ARS | Publication request: Colloid-Facilitated Solute Transport in Variably-Saturated Porous Media: Numerical Model and Experimental Verification

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 22, 2005
Publication Date: August 24, 2006
Reprint URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2113.pdf
Citation: Simunek, J., He, C., Pang, L., Bradford, S.A. 2006. Colloid-facilitated solute transport in variably-saturated porous media: numerical model and experimental verification. Vadose Zone Journal. Vol 5:1035-1047

Interpretive Summary: Many hazardous chemicals that are found in soils and environmental systems can be strongly adsorbed to soil particles, and are therefore typically assumed to be immobilized and pose a low risk of contaminating water supplies. Recent experimental observations, however, have shown that the transport of these chemicals can be enhanced by the presence of small soil and organic particles (colloids) that are commonly found in surface water and groundwater. To address this problem, we have developed a one-dimensional numerical model based on the HYDRUS-1D software package that incorporates state-of-the-art mechanisms associated with colloid-facilitated transport of these chemicals in soil environments. Example simulations are presented to illustrate the utility of this model to describe experimental data and to highlight the sensitivity to various model parameters.

Technical Abstract: Strongly sorbing chemicals (e.g., heavy metals, radionuclides, pharmaceuticals, and explosives) in porous media are associated predominantly with the solid phase, which is commonly assumed to be stationary. However, recent field- and laboratory-scale observations have shown that, in the presence of mobile colloidal particles (e.g., microbes, humic substances, clays and metal oxides), the colloids could act as pollutant carriers and thus provide a rapid transport pathway for strongly sorbing contaminants. To address this problem, we have developed a one-dimensional numerical model based on the HYDRUS-1D software package that incorporates mechanisms associated with colloid and colloid-facilitated solute transport in variably saturated porous media. This numerical model accounts for transient variably saturated water flow and for both colloid and solute movement due to convection, diffusion, and dispersion, as well as for solute movement facilitated by colloid transport. The colloid transport module additionally considers processes of attachment/detachment to/from the solid phase and/or the air-water interface, straining, and/or size exclusion. Various blocking and depth dependent functions can be used to modify the attachment and straining coefficients. The solute transport module uses the concept of two-site sorption to describe nonequilibrium adsorption-desorption reactions to the solid phase. The module further assumes that the contaminant can be sorbed onto surfaces of both deposited and mobile colloids, fully accounting for the dynamics of colloids movement between different phases. The application of the model is demonstrated using the experimental data from column experiments, conducted to investigate the adsorption of cadmium (Cd) onto Bacillus subtilis spores and Escherichia coli vegetative cells and Cd transport in alluvial gravel aquifer media in the presence of these bacteria. Numerical results simulating bacteria transport, as well as the colloid-facilitated Cd transport are compared with experimental results. Sensitivity analysis of the model to various parameters is also presented.