Russo, D., Department of Environmental Physics and Irrigation, Volcani Center, Institute of Soils, Water and Environmental Sciences, Bet Dagan 50250, Israel

The transport of a conservative tracer (bromide) in a three-dimensional, heterogeneous combined vadose zone-groundwater flow system was analyzed through a series of detailed numerical simulations. The scope of the present study was to analyze the effect of both the soil type and the pulse application date on solute movement and spreading in the combined flow system subject to time-dependent, external forcing conditions, F(t) (characterized by a time period, τp), imposed on a flat soil surface. Of particular interest were the suitability of the time-invariance assumption of the solute travel time distribution and the related issue of the capability of an equivalent, steady state vadose zone flow model to describe solute transport in a realistic flow system. Considering flow systems in which the water table is located at sufficiently large distance from the flat soil surface, the main results of this study suggest that the velocity associated with the wetting front position, which may be considered as an "effective" velocity, is soil- and calendar date-dependent. Consequently, characteristics of the transport (i.e., solute displacement and spreading, first- and peak-arrival times) are soil- and pulse release date-dependent. The soil-dependent solute travel time PDF at a CP located in the vicinity of the water table, however, may be considered as essentially independent of the pulse release date, particularly in the fine-textured (clay) soil associated with mean travel time, τ0 that substantially exceeds τp. Furthermore, for τ0 > τp, the equivalent steady state definition of the flow problem may be quite effective in describing the solute travel time PDF of the actual transport process occurring under nonmonotonous, transient flow conditions.© 2011 by the American Geophysical Union.