Collaborative Research: Chemotaxis in Porous Media--Experimental Observations and Upscaling for Development of a Descriptive Theory

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Chemotaxis is the ability of bacteria to sense chemical concentration gradients in their local surroundings and swim toward higher concentrations of chemicals, which they perceive to be beneficial to their survival. In this work, we propose to examine how the pore-scale process of chemotaxis influences the Darcy-scale observation of bacterial transport in porous media. In the subsurface, bioremediation is often hindered by the inability to achieve good mixing between injected substances and the resident contaminants. In such situations, chemotaxis might be exploited to enhance the mixing of bacterial populations within contaminated zones. This would be a multiscale phenomenon in which bacteria migrate in response to pore-scale variations in pollutant concentration that results in greater mixing at the field-scale. There are two closely related open questions for this scheme: (1) how does one relate the pore-scale description of chemotaxis to the effective dispersion tensor that is used to predict bacterial spreading in Darcy-scale applications, and (2) can one also predict the spatial variance of the bacterial concentration to predict the microscale mixing that has occurred within the porous medium. Objective and Approach: The overall goal of this project is to quantify the impact of pore-scale chemotaxis on bacterial transport at the Darcy-scale. We will accomplish this through a combination of theory development and laboratory experimentation in the following steps. 1. Derive a Darcy-scale transport equation for bacteria that accounts for chemotactic responses to local chemical gradients via volume averaging. This will be used to predict (i) the effective dispersion tensor, and (ii) the Darcy-scale spatial variance of the concentration of bacteria.2. Compare theory and experiment for several simplified test cases in which the velocity field and chemical gradients are well-defined. Represent these results as engineering correlations that relate dispersion to dimensionless groups such as the Peclet number and chemotactic driving force.3. Test these correlations experimentally for more complex porous media systems by using microfluidic devices, which allow for direct visualization of fluid flow patterns and bacterial distributions at the pore- and Darcy-scales. Intellectual Merit: To understand the impact of chemotaxis on the biological degradation of chemical contaminants in groundwater systems requires a quantitative analysis that relates the chemotactic response to local chemical gradients over length scales of millimeters to bacterial dispersion over length scales of meters. In our approach we build on current work of hydrologists to model transport of chemical contaminants in groundwater and extend it to motile colloids that are transported due to chemical gradients in addition to hydraulic gradients. The potential for exploiting chemical gradients as a driving force to control the migration of bacterial populations is intellectually appealing. State-of-the-art approaches both in mathematical modeling (upscaling by volume-averaging) and experimental design (microfluidic devices) will be employed in the project.Broader Impact: A new partnership between researchers at the University of Virginia and Oregon State University brings together expertise in the design of experimental systems to quantify bacterial migration and mathematical modeling to relate pore-scale phenomena to field-scale observations. We will encourage students to broaden their experiences through international student exchange and collaboration. Investigators with a proven commitment to recruiting underrepresented groups in engineering will broadly educate one graduate student and one post-doctoral associate in an interdisciplinary context to meet critical needs for environmental engineers in our national workforce. Outreach to local high schools will provide hands-on laboratory experience in genetic engineering for Honors Biology students and their teachers to engage the next generation in the wonder of science. Results from this study will yield engineering correlations to better inform decision-makers about the feasibility of monitored natural attenuation as a treatment option at polluted sites where biological degradation has been documented.