Effects of hydrophilic macropore fillings and coatings on the infiltration into water repellent porous media

Macropores generate rapid flow paths in the surface soils by their high permeability under saturated/near-saturated moisture conditions. In natural soils, some macropores are filled/coated with various materials including decayed plant roots (Meek et al., 1989), exudates from plants/soil organisms (Jegou et al., 2001), iron oxides or other precipitates from preferentially-introduced solutes/colloids to the macropores (Rasmussen et al., 2001), or the surrounding soils with reduced bulk density (Ela et al., 1992). When we expect infiltration into water repellent soils through macropores or hydrophilic patches created from the macropore cementation processes, hydrophilicity of the macropore fillings/coatings should be understood. In the present study, we conducted an infiltration experiment with water repellent porous media and some macropore fillings/coatings, in order to clarify the roles of hydrophilic macropore fillings/coatings in infiltration. Ponding depth and flow distribution were monitored with a micro-focus X-ray computational tomography apparatus (SMX-90CT, Shimadzu Corp., Kyoto, Japan) at 90 kV and 110 μA. Dilute CsCl(aq) (density: 1.04 Mg m-3) was used as the contrast media to avoid density-driven alteration of the flows. Water repellency of the samples was evaluated by the water drop penetration time (WDPT, Van't Woudt, 1959). A glass beads (mean diameter: 0.46 mm, BZ-04, ASONE Corp., Osaka, Japan) was used as water repellent porous media. The glass beads sample was packed in 50-mL polypropylene centrifugation tubes at 1.55 Mg m-3 bulk density. A 2-mm hole was made at the bottom of each centrifugation tube for ventilation. The hole was covered with mesh cloth. Macroporous structure was made at the center of each tube from the surface. Each macroporous structure had 4-mm diameter and 30-mm length. Six types of macropores were prepared including 1) no macropore, 2) empty macropore, 3) an aluminum (Al) pipe (4-mm inner diameter, 5-mm outer diameter), 4) a water-repellent Al-pipe (treated with HIREC1450NF, NTT AT Corp., Tokyo, Japan), 5) macropore with bamboo fiber, and 6) macropore with palm fiber. Rainfall intensity was set at 60 mm h-1 to simulate ponding water formation by a storm event. The empty macropore was made by inserting an aluminum lod (4-mm diameter) to 30-mm depth after the simulated rainfall. The WDPT (H2O) of the glass beads exceeded 16 h, while the plant fibers showed less WDPTs (15 s for the bamboo fiber, 144 s for the palm fiber). The WDPTs of the samples were prolonged by CsCl, so we focused on comparizon among the samples. The results of the infiltration experiment showed the greatest efficiency of ponding-water removal by the Al-pipe, but the water-repellent Al-pipe did not show any effect on ponding-water removal. The CT images showed air-entrapment in the water-repellent pipe. The empty macropore also had little effect on ponding-water removal, but the macropore was filled with the contrast media. Therefore, inertial force of the flow in the hydrophilic pipe would be effective in exceeding water entry pressure of the porous media. The fiber fillings gradually infiltrated the ponding water. The bamboo fiber showed higher effciency for ponding water removal than the palm fiber. These facts have shown the effectiveness of hydrophilic macropore fillings/coatings in infiltration.