The investigation of the transport and retention of bacteria in porous media has a great practical importance in environmental applications, such as protection of the the surface and groundwater supplies from contamination, risk assessment from microorganisms in groundwater, and soil bioremediation. While bacteria transport in homogeneous porous media has extensively been studied, one drawback associated with the current body of literature is the limited number of studies examining the pore size distribution of the porous media and its effect on bacteria transport and retention. Recent publications have demonstrated that pore size can strongly affect the transport and retention of colloidal particles in heterogeneous porous media constituted by mixing of sands with different grain size or macropore insertion on the homogenous sand. Conversely to inert colloids, differences in bacteria cell properties like cell geometry, hydrophobicity and motility may greatly affect the transport in porous media. Though, a number of studies performed in homogenous sandy media have demonstrated their role in transport and deposition, another limitation of the current literature is the lack of studies examining the couple effect of cell properties like motility and pore size distribution of the porous media.
Laboratory tracer and bacteria transport experiments were performed in three porous media with distinct pore size distribution in order to investigate and quantify water and bacteria transport process under steady state flow conditions. A conservative solute was used as water tracer, and this work was restricted to the study of physical non-equilibrium mechanisms. A gram negative motile Escherichia coli, a commonly used indicator organism, and gram negative non-motile Klebsiella sp., detected in environmental sources such as surface water, soil and plants, were selected for the transport experiments. HYDRUS-1D code was used to simulate tracer and bacteria elution curves and to identify transport parameters by inverse modelling. To account for preferential flow and bacteria transport in such heterogeneous porous media, a two-region model known as the MIM (M-mobile/ IM-immobile water) transport model was used to model tracer and bacteria experimental elution. Results obtained through both laboratory experiments and numerical simulations outlined non-uniform flow pathways, which were dependent on both grain/pore size as well as pore size distribution of the porous media. Bacteria transport pathways were different from the tracer aqueous transport, due to size exclusion of bacteria from smaller pore spaces and bacteria motility. Bacteria retention was greatly influenced by pore network geometry and cell properties. The differences between motile and non-motile bacteria strains, made difficult to reach global conclusive results about cell transport and deposition, and highlighted the role of the coupled influence of cell properties, pore size distribution and hydrodynamics of the porous media.
 

bacteria transport, porous media, pore size distribution, cell properties