PREFERENTIAL FLOW IN WATER-REPELLENT SANDS

Water-repellent soils occur all over the world and affect both ground, vater pollution and crop yield. The finger-like wetting patterns in th ese soils have many similarities with unstable wetting fronts in coars e-grained sandy soils. Our objectives were to study the water movement in,vater-repellent sand and to examine how the theory for unstable we tting fronts applies to water-repellent sands. infiltration experiment s, in which moisture content and matric potentials were measured, were carried out in slab chambers with identical sands but with different levels of water repellence. Soil water characteristics were determined in separate experiments. Infiltration in the hydrophilic soil resulte d in a uniform and horizontal front. All water-repellent sands showed a fingered now pattern. For negative water-entry values, water infiltr ated without delay, For positive values, water entered the soil only a fter the depth of the ponded mater equaled or exceeded the water-entry pressure, which increased with increasing repellency: The finger widt hs predicted with the unstable now theory agreed rather well with the observed values. In general, the research showed that the wetting patt erns of water repellent sands depended directly on the soil water char acteristic curve, This implies that the type of wetting front and risk to groundwater pollution can be predicted based on laboratory-measure d soil hydraulic properties.