Flow and Distribution of Water in Fractional Wettability Unsaturated Porous Media

In recent years, interest in fluid flow and transport in the unsaturated zone has gained more attention, due to growing concerns that the quality of the subsurface environment is adversely affected by agricultural, industrial, and municipal activities. The wettability properties (e.g., water repellency) of soil in the unsaturated zone play an important role in determining fluid movement and ultimate distributions. A number of studies have shown that preferential flow and spatially variable moisture content is likely to occur in water repellant soils. Development of appropriate constitutive relations for numerical modeling becomes even more difficult in systems that contain water repellant soils. First, flow through fractionally-wet systems often follows preferential flow paths. Preferential flow can result in fast and deep infiltration of water and may impact solute and colloid/virus transport and plant growth. Second, it is difficult to incorporate pore- and centimeter-scale processes that result in irregular water flow and distribution during and following drainage. The resulting small-scale heterogeneities may impact subsequent infiltration and evaporation/volatilization processes. In this research, mm- and cm-scale capillary pressure¡Vwater content experiments and computed x-ray microtomography (CMT) were used to obtain quantitative data describing drainage and the irreducible water distribution in fractionally-wet systems. The findings from this research showed that wettability and pore-size distribution affected the capillary pressure-water content relationship in uniform and well-graded sand. As expected, an increase in the fractional wettability caused a decrease in the air entry pressure for all the sands tested. As the fractionally wettability increased, the slope of the capillary pressure-water content became steeper for the uniform sands and shallower for the well-graded sand. Comparison of mm- and cm-scale drainage capillary pressure-water content experiments showed that columns designed for CMT experiments can be used for low values of fractional wettability (less than 25%). CMT was successfully used to image the heterogeneous distribution of water during and following drainage; water content values obtained from the images when combined with the corresponding capillary pressure head values matched the laboratory experimental data. Finally, CMT was shown to be a highly effective technique to quantitatively characterize ƒÝm-scale grain, pore, and fluid properties