Work to date has raised awareness regarding the importance of transport of colloids in the vadose zone, as it is the critical connection between shallow contaminant sources and the deeper groundwater. Existing models do not predict accurately the physical conditions under which such particle transport occurs. Recent theory on capillary and friction forces acting at the air–water meniscus–solid (AWmS) interface suggests that grain roughness is an important factor in colloid retention for unsaturated media. Our main objective was to investigate the role of surface roughness in governing colloid transport in the vadose zone. Unsaturated flow cell experiments with sands of different grain roughness were performed to determine colloidal retention at the postulated AWmS interface. Two rectangular acrylic vertical flow cells (2 by 2 by 5 and 2 by 2 by 10 cm) were built to collect visual and numerical data under the effects of gravity. Colloid behavior was visualized in situ with digital bright field microscopy, effluent concentrations of colloids were measured concomitantly with spectrophotometry, and retention was quantified with mass balance analysis. From visual analysis, significant retention was observed at apparent AWmS interfaces and in zones of immobile water. An equilibrium deterministic convective–dispersion model was used and fitted to our data with fair accuracy. This study's visual, quantitative, and statistical results show that: more particles are retained at lower water contents, smoother grain surfaces retain fewer colloids in the porous media under both saturated and unsaturated conditions, and the effects of surface roughness diminish with increasing grain surface roughness.

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