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Impact of the Capillary Fringe on Local Flow, Chemical Migration, and Microbiology

^a Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100 Israel
^b Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556

View larger version (111K): [in a new window]   Fig. 1. Initial experiment of Silliman et al. (2002) demonstrating transport within a homogeneous sand. Flow is from left to right and the dye is introduced at the locations shown by the circles. Transport above the water table (the water table is shown by the dotted line) is evident in the motion of the dye spots. The arrows show the association between each dye spot and the location of injection of that spot. The time reported on the figure is time since the introduction of the dye tracer. (After Silliman et al., 2002.)

View larger version (103K): [in a new window]   Fig. 2. Second experiment of Silliman et al. (2002) demonstrating infiltration within a homogeneous sand. The average groundwater flow is from left to right. The water table is marked with the dashed line. The solid line indicates the approximate upper limit of the capillary fringe. The dye was added as a slug input within the small injection region labeled "Dye Input" in the figure. The slug was flushed into the sand under a uniform infiltration distributed across the surface of the sand. (After Silliman et al., 2002.)

View larger version (120K): [in a new window]   Fig. 3. The experimental apparatus used by Dunn and Silliman (2003) to study the impact of air-entry barriers. The numbers represent the location of TDR probes, and the circle represents the location of a pressure transducer located within the sand. The coarse sand appears as the darker sand in the image. The sand was packed under saturated conditions. The first experiment involved lowering the water table to the base of the tank. The second experiment involved raising the water table to the upper surface of the sand.

View larger version (26K): [in a new window]   Fig. 4. Comparison of moisture content in the right and central coarse sand inclusions in Fig. 3 during drainage. "Height of WT" refers to height of the water in the reservoirs relative to the base of the tank. "Pressure Head" refers to the equivalent pressure head at the measurement point assuming hydrostatic conditions. Within the right sand inclusion, free movement of the air phase allows sequential desaturation at the four TDR locations. In the center sand inclusion, prevention of the invasion of the air phase (due to overlying fine sand) leads to delayed drainage of the upper TDR locations until air penetration of the overlying fine sand. (After Dunn and Silliman, 2003.)

View larger version (27K): [in a new window]   Fig. 5. Comparison of moisture content in the right and central coarse sand inclusions in Fig. 3 during imbibition. "Height of WT" refers to height of the water in the reservoirs relative to the base of the tank. "Pressure Head" refers to the equivalent pressure head at the measurement point assuming hydrostatic conditions. Within the right sand inclusion, free movement of the air phase allows sequential resaturation at the four TDR locations. In the center sand inclusion, resaturation continues as in the right zone until the overlying fine sand becomes saturated. At that point, air is trapped within the coarse sand and no further significant increase in saturation is observed at the upper two TDR locations despite a rise in the water table to the upper surface of the sand. (After Dunn and Silliman, 2003.)

View larger version (100K): [in a new window]   Fig. 6. Migration of fluorescent microbes (the bright regions in the figure) in a heterogeneous sand. Flow is from left to right, with microbes added only below the water table in the left reservoir. The bright region in the upper right represents microbes being preferentially transported through a coarse sand lens above the water table. The dark region slightly left of center in this image is a zone of trapped air in a coarse sand and demonstrates the inability of microbes to enter the upper portion of this sand lens due to the very low moisture content.