(354d) Transport of Chemotactic Bacteria in Granular Media Containing a Distribution of Non-Aqueous Phase Liquid Contaminants

Chemotaxis describes the transport of a population of self-propelled bacteria towards or away from regions of increasing chemical concentration. Bacterial cells can be characterized as colloidal particles given their typical size distribution; however the added motility and directional bias that chemotactic cells exhibit often yield interesting particle transport phenomena that can be positively exploited. One particular area of application in which bacterial chemotaxis plays an important role is that of bioremediation of non-aqueous phase liquid (NAPL) contaminants. NAPL contaminants, typically introduced into groundwater systems via oil spills and industrial waste streams, are difficult to remediate due to their low aqueous solubility and the physically heterogeneous nature of subsurface environments. As a result, NAPL contaminants can be trapped in the subsurface for long periods of time while they slowly dissolve into surrounding groundwater. Bioremediation of groundwater occurs via pollutant-degrading bacteria, many of which exhibit chemotaxis. That is, the bacteria can detect the presence of organic contaminants (which serve as a carbon source for them) and preferentially swim towards regions of higher chemical concentration because it is advantageous for them to do so. The movement of chemotactic bacteria towards the contaminant source, irrespective of whether it is trapped in the interstices of the soil matrix, leads to increased pollutant accessibility and biodegradation.

This experimental study employed a laboratory-scale continuous flow sand column to better understand chemotactic transport in response to a uniform distribution of entrapped NAPL. The presence of NAPL within the column created localized concentration gradients. 2,2,4,4,6,8,8-heptamethylnonane (HMN) containing dissolved naphthalene was used as a model NAPL. An equal concentration of Pseudomonas putida G7, which is chemotactic to naphthalene, and Pseudomonas putida G7 Y1, a non-chemotactic mutant strain, was introduced simultaneously into the sand column and effluent bacterial concentrations were measured with time. To aid differentiation between bacterial strains, each cell type was stained with a fluorescent label prior to injection into the column. Experiments with sand only and sand/HMN-NAPL with no dissolved naphthalene were also conducted to serve as controls. We tested the hypothesis that due to chemotaxis, an increase in the retention of P. putida G7 will be observed within the sand column containing the HMN-naphthalene-NAPL (as compared to the control cases). This increase in chemotactic bacterial retention within the polluted porous media is expected to lead to an increase in contaminant biodegradation. Moment analysis parameters, such as percent recovery and mean travel time, were obtained from species breakthrough curves. Additionally, a one-dimensional advection-dispersion equation was fitted to species breakthrough curves and the influence of chemotaxis on apparent model parameters was analyzed. Initial experiments in which solid naphthalene crystals were evenly distributed throughout the sand column resulted in a 43% decrease in percent recovery of P. putida G7 from the column, compared to the control case with no naphthalene present, thus supporting our hypothesis. In contrast, transport of the non-chemotactic mutant strain was not influenced by the presence of naphthalene. The results of this experimental study are important for understanding how bacterial chemotaxis may influence the accessibility of contaminants present within NAPL contaminated aquifers.