Macropore flow and transport dynamics in partially saturated low permeability soils

Near-surface sediments play an important role in governing the movement of water and contaminants from the land surface through the vadose zone to groundwater. Generally, low permeability surficial soils restrict water flow through the vadose zone and form a natural protective barrier to migration of surface applied contaminants. These types of fine-grained soils commonly contain macropores, such as fractures, animal burrows, and root holes, that have been identified as preferential flow pathways in the subsurface. Accordingly, macropores have the potential to influence groundwater recharge rates and compromise the protective capacity of surficial soils, particularly where the overburden is thin and aquifers are close to the surface. Partially saturated flow and transport in these environments is inherently complex and not well understood. The objective of this thesis was to examine preferential flow processes and the associated movement of contaminants in macroporous, low permeability soils. This was accomplished by conducting numerical and field experiments to investigate and describe the dynamics of macropore flow during episodic infiltration through the vadose zone and evaluate the corresponding influence of macropores on vertical water flow and contaminant transport.;Model runs designed to assess transport variability under a variety of different physical settings, including a wider range of soil types, were also completed. Vertical contaminant fluxes were examined at several depths in the soil profile. The results showed that the presence of macropores (in the form of fractures) was more important than matrix permeability in controlling the rate of contaminant migration through soils. The depth of contaminant migration was strongly dependent on the antecedent moisture content and the presence of vertically connected fractures. Soil moisture content played a pivotal role in determining the onset and extent of preferential flow, with initially wet soils much more prone to macropore flow and deep contaminant migration. Simulations showed that surface applied tracers were able to reach the base of 2 m thick fractured soil profiles under wetter soil conditions (i.e., shallow water table). Likewise, long-duration, low-intensity rainfall events that caused the soil to wet up more resulted in proportionately more contaminant flux at depth. Fractured soils were particularly susceptible to rapid colloid movement with particle travel times to depths of 2 m on the order of minutes. The main implication is that the vulnerability of shallow groundwater is related more to vertical macropore continuity and moisture conditions in the soil profile, rather than traditional factors such as soil thickness and permeability.;Macropore flow and transport processes under field conditions were investigated using small-scale infiltration experiments at sites in Elora and Walkerton, Ontario. (Abstract shortened by UMI.).;Numerical simulations were conducted to identify the important physical factors controlling flow and transport behaviour in partially saturated, fractured soils. A three-dimensional discrete fracture model, HydroGeoSphere, was used to simulate infiltration into homogeneous soil blocks containing a single vertical rough-walled fracture. Relatively large rainfall events with return periods ranging from 5 to 100 years were used, since they are more likely to generate significant preferential flow. Initial results showed that flow system dynamics were considerably more sensitive to matrix properties, namely permeability and antecedent moisture content, than fracture properties. Capillary forces, combined with the larger water storage capacity in the soil matrix, resulted in significant fracture-matrix interaction which effectively limited preferential flow down the fracture. It is also believed that fracture-matrix interaction reduced the influence of fracture roughness and other related small-scale fracture properties. The results imply that aperture variability within individual fractures may be neglected when modeling water flow through unsaturated soils. Nevertheless, fracture flow was still an important process since the fracture carried the majority of the water flow and virtually all of the mass of a surface applied tracer to depth in the soil profile.