Colloids movement at the wetting front during infiltration to sand

Both preferential flow and colloid-sized particles facilitate groundwater pollution in the vadose zone. Preferential flow in sandy soils that overlays most aquifers is through unstable finger-like features. Studying colloid movement in these preferential flow paths is a crucial step toward better strategies for dealing with this pollution. In this study, we aimed to examine how the wetting front of fingers affects colloid mobilization and movement in dry sands. We postulate that the discontinuous pressure at the finger front results in the wetting front to move one pore at a time, causing high pore velocities with increased interfacial contact angles according to the Hoffman-Jiang equation. In a series of flow cell infiltration experiments, we used a high-speed camera (500 fps) to capture the colloid movement at the wetting front as water infiltrated acid-washed sands (2 mm in diameter). Four hundred and fifty milligrams of the sand particles were packed in a 0.6 x 0.2 x 2 cm channel and flushed with red-colored deionized water at 10 μl/min. Colloids were introduced by applying to the sand hydrophilic blue carboxylated microspheres (10.3 μm) or water-repellent polystyrene microspheres (10.2 μm) at a concentration of spheres/gram of sand before cell packing. Frame-by-frame image analysis was used to determine the position and velocity of the colloid movements, wetting front, and the advancing front contact angle. The results of the different experiments showed that the water velocity in the pore behind the front, based on the colloid velocity, is often quite different from the wetting front velocity and lacks a direct connection to it. In some cases, the wetting front advancement speed was 20 mm/s, four times faster than colloids. In others, the velocity of colloids could achieve around 10 mm/s while the wetting front’s velocity was 2 to 3 times less. The results also show that the changes in contact angle between the wetting front and particle surface are consistent with the Hoffman theory and are close to the value derived from the Hoffman Jiang equation. It confirmed that the change in contact angle during infiltration should be considered when studying water and colloid transport in sands.