ON THE STRUCTURE AND FLOW PROCESSES IN THE CAPILLARY-FRINGE OF PHREATIC AQUIFERS

The water distribution in the capillary fringe (CF) reflects the inter action of a strongly wetting fluid in a heterogeneous porous medium. F ield profiles of gravimetric water content of the CF for a 30 m deep, sandy, phreatic aquifer in Israel are critically analyzed in the conte xt of the possible wetting and drainage processes in these sediments. A highly plausible explanation of the profiles is based on the spatial configuration of the CF surface determined from a model of the moveme nt of water within the porous medium. The structural types of CF that can arise from a number of competing pore-scale displacement mechanism s, in the presence of gravity, are characterized by the model. We diff erentiate between two generic types of CF structures: a tenuous invasi on-percolation type and a compact type. Flow, in response to a horizon tal pressure gradient, associated with each structure is analyzed. Our interpretation of the field data supports the compact structure with a spatial variation in the height of the CF surface, above the water t able, on the order of 1 m. In this compact structure horizontal how is characterized by stagnant regions in the CF above a critical height h (c) and flow only for regions below h(c). The field water content (at h(c)) may be used to predict the onset of lateral water flow in the CF .