From local hydraulic properties to effective transport in soil

The inability to predict flow and transport behaviour based on a priori inf ormation demonstrates the lack of knowledge we currently have concerning tr ansport-relevant properties or processes, or both. We present an approach i n which the behaviour of effective solute transport at the scale of a soil column (100 mm) is predicted by taking into account the spatial structure o f the hydraulic properties at the local scale (1 mm). The local absorption coefficients obtained from X-ray tomography, which are linearly related to bulk density, are used as local proxy for hydraulic properties. As a first approximation, two density classes were distinguished, and the three-dimens ional structure of the hydraulic properties was implemented in a model of f low and transport. The local hydraulic properties were obtained from a netw ork model, except for the absolute value of the hydraulic conductivity func tion which was measured. Model simulations were compared with a measured br eakthrough curve determined on the same soil sample. The two agreed well, a lthough the local hydraulic properties and parameter structure were determi ned independently with respect to a breakthrough experiment. Predictions of solute transport at the column scale were sensitive to the difference in s aturated hydraulic conductivities of both materials, but not to the local d ispersivities. The simulations demonstrate that (i) assuming validity of th e Richards equation and the convection-dispersion equation on the local sca le leads to a good description of the effective flow and transport behaviou r at the column scale without making any assumptions about the governing pr ocesses at that scale; (ii) the dispersion parameters, which are notoriousl y difficult to determine, need not be determined since their effect is incl uded explicitly; and (iii) local absorption coefficients can be used as a l ocal proxy for the parameter field of the hydraulic properties.