Transport of chemotactic bacteria in porous media with residual sources of oil-phase chemical pollutants

The removal of hydrocarbon contaminants from groundwater aquifers is particularly challenging due to their low solubility and the entrapment of residual oil-phase ganglia in layers with low permeability. An alternative to conventional clean-up processes is in situ bioremediation, which relies on microorganisms to biodegrade the hydrocarbons. Critical to the success of in situ bioremediation is getting the microorganisms in close proximity to residual hydrocarbon sources. Thus, limited access and poor mixing in zones of low hydraulic conductivity limits bioremediation. To overcome this limitation some microorganisms are self-propelled and have sensors that allow them to detect micromolar levels of hydrocarbons as they move about in their surroundings. This ability of microbial populations to sense and swim toward chemicals is termed chemotaxis; it provides a fitness advantage as many hydrocarbons are a source of food and are beneficial to the growth and survival of the population.

Bacteria that are naturally present in aquifers are carried along by groundwater flow through highly permeable zones in the subsurface environment. Oil-phase hydrocarbons trapped within the interstices of the soil matrix slowly release their components into the surrounding aqueous phase. This dissolution process creates chemical gradients at the pore scale, which drives nearby populations of chemotactic bacteria toward the increasing concentrations. Because motile bacteria can swim independently of the groundwater flow, they can reach areas that may otherwise be inaccessible. By increasing the number of bacteria near the hydrocarbon sources via chemotaxis and motility, the rate of biodegradation increases and enhances the overall success of in situ bioremediation.

There are many processes that affect the access of bacteria to hydrocarbons in groundwater aquifers: solubility, advection, dispersion, sorption, and others. Assessing if chemotaxis plays a significant role relative to these other processes will aid in evaluating various treatment options. 

Figure 1 (A-G) illustrates a multi-scale approach that combines experiments and computer simulation to connect chemotaxis of individual bacteria toward pore-scale naphthalene gradients with column-scale transport as revealed by bacterial breakthrough curves. A flow-through sand-packed column (C) contained discrete sources of naphthalene in oil-phase ganglia (B) that were distributed randomly throughout the column to represent a contaminated aquifer (Adadevoh et al., 2018). A mixture of chemotactic and non-chemotactic bacteria (A) was introduced under flow to examine differences in the breakthrough curves (E) that reflected interactions between chemotactic bacteria and residual naphthalene contamination. A greater portion of chemotactic bacteria were retained within the contaminated column relative to the non-chemotactic strain (Adadevoh et al., 2018). This result suggested that chemotaxis had a measurable effect on bacterial transport. Two approaches were undertaken to examine the underlying mechanism: (1) modeling chemotaxis and visualizing the impact through computer simulation of the column (e.g. Adadevoh et al., 2017) and (2) conducting experiments in microfluidic devices that enabled direct visualization of bacterial distributions in the pore space near oil-phase contaminants (e.g. Wang et al., 2016). Computer simulations (D) revealed very different distributions of chemotactic (ii) and non-chemotactic bacteria (i) within the sand column underflow as chemotactic bacteria accumulated around oil-phase ganglia (B) while non-chemotactic bacteria passed by. The distribution of non-chemotactic bacteria appears Gaussian (Di) following dispersion of an initial pulse input while the chemotactic population is patchy and more broadly distributed throughout the column (Dii). Direct observation by widefield microscopy in 2-D porous micromodels (F) also shows accumulation of chemotactic bacteria near oil ganglia (G) and is consistent with simulation predictions in the 3-D column. 

Chemotactic microorganisms have unique features (chemical sensors and motility) that can be exploited to overcome limited bioavailability of hydrocarbon pollutants in groundwater aquifers. Quantitative evaluation of chemotaxis across multiple scales is needed to determine the conditions under which it provides an advantage to bioremediation schemes.