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Title: Colloid transport in dual-permeability media

item LEIJ, FEIKE - California State University
item Bradford, Scott

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2013
Publication Date: 4/19/2013
Citation: Leij, F.J., Bradford, S.A. 2013. Colloid transport in dual-permeability media. Journal of Contaminant Hydrology. 150:65-76.

Interpretive Summary: Preferential transport pathways in soils and aquifers can increase the risks of disease causing microbes to contaminant water supplies. A model was developed to simulate microbe transport in soils with preferential flow. Simulation results provided an adequate description of transport data for microbe sized microspheres in well defined preferential flow systems. Furthermore, the sensitivity of transport behavior to model parameters variations was demonstrated. The findings from this study will be of interest to scientists and engineers concerned with predicting the fate of microbes in heterogeneous soils with preferential flow.

Technical Abstract: It has been widely reported that colloids can travel faster and over longer distances in natural structured porous media than in uniform structureless media used in laboratory studies. The presence of preferential pathways for colloids in the subsurface environment is of concern because of the increased risks for disease caused by microorganisms and colloid-associated contaminants. This study presents a model for colloid transport in dual-permeability media that includes reversible and irreversible retention of colloids and first-order exchange between the aqueous phases of the two regions. The model may also be used to describe transport of other reactive solutes in dual-permeability media. Analytical solutions for colloid concentrations in aqueous and solid phases were obtained using Laplace transformation and matrix decomposition. The solutions proved convenient to assess the effect of model parameters on the colloid distribution. The analytical model was used to describe effluent concentrations for a bromide tracer and 3.2- or 1-µm-colloids that were observed after transport through a composite 10-cm long porous medium made up of a cylindrical lens or core of sand and a surrounding matrix with sand of a different grain size. The tracer data were described very well and realistic estimates were obtained for the pore-water velocity in the two flow domains. An accurate description was also achieved for most colloid breakthrough curves. Dispersivity and retention parameters were typically greater for the larger 3.2-µm-colloids while both reversible and irreversible retention rates tended to be higher for the finer sands than the coarser sand. The relatively small sample size and the complex flow pattern in the composite medium made it difficult to reach definitive conclusions regarding transport parameters for colloid transport.