Abstract. 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 qbX174 and MS-2 and Brtracer 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 qbX174 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.

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Viruses from contaminant sources can be transported through porous media to drinking water wells. The objective of this study was to investigate inactivation and sorption of viruses during saturated and unsaturated transport in different soils. Bacteriophages phiX174 and MS-2, and Br- tracer in a phosphate-buffered saline solution were introduced into saturated and unsaturated soil columns as a step function under constant flow rate and hydraulic conditions. Results showed that significantly greater virus removal occurred in the unsaturated columns than in the saturated columns in the two soils containing high metal oxides content. However, the increase in virus retention under unsaturated conditions was not significant in two other soils having high phosphorus and calcium contents and high pH, and in another soil with high organic matter content. The results imply that the extent of water content effect on inactivation and sorption of viruses can range from significant to minimal depending on the properties of the transport medium. We found that the presence of in situ metal oxides was a significant factor responsible for virus sorption and inactivation. Therefore, soils with high metal oxides content may have the potential to be used as hydrological barriers in preventing microbial contamination in the subsurface environments. We also found that the water content effect on virus removal and inactivation strongly depended on solid properties of the testing medium.

To accurately examine the mechanisms that control virus retention and transport in porous media, it is essential to perform experiments under environmentally relevant conditions. Laboratory studies of virus transport are generally conducted using buffered solutions. However, there are no standards for the concentration and composition of the solution, making comparisons between different studies difficult. Our objective was to systematically investigate the effects of the ionic strength and composition of buffer solutions on adsorption of viruses during transport through saturated sand columns. Two phosphate buffered saline solutions (PBS and PBS2) and one artificial ground water (AGW) were used and experiments were conducted under saturated, steady state flow conditions. Bromide tracer and bacteriophages φX174 and MS‐2 were introduced into saturated sand columns as step inputs and their concentrations in the outflow samples were determined. Change in ionic strength did not affect the behavior of φX174, whereas MS‐2 was largely removed in the high ionic strength PBS buffer (0.16 M) but moved through the columns in the low ionic strength PBS2 and AGW buffers (0.002 2M). An additional experiment conducted under an ionic strength gradient (from 0.002 to 0.2 M) using PBS indicated that the outflow concentration of MS‐2 decreased slowly from an ionic strength of ∼0.03 M, reached a minimum at an apparent critical value of ∼0.1 M, and then increased gradually. This study also suggests that metals or metal oxides on sand surfaces can cause significant virus removal under high ionic strength conditions.

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