  Understanding flow and transport of bacteria in porous media is crucial to
technologies such as bioremediation, biomineralization or enhanced oil
recovery. While physicochemical bacteria filtration is well-documented, recent
studies showed that bacterial motility plays a key role in the transport
process. Flow and transport experiments performed in microfluidic chips
containing randomly placed obstacles confirmed that the distributions of
non-motile particles stays compact, whereas for the motile strains, the
distributions are characterized by both significant retention as well as fast
downstream motion. For motile bacteria, the detailed microscopic study of
individual bacteria trajectories reveals two salient features: (i) the
emergence of an active retention process triggered by motility, (ii)
enhancement of dispersion due to the exchange between fast flow channels and
low flow regions in the vicinity of the solid grains. We propose a physical
model based on a continuous time random walk approach. This approach accounts
for bacteria dispersion via variable pore-scale flow velocities through a
Markov model for equidistant particle speeds. Motility of bacteria is modeled
by a two-rate trapping process that accounts for the motion towards and active
trapping at the obstacles. This approach captures the forward tails observed
for the distribution of bacteria displacements, and quantifies an enhanced
hydrodynamic dispersion effect that originates in the interaction between flow
at the pore-scale and bacterial motility. The model reproduces the experimental
observations, and predicts bacteria dispersion and transport at the macroscale.
