Virus retention and transport in chemically heterogeneous porous media under saturated and unsaturated flow conditions.

Retention and transport of colloids and microorganisms are complex processes, especially in the vadose zone due to the more complicated water flow regime and additional interfacial reactions involved. In this study, we examined the retention and transport behavior of two bacteriophages, MS-2 and phiX174, in homogeneous and chemically heterogeneous media under variably saturated conditions. Column experiments with glass beads (treated to have either hydrophilic or hydrophobic surface properties) were conducted using a phosphate-buffered saline solution at different pore water ionic strengths ranging from 0.025 to 0.163 M. In columns packed with 100% hydrophilic glass beads, retention of the viruses increased with decreasing water content and increasing ionic strength, a result similar to those reported in the literature. However, greater retention of both MS-2 and phiX174 was observed in saturated columns than in unsaturated columns packed with a 1:1 mixture of hydrophilic and hydrophobic glass beads, especially at high ionic strengths. This result contradicts the common belief that viruses (and colloids in general) are subject to greater removal in unsaturated media. Our study suggests that while the mechanisms controlling colloid interfacial interactions (i.e., attachment on solid-water and air-water interfaces and film straining) on the pore scale are relevant, nonuniform wetting conditions due to heterogeneous grain surface hydrophobicity can strongly influence water flow and phase interconnection. Under these conditions, hydrodynamic effects on the mesopore scale will dominate pore-scale interfacial reactions in controlling the extent of colloid retention and movement in unsaturated media.