Modeling bacterial transport and fate: Insight into the cascading consequences of soil water repellency and contrasting hydraulic conditions

The mechanisms governing bacteria transport and fate rely on their hydrophobicity and the wettability of porous media across a wide range of soil moisture conditions, extending from extreme dryness to highly saturated states. However, it largely remains unknown how transport, retention, and release mechanisms change in natural soil systems in such conditions. We thus optimized our previously published unique transport data for hydrophilic Escherichia coli (E. coli) and hydrophobic Rhodococcus erythropolis (R. erythropolis) bacteria, and bromide (Br^-) in two distinct wettable and water-repellent soils at column scale. The soils were initially dry, followed by injecting influents in two pulses followed by a flushing step under saturated flow conditions for six pore volumes. We conducted simulations for each pulse separately and simultaneously for soils. There were differences in hydraulic properties of the soils due to their contrasting wetting characteristic in separate and simultaneously modeling of each pulse affecting Br^- and bacteria transport fate. Bacteria attachment was the dominant retention mechanism in both soils in these conditions. Notably, the 82.4 min^-1 attachment rate in wettable soil was almost 10× greater than in the water-repellent soil and it governed optimization of bacteria die-off. Physicochemical detachment and physical release unraveled the effect of bacteria size and hydrophobicity interacting with soil wettability. The smaller and hydrophobic R. erythropolis detached more easily while hydrophilic E. coli released; the rates were enhanced by soil water repellency. Further research is needed to reveal the effects of surface wettability properties on bacteria survival especially at the nanoscale.