal averages. We suggest that most such seepage proceeds in an unsteady, episodic manner in localized preferential pathways. Mechanisms are proposed and explored which permit water to flow relatively freely in networks of interconnected fractures in the presence of strong suction pressures from an unsaturated rock matrix. Imbibition into the rock matrix is reduced by limited wetted area, the episodic nature of flow, and possibly also by the presence of mineral coatings of low permeability on the fracture walls. These effects and conditions combine to limit total imbibition rates to values less than saturated matrix hydraulic conductivity, leaving the rock matrix in partially saturated conditions even as water is flowing freely in portions of the fracture network. The proposed conceptual model is elaborated and substantiated with analytical estimates and numerical simulation studies. Mechanisms are demonstrated that can funnel distributed seepage into spatially localized preferential paths. Subsequent horizontal broadening from dispersive mechanisms is found to be a slow process for fractures of high intrinsic permeability. The presence of an unsaturated rock matrix provides constraints on fracture‐matrix interface areas for imbibition and on the frequency of episodic deep seepage events. Further studies are needed to test the proposed model.