Annette Dathe (1), Yuniati Zevi (2), Brian K. Richards (2), Bin Gao (2), Jean-Yves Parlange (2), Tammo S. Steenhuis (2)
(1) The City College of New York, Earth and Atmospheric Sciences, New York, NY 10031,
(2) Cornell University, Biological and Environmental Engineering, Ithaca, NY 14853,

Colloid transport through the vadose zone is of growing concern in recent years. Non-water-soluble contaminants can enter an aquifer very quickly (colloidal facilitated transport) or colloids can be pathogens itself (for example Cryptosporidium parvum), thus yielding the risk of polluting drinking water. Methods for visualization and modeling of colloid transport and retention on the pore scale will be presented and its implication for large scale processes will be discussed.

Flow experiments were run in a horizontal flow chamber using clean quartz sand as porous medium and synthetic fluorescent microspheres or bacteria (Escherichia coli, containing a green florescent protein) as colloids. The water phase was stained with Rhodamine B. The chamber was mounted under a Laser Scanning Confocal Microscope (Leica TCS SP2) which allows the acquisition of time series or image stacks in the z-direction. Three spectral channels were used, detecting three 8 bit images simultaneously for every time step.

The 3D z-stacks reveal that the colloids are attaching at the air/water meniscus/solid (AWmS) interface, where the water menisci diminish into a thin film covering the grains. Methods of digital image analysis are presented for quantification of the number and area of moving and retained colloids. The results show that once the first colloid is attached at the AWmS interface, the attachment rate increases until the number of possible attachment sites becomes limiting. The attachment continues until there is no space for the colloids to attach anymore. A theoretical model is presented that is capable of predicting observed colloid attachment processes.