Virus transport in porous media is affected by the water flow regime. During transient, variably saturated flow, fluctuating flow regimes can enhance virus detachment from both solid–water interfaces (SWIs) and air–water interfaces (AWIs). The objective of this study was to simulate the influence of drainage and imbibition events on the remobilization of attached viruses. Three different modeling approaches were examined. In the first approach, all attachment and detachment coefficients were assumed to be constant, but the values of the detachment coefficients were increased drastically for the duration of transient, unsaturated flow. The second and third modeling approaches involved extensions of the model of Cheng and Saiers, who assumed enhanced detachment of viruses to be proportional to the time rate of change in the water content. Their model was extended to include separate terms for virus attachment–detachment on SWIs and AWIs. In our second approach, we assumed kinetic sorption onto the AWI, with the desorption rate being described as a function of temporal changes in the air content. This approach did not explicitly account for the specific air–water interfacial area. Thus, in our third approach we explicitly included the presence and variation of air–water interfaces and assumed AWI attachment–detachment to be an equilibrium sorption process. The available air–water interfacial area was assumed to be a function of fluid saturation. The models were used to simulate a series of saturated–unsaturated virus transport experiments reported in the literature for conditions of both drainage and imbibition. The most promising results were obtained with the third approach, which explicitly accounts for adsorption to air–water interfaces and assumes equilibrium sorption on the available air–water interfacial area.

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