Surface-zone flow along unsaturated rock fractures

Although fractures in rock are well recognized as pathways for fast percola tion of water, processes which permit fast flow along unsaturated fracture pathways remain to be identified and understood. Earlier aperture-based mod els of flow in partially saturated fractures permit fast flow only through a continuous network of locally saturated segments. Film flow was recently identified as a mechanism capable of sustaining fast Row along truly unsatu rated fractures when the matric potential is very close to zero. Another me chanism for fast flow along unsaturated fractures is introduced in this stu dy, "surface-zone flow," which can be important when the permeability of th e rock along fractures (fracture skin) is significantly greater than that o f the bulk rock matrix. In such systems the fracture surface zone provides low resistance pathways through which fast flow (relative to the bulk matri x rock) can occur, even when the fractures are at very low water saturation . Initial experimental tests of surface-zone fast flow were performed. Surf ace-zone fast imbibition of water was measured on a welded tuff and a rhyol ite. However, because (1) imbibition rates are also strongly influenced by rock wettability, (2) these initially air-dry rocks exhibited finite contac t angles upon exposure to water, and (3) we lack methods to reliably measur e permeabilities of thin regions on rock surfaces, quantification of permea bility contrasts was not possible in these initial tests.