Colloid Retention in Unsaturated Porous Media due to Capillary Forces

Colloid transport is an important mechanism in the movement of contaminants through the vadose zone to groundwater. However, direct observations of colloid transport in unsaturated soils have typically been limited. As a result, differing and conflicting colloid transport mechanisms have been proposed for the effects that the air phase plays on the retention of colloids, with little predictive ability to date. Proposed are pore-scale visualization studies of colloid movement and retention in unsaturated natural sand. These observations, coupled with breakthrough measurements, will allow greater understanding of the actual mechanisms governing colloidal transport in unsaturated media. Our hypothesis is that capillary forces determine the unique features of colloid retention and release in the thin water films where the air-water and water-solid interfaces closely approach each other in unsaturated porous media. We will investigate how the nature and magnitude of interfacial interaction energies through changes in properties of both the porous media such as surface roughness and the colloid properties will result in observable and predictable changes in capillary force-dependent retention behavior including water meniscus contact angle, extent and location of retention at the air-water-solid interface; 3 D reconstructions of images obtained using bright-field microscopy and confocal scanning laser microscopy will be analyzed digitally and used to quantify the movement and retention, rates of colloids. These experimental results will then be used to test and quantify the mechanisms that governing colloid retention and transport in unsaturated porous media.

Recent experience confirms that the role of colloidal transport in groundwater contamination is typically ignored in the environmental regulatory and assessment sphere. The research will improve the knowledge on how micrometer-size colloids move in unsaturated hydrological systems and thereby will help improve models in order to make possible more accurate predictions of conditions under which groundwater contamination can take place.