Preferential flow in water repellent and moist sands

Water repellency in soils affects both groundwater pollution and crop yield. In water repellent soils, finger-like wetting patterns form that have many similarities with the unstable wetting fronts formed in dry coarse-grained sandy soils. The objectives were to examine the effect of both the degree of repellency and the initial moisture content on the wetting front pattern in sandy soils and to examine how the theory for unstable wetting fronts applies to water repellent sands. Three sets of infiltration experiments were performed. The first set involved measuring moisture content and matric potential in a slab chamber, using sands that were treated with OTS (octadecyltrichlorosilane) to produce varying levels of water repellency. In the second set water with and without surfactant was infiltrated in both naturally water repellent soils and in the same soils with the organic matter removed. A third set of infiltration experiments was performed in slab chambers with coarse sands having different levels of initial moisture content. In all experiments moisture contents were determination with a time resolution of seconds with either synchrotron radiation or light transmission. The results of the first and second sets of experiments showed that the wetting patterns of water repellent sands depended directly on the soil water characteristic curve. Using these curves, unstable flow theory developed for coarse-grained soils can predict the shape and water content distribution of the wetting front. The third set of experiments found that at moisture contents below 2%, unstable wetting fronts formed with both matric potential and water content decreasing behind the finger tip. Above 2%, a Richards' type wetting front was observed with the matric potential and water content increasing (slightly) behind the wetting front. These findings allow us to predict wetting pattern in sands with laboratory-measured soil hydraulic properties.