Hydraulic Properties of Unsaturated Soil: The Effect of Microbial Activity.

Classic treatment of soil-water flow is described by the unsaturated version of Darcy's law and Richards' equation, assuming time invariant hydraulic properties. However, Microbial activity takes place in all soils and in specific cases, such as wastewater irrigation, soil-aquifer treatment facilities and bioremediation, can be significant. Biological activity constantly alters the pore-scale structure, leading to macro-scale alteration of the hydraulic properties. In this work we explore the dynamic alteration of soil hydraulic properties by a combination of column experiments, direct and indirect measurement of hydraulic properties, and pore-network modeling. We experimentally demonstrate how biological activity clogs an unsaturated soil column and reduces its hydraulic conductivity, while a similar column where biological activity is limited does not clog. Further, we demonstrate that the clogging is preferential to the nutrient input. Next, we develop a pore-network model that uses triangular shape channels. This allows a dual occupancy (water-air) of each channel and high connectivity. The model considers the biofilm as a separate phase that interacts with the surrounding liquid phase.