[1] We present results from pore-scale observations of colloid transport in an unsaturated physical micromodel. The experiments were conducted separately using three different sizes of carboxylate polystyrene latex spheres and Bacteriophage MS2 virus. The main focus was to investigate the pore-scale transport processes of colloids as they interact with the air-water interface (AWI) of trapped air bubbles in unsaturated porous media, as well as the release of colloids during imbibition. The colloids travel through the water phase but are attracted to the AWI by either collision or attractive forces and are accumulated at the AWI almost irreversibly, until the dissolution of the air bubble reduces or eliminates the AWI. Once the air bubbles are near the end of the dissolution process, the colloids can be transported by advective liquid flow, as colloidal clusters. The clusters can then attach to other AWI down-gradient or be trapped in pore throats that would have allowed them to pass through individually. We also observed small air bubbles with attached colloids that traveled through the porous medium during the gas dissolution process. We used Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to help explain the observed results. The strength of the force that holds the colloids at the AWI was estimated, assuming that the capillary force is the major force that holds the colloids at the AWI. Our calculations indicate that the forces that hold the colloids at the AWI are larger than the energy barrier between the colloids. Therefore it is quite likely that the clusters of colloids are formed by the colloids attached at the AWI as they move closer at the end of the bubble dissolution process. Coagulation at the AWI may increase the overall filtration for colloids transported through the vadose zone. Just as important, colloids trapped in the AWI might be quite mobile when the air bubbles are released at the end of the dissolution process, resulting in increased breakthrough. These pore-scale mechanisms are likely to play a significant role in the macroscopic transport of colloids in unsaturated porous media.INDEX TERMS: 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport; 5139 Physical Properties of Rocks: Transport properties; KEYWORDS: air-water interface, DLVO, hydrophobic, colloid, unsaturated, micromodel Citation: Sirivithayapakorn, S., and A. Keller, Transport of colloids in unsaturated porous media: A pore -scale observation of processes during the dissolution of air-water interface, Water Resour.

[1] We present experimental evidence of the effect of colloid exclusion from areas of small aperture sizes, using direct observations at the pore-scale using a realistic micromodel of porous media. Four sizes of hydrophobic latex spheres in aqueous suspension, from 0.05 to 3 mm, were introduced into the micromodel at three different pressure gradients. We observed the frequency of occurrence of the size exclusion effect and the influence of relative size of pore throats and colloids (T/C ratio) and flow velocity. From our observations the smallest T/C ratio entered by these different colloids was 1.5. We also observed certain preferential pathways through the pore space for different colloid sizes, such that size exclusion eventually results in distinct pathways. These preferential paths become more important for larger colloids and for greater pressure gradients. Measured colloid velocities were 4-5.5 times greater than estimated pore water velocities. Acceleration factors (ratio of colloid to water velocity) increased for all colloid sizes with increasing pore-scale Pe. Smaller particles appeared to travel along faster streamlines in pore throats, while larger particles travel along with a number of streamlines, thus at a slightly lower velocity than the small colloids. At larger scales the acceleration factor is decreased owing to Brownian motion, adsorption, colloid and straining filtration, and other factors, but these pore-scale results shed light on the size exclusion effect and its role in determining early colloid breakthrough.

1] We conducted column-scale experiments to observe the effect of transport velocity and colloid size on early breakthrough of free moving colloids, to relate previous observations at the pore scale to a larger scale. The colloids used in these experiments were bacteriophage MS2 (0.025 mm), and 0.05-and 3-mm spherical polystyrene beads, and were compared with a conservative nonsorbing tracer (KCl). The results show that early breakthrough of colloids increases with colloid size and water velocity, compared with the tracer. These results are in line with our previous observations at the pore scale that indicated that larger colloids are restricted by the size exclusion effect from sampling all paths, and therefore they tend to disperse less and move in the faster streamlines, if they are not filtered out. The measured macroscopic dispersion coefficient decreases with colloid size due to the preferential flow paths, as observed at the pore scale. Dispersivity, typically considered only a property of the medium, is in this case also a function of colloid size, in particular at low Peclet numbers due to the size exclusion effect. Other parameters for colloid transport, such as collector efficiency and colloid filtration rates, were also estimated from the experimental breakthrough curve using a numerical fitting routine. In general, we found that the estimated filtration parameters follow the clean bed filtration model, although with a lower filtration efficiency overall.Citation: Keller, A. A., S. Sirivithayapakorn, and C. V. Chrysikopoulos (2004), Early breakthrough of colloids and bacteriophage MS2 in a water-saturated sand column, Water Resour. Res., 40, W08304,

[1] We conducted column-scale experiments to study the transport of colloids (latex particles and bacteriophage MS2) under water-unsaturated conditions. The objective was to draw connections between observations at the pore scale and the results obtained from column-scale experiments. The same system had been previously operated under saturated conditions to determine colloid collision efficiency. Breakthrough of colloids was first evaluated under unsaturated but steady water content conditions, with constant trickling flow. After monitoring the steady breakthrough of the colloids, the column was flushed with water at higher flow rate to increase the water content up to a saturated condition. Colloid breakthrough was monitored during the entire experiment, as water content increased. Colloid removal increases significantly with decreasing initial water saturation, reflecting retention at the air-water interface and straining in thin-water films. Colloid breakthrough occurs earlier than a conservative tracer even under unsaturated conditions, although the colloid concentrations are much lower than the tracer. After flushing at similar flow rates, there is increased colloid retention under unsaturated conditions even as the system approaches water saturation, indicating that additional removal is occurring, due possibly to the formation of colloidal clusters. These results can be explained to a great extent by pore-scale observations of retention and remobilization mechanisms.

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