Bacterial Enhancement of Contaminant Bioavailability: Effects of Random Motility and Chemotaxis

Understanding bacterial and contaminant transport in subsurface porous media is critical both in preventing contamination of drinking-water supplies and in successful implementation of bioremediation. Groundwater bioremediation is often limited by inadequate distribution of bacteria within the contaminated sites, which reduces the bioavailability of contaminants. Bacterial motility mechanisms, random motility and chemotaxis (directed movement toward or away from chemicals), can potentially help improve these limitations, thereby enhancing in situ bioremediation.;Microfluidic devices (MFDs) were designed and fabricated to simulate two-dimensional groundwater aquifer contamination scenarios. Three major studies were performed. First, the effect of bacterial random motility on contaminant mixing was studied in three different pore structure designs at four different flow rates. In uniform grain size with large pore throats (MFD-I) and non uniform grain size with restricted pore space (MFD-II) devices, the motile bacteria contributed to a nearly 2.3-fold and 3-fold increase in measured apparent transverse dispersivity (alphaapp), respectively. No appreciable change was observed in a uniform grain size device with smaller pore throats (MFD-III).;In the second study, bacterial (Escherichia coil (E. coli) HCB33) chemotactic migration (in response to the chemoattractant DL-aspartic acid) toward a low permeability region of dual-permeability MFD (MFD-IV) was studied. Chemotaxis significantly increased (1.09 to 1.74 times) observed bacterial counts in the low permeability region of the MFD. An accumulation of chemotactic bacteria was observed at the interfaces between the two permeability regions.;The third study focused on evaluating viable bacterial (Pseudomonas putida F1 (P. putida F1)) distribution in chemical gradients and the toxic effects of prolonged exposure to high concentrations of model contaminants (trichloroethylene (TCE) and toluene). Viable bacteria were observed to move away from high concentration of TCE. Toxicity of TCE and toluene to P. putida F1 were best described by exponential and linear viability-decay models, respectively, with viability-decay constants kTCE = 0.025 h-4.95 (r 2 = 0.965) and ktoluene = 0.198 h -1 (r2= 0.972).;This study presents the first attempt to incorporate mixing due to bacterial motility in contaminant transport as well as the first experimental demonstration of bacterial chemotactic migration toward a contaminant source trapped in a low-permeability region. Results obtained will advance current understanding of the processes that may help improve mass transfer limitations of bioremediation strategies for contaminated sites.