Does Water Content or Flow Rate Control Colloid Transport in Unsaturated Porous Media?

• Published in: Environmental science & technology Vol. 48; no. 7; pp. 3791 - 3799
• Main Authors: Knappenberger, Thorsten, Flury, Markus, Mattson, Earl D, Harsh, James B
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.04.2014
• Subjects: Applied sciences; Biological and physicochemical properties of pollutants. Interaction in the soil; colloid transport; colloids; Colloids - chemistry; Earth sciences; Earth, ocean, space; energy; Engineering and environment geology. Geothermics; Exact sciences and technology; Flow velocity; Fluid dynamics; GENERAL AND MISCELLANEOUS; Models, Theoretical; Nitrates - analysis; Pollution; Pollution, environment geology; Polystyrene; polystyrenes; Porosity; Porous materials; porous media; Rheology; Sediment transport; soil; Soil and sediments pollution; Soil contaminants; Water - chemistry; water content; Water Movements; Transport process; Unsaturated zone; Flow rate; Porous medium flow; Limiting factor; Water content; Soil pollution; Unsaturated soil; Colloid particle
• ISSN: 0013-936X; 1520-5851; 1520-5851
• DOI: 10.1021/es404705d

Mobile colloids can play an important role in contaminant transport in soils: many contaminants exist in colloidal form, and colloids can facilitate transport of otherwise immobile contaminants. In unsaturated soils, colloid transport is, among other factors, affected by water content and flow rate. Our objective was to determine whether water content or flow rate is more important for colloid transport. We passed negatively charged polystyrene colloids (220 nm diameter) through unsaturated sand-filled columns under steady-state flow at different water contents (effective water saturations S e ranging from 0.1 to 1.0, with S e = (θ – θr)/(θs – θr)) and flow rates (pore water velocities v of 5 and 10 cm/min). Water content was the dominant factor in our experiments. Colloid transport decreased with decreasing water content, and below a critical water content (S e < 0.1), colloid transport was inhibited, and colloids were strained in water films. Pendular ring and water film thickness calculations indicated that colloids can move only when pendular rings are interconnected. The flow rate affected retention of colloids in the secondary energy minimum, with less colloids being trapped when the flow rate increased. These results confirm the importance of both water content and flow rate for colloid transport in unsaturated porous media and highlight the dominant role of water content.