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Larry Lanning
Mira Olson (Civil Engineering)
Louise Parsons

One difficulty with using conventional pump-and-treat technology to clean-up polluted groundwater is that contaminants are often trapped within the soil matrix in areas that are not readily accessible to groundwater flow. Once pumping ceases, residual contamination gradually diffuses from these less permeable areas into more permeable areas where it is carried downstream by the groundwater to monitoring wells or drinking water supplies. Thus, it is often residual contamination in regions of low permeability that presents the most persistent groundwater problems that are especially challenging and costly to remediate.

An important mechanism by which the residual contamination may be removed is through biological degradation by motile bacteria...

An important mechanism by which the residual contamination may be removed is through biological degradation by motile bacteria that preferentially migrate toward increasing concentrations of chemical contaminants (a phenomenon known as chemotaxis). It has been observed that P. putida F1 bias their swimming behavior in response to gradients in benzene concentration. Computer simulations of bacterial swimming behavior near a nonaqueous phase liquid (NAPL) ganglion showed a significant increase in the residence time of bacteria in the vicinity of the NAPL-water interface. An increase in contact time between the bacteria and the contaminant will increase the rate of degradation. As bacteria consume the contaminant, they help to maintain a large difference in contaminant concentration at the water-NAPL interface. This concentration gradient provides a driving force for diffusion of the contaminant into the bulk fluid where it can be consumed by bacteria or removed by pumping.

The goal of this project is to quantify the strength of this bacterial chemotactic response in order to judge its significance relative to other processes affecting contaminant degradation and removal from groundwater. We will study this behavior experimentally in porous media containing residual NAPL. We have developed unique experimental capabilities in collaboration with Prof. Erik Fernandez that use magnetic resonance imaging (MRI) to visualize the benzene distribution within packed sand columns. Furthermore, by tagging bacteria with superparamagnetic iron particles we have been able to observe the bacterial distribution within a stagnant column diffusing from an initial step-change in bacterial density.

This research is sponsored in part by The Petroleum Research Fund administered by the American Chemical Society.


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Department of Chemical Engineering
University of Virginia
Charlottesville, VA 22904