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Abstract

Dispersive processes that diminish contaminant concentrations originating from an unsaturated source zone on the way to groundwater, were investigated. Simulations using the numerical model MIN3P were performed for a non-volatile, non-degrading contaminant from a persistent source after reaching a steady state. A 2-D vertical cross-section was used as geometry. Two different types of sandy sediment were simulated: a rather coarse sand with a capillary rise of 90% water saturation to 4 cm above the water table, and a silty sand showing a capillary fringe of 30 cm height (90% water saturation). Major dispersive fluxes were found to take place below the water table, thus dilution and concentration reduction at and above the water table is not very significant.

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... In this approach, the assumption was made that the subsurface of the Hanford Site Z-9 Trench is sufficiently characterized such that a conceptual model as depicted in Fig. 1 could be derived with a reasonable level of confidence, including the hydraulic properties of the major porous media. In this context, it is recognized that the capillary fringe configuration may have a considerable effect on mass flux into the water table (Klenk and Grathwohl, 2002;Mccarthy and Johnson, 1993), including proper discretization of this zone in numerical models (Maier et al., 2008a(Maier et al., , 2008bOostrom et al., 2010). However, given that capillary fringe effects on contaminant transport are not yet fully understood, variations in capillary fringe configurations were not included in the current analysis approach. ...
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