Previous studies suggest that sorption of colloidal particles onto the air-water interface is an important mechanism for enhanced retention and retardation during transport in unsaturated systems. In this study, bacteriophages ϕX174 and MS-2 and Br- tracer were introduced into sand columns of various water contents as a step function under constant flow rates. The results showed that when a reactive (water washed) sand was used, the retention of both ϕX174 and MS-2 increased significantly at low water saturation levels. However, when an inert (metals and metal oxides removed) sand was used, the effect of water content was minimal, although observable. These results suggest that in the presence of reactive solid surfaces, increased reactions at the solid-water interface rather than at the air-water interface dominates in virus removal and transport under unsaturated conditions. A model that incorporated reactions at both the solid-water and air-water interfaces was developed and successfully applied to the data. ¿ 2001 American Geophysical Union