Abstract
Groundwater pollution is of major concern as contaminants are continuously released into the subsurface via accidental spills, leaking oil pipelines, agricultural runoff, etc. Often times these contaminants become trapped within regions of the soil matrix that are characterized by low hydraulic conductivity making them difficult to remediate by conventional approaches. In bioremediation strategies, chemotaxis – a phenomenon in which pollutant-degrading bacteria have the ability to detect chemical concentration gradients and move preferentially toward the contaminant source – 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 dissertation work aimed to utilize experimental, modeling and computational techniques to investigate, predict, and better understand the migratory response of chemotactic bacteria to contaminant microniches within a model subsurface environment. Focus was placed on a continuous-flow fully saturated sand-packed column which contained randomly distributed naphthalene sources as the chemoattractant. Two experimental studies were conducted in this work. In the first, a uniform distribution of solid naphthalene crystals created distributed sources of dissolved phase contaminant within the sand column. The second experimental study was focused on non-aqueous phase liquid (NAPL) contaminants which are more typical of oil pollution; naphthalene dissolved in 2,2,4,4,6,8,8-heptamethylnonane (i.e. HMN, a model NAPL) created multiple residual oil ganglia and localized attractant gradients within the column. For both experimental set-ups, equal concentrations of Pseudomonas putida G7 (PpG7), a bacterial strain that exhibits chemotaxis towards naphthalene, and Pseudomonas putida G7 Y1 (PpG7 Y1), a non-chemotactic mutant strain, were simultaneously introduced into the column as a pulse injection. Breakthrough curves obtained from experiments conducted with and without naphthalene were used to quantify the effect of chemotaxis on transport parameters. Experimental results revealed an increased retention of chemotactic bacteria in the contaminated porous media with a 30-45% decrease in cell percent recovery from the sand column, compared to control experiments.
Furthermore, a modified advection-dispersion equation containing an additional advection-like term to describe chemotaxis was used to model bacterial transport in a column system analogous to that of the first experimental study. The chemotactic velocity is a function of the attractant concentration, its concentration gradient, and bacterial chemotaxis properties. Simulation results revealed a distinct motion bias of chemotactic bacteria within the sand column. Additionally, results showed that when chemotaxis was occurring, bacterial transport in the direction of aqueous 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 our experimental observations.
Within the context of bioremediation, chemotaxis may work to enhance bacterial retention in zones of contamination and thereby improve treatment. The results of this dissertation work demonstrated the significance of chemotaxis in augmenting cell retention at the site of contamination. This increased retention of pollutant-degrading cells, which proliferate at an exponential rate upon contaminant consumption, is expected to lead to greater pollutant accessibility and biodegradation. Chemotaxis could potentially allow for more rapid aquifer restoration.
