Colloid transport in unsaturated porous media is not yet well understood. Both water content and flow rate affect colloid transport, but their relative importance is not known. Standard laboratory column studies and field tests cannot elucidate the change of colloid retention due to changes in water content or flow rate independently, because the change of one variable results in the change of the other. Laboratory transport studies conducted in a geocentrifuge allow independent control of water content and flow rate.

We conducted colloid transport experiments under varying water contents and pore water velocities. Two carboxylate-modified polystyrene colloids (26 and 220 nm diameter) were introduced to unsaturated columns under steady-state flow. The solution chemistry was chosen so that colloid transport occurred under unfavorable attachment conditions. We compare colloid transport under gravity conditions and under centrifugation at different water contents and pore water velocities.

In general, decreasing water content led to increased colloid retention inside the column for both colloids. A fraction of the retained colloids could be released by changing the solution chemistry, indicating that these colloids had been retained in the secondary energy minimum. We attribute the remaining unrecovered colloids to retention in the primary energy minimum and the liquid-gas interface. Overall the retention of the 26 nm colloids was stronger than the retention of the 220 nm colloids. Transport of 220 nm colloids was affected by water content but not pore water velocity. These results suggest that the 220 nm colloids are transported through pendular rings whereas the 26 nm colloids are transported in adsorbed water films.