(540g) Modeling Chemotactic Bacterial Transport in Physically Homogeneous Groundwater Systems Containing Distributed Contaminant Sources

Authors: 
Adadevoh, J. S. T., University of Virginia
Ford, R. M., University of Virginia
Groundwater pollution is of major concern as contaminants are continuously released into the subsurface via oil spills, chemical waste streams, and other means. Often times these contaminants become trapped within regions of the soil matrix that are characterized by low hydraulic conductivity making them difficult to remediate. In bioremediation strategies, chemotaxis â?? a phenomenon in which pollutant-degrading bacteria have the ability to detect chemical concentration gradients and swim toward the contaminant source â?? may enhance the transport of bacteria to sources of contamination and concomitantly lead to increased pollutant accessibility and biodegradation, even in regions with low water permeability. The influence of chemotaxis on bacterial transport in aquifers containing a distribution of contaminants with localized concentration gradients is not yet fully understood. This study aimed to utilize computational techniques to predict and better understand the migratory response of chemotactic bacteria to contaminant microniches within a model subsurface environment. Focus was placed on a sand-packed column containing randomly distributed pollutant sources. The bacterial population was introduced as a pulse input at the column inlet. A modified advection-dispersion equation containing an additional advection-like term to describe chemotaxis was used to model bacterial transport. The chemotactic velocity is a function of the chemotactic sensitivity coefficient, which describes the strength of chemotaxis, and the chemotactic receptor constant, which is the concentration at which the chemotactic velocity of the bacterial population is equal to one-half of its maximum value. Sensitivity analyses were performed on the chemotactic sensitivity coefficient and receptor constant to determine their influence on bacterial transport. It was equally important to predict the pollutant concentration profile within the porous media, via a standard advection-dispersion equation, because the chemotactic velocity is also a function of the pollutant concentration and concentration gradient. Our results showed that when chemotaxis was occurring, bacterial transport in the direction of groundwater flow was retarded and the bacterial population was retained within the contaminated porous media for an extended period of time, compared to when chemotaxis was not occurring. Predictions from our numerical simulations were consistent with experimental observations from a sand column system analogous to our model set-up. Modeling chemotactic bacterial transport in porous media systems containing a distribution of contaminant sources is important for assessing the significance of chemotaxis in enhancing the retention of bacteria in sites of contamination.