Water Flow and Solute Transport in Unsaturated Fractured Chalk

Abstract

The nature of flow and transport through fractures crossing an unsaturated chalk formation were investigated in the northern Negev desert, Israel. An experimental setup was developed to allow controlled infiltration experiments through discrete, in situ fractures. Percolation experiments showed significant spatial and temporal flow variability through the fractures. Steady state flow was not reached for the duration of the experiments, either through individual small regions or across the entire flow domain, although the boundary conditions were kept relatively constant. Moreover, flow trajectories within the fracture plane, defined by tracer tests, varied over time. Over 70% of the fluid flux was transmitted through less than 20% of the studied fracture openings. Water flow through the fracture was focused into dissolution channels, which were typically associated with fracture intersections. The flow through these channels was governed primarily by the mineralogical composition of the filling material and the inner structure of the fracture voids. In particular, salt dissolution, solid-particle migration and clay swelling were found to be the main processes controlling flow through these dissolution paths; these processes account for the observed unstable flow regime. Our results suggest that models for simulating water percolation through fractures in the vadose zone, at least in chalk formations, should consider the mapping of fracture intersections, in addition to the more common mapping of fractures. Moreover, detailed characterization of these fracture geometries is not the sole key parameter determining fracture flow patterns - flow is also strongly controlled by physical variations in the filling materials during wetting and drying cycles.