PARTITIONING OF CHEMOTACTIC BACTERIA AT INTERFACES OF POROUS MEDIA AND AQUEOUS SOLUTIONS

Chemical contaminants released into groundwater tend to accumulate within clay lenses that are less permeable to flow; these clay lenses then become long-term sources of contamination as contaminants slowly leach out over time into the surrounding, more permeable sandy aquifer. Bacteria that are capable of degrading the contaminants may be transported to these sources by flow through more permeable, sandy layers. We investigated the distribution of motile bacteria at an interface between a viscous aqueous solution of methylcellulose (MC) and two types of porous media; one a cross-linked polymer network to resemble the amorphous structure of clay and the other a glass-bead pack to resemble grains of sand. An initially uniform distribution of bacteria across an interface of a MC solution and Gelrite network increased in concentration on the Gelrite-side and decreased on the MC side. This result cannot be explained by a difference in the effective diffusivities of motile bacteria caused by structural differences in the two media because diffusive processes tend to reduce concentration differences over time rather than generate them. Instead, we propose that swimming bacteria are obstructed by the more densely cross-linked regions of the gel network and the glass surfaces of the bead-packs that leads to an extended residence time at the surface. The association with the surface is accounted for as an adsorption-like process even though direct observations of individual swimming bacteria do not suggest that they are physically attaching to the surfaces. A mathematical model that incorporates this adsorption-like term in the transport equation produces bacterial concentration profiles that are consistent with experimental observations for several configurations of the interface and initial bacterial distributions. In the presence of a gradient in contaminant concentration chemotactic bacteria are able to bias their migration in the direction of increasing concentration of the contaminant. We also investigated partitioning of chemotactic bacteria across interfaces with a chemical gradient and observed an enhanced partitioning effect. With the addition of a chemotactic term to the mathematical model, we found good agreement between model predictions and experimentally observed bacterial profiles.