Various flow mechanisms occur in an unsaturated rock matrix-fracture system. For example, the fracture may act as a capillary barrier, or it may have flow in relatively thick films along the fracture surface. The topic of unsaturated flow in fractures has received substantial scientific attention only in the last few years. Broadly applicable quantitative models for predictive treatment of unsaturated flow in fractured media, analogous to those based on Darcy's law and Richards' equation for nonpreferential unsaturated flow, are yet to be developed. Basic knowledge of these mechanisms is critical to the development of the quantitative models needed for research and management issues concerning fractured media.
In soils, macropores such as root holes, animal burrows, and desiccation or cooling cracks can lead to rapid flow rates, especially under very wet conditions. In complex unsaturated zones, macropore structure and texturally contrasting layers may have competing influences on water flow. Contrasting layers generally impede unsaturated flow, as unsaturated hydraulic conductivities decrease with the necessary adjustment of the unsaturated state of each layer to that of adjacent layers.
The unsaturated medium of the 200-m-deep unsaturated zone at our Idaho National Engineering and Environmental Laboratory (INEEL) field site consists of fractured basalts that are interbedded by thinner layers of fine sediments. These interbeds, depending on the magnitude and prevalence of critical hydraulic processes within them, may have the effect of (1) accelerating contaminant transport or retarding it, (2) concentrating contaminants or diluting them, and (3) determining whether the dominant movement is vertical or horizontal. The surficial soil at this site, especially where it has not been disturbed for landfill construction, has impeding layers and significant macropore structure. For this soil we developed a numerical simulation of unsaturated flow in 2 dimensions, for 9 surficial layers, based on measured hydraulic properties. We successfully applied the VS2DT code, modified for hysteresis, using wetting curves determined from minimal data using various hysteresis models. Hysteresis showed less effect than macropore flow. We represented macropore effects by developing a technique involving time-staggered initial conditions. Macropore flow modeled through time-staggered initial conditions shows that 3 hours of data from ponded infiltration embodies essential information concerning macropores
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