| Tightly packed soil presents a barrier against migration of hazardous waste towards the groundwater that may infiltrate through the normally unsaturated vadose zone. Cracks or other physical irregularities, which allow accelerated transport of waste through them, can compromise the barrier and are thus avoided in normal engineering design, for example in waste burial facilities. However, unsaturated soils that are apparently homogeneous on a macroscopic spatial scale can nevertheless allow accelerated flow pathways under conditions of flow instability. The combined influences of gravity, surface tension and random variation of void ratio drive this process on a spatial scale of a few times the dimensions of individual soil particles. Flow then occurs mainly through “fingers” that can accelerate migration of contaminants through the soil.; To determine the threat of an accidental surface spill of contaminants or discharges from underground storage tanks and landfills, the distribution of the chemicals in the vadose zone must be known or confidently predicted. This preferential flow presents a mechanism for accelerated transport of water and solutes through the vadose zone that has the potential to contaminate large quantities of groundwater. These fingers enable the liquids to travel faster than predicted by traditional sharp wetting-front models while bypassing a large portion of the unsaturated soil medium.; An infiltration chamber and associated techniques along with laboratory experiments were developed to study such fingering. The mean grain size and the density of the soil affected the properties of the fingers. The coarser and denser soil decreased the width while increasing the velocity of the fingers. The stability and persistence of preferential flow paths in sands have been shown to determine the flow paths of subsequent infiltration events. Since existing numerical and analytical models have been shown to be inadequate in describing the unstable wetting front a new analytical model, based on the results of the experiments, was developed to attempt to predict the properties of the fingers. The results of these experiments could also be used to determine which soils and infiltration rates may cause instabilities in the wetting front. |
