Predicting contaminant migration within the vadose zone, for performance or risk assessment, requires estimates of unsaturated hydraulic conductivity for field soils. Hydraulic conductivities, K, were experimentally determined as a function of volumetric moisture content, &thgr;, for Hanford sediments. The steady state head control method and an ultracentrifuge method were used to measure K(&thgr;) in the laboratory for 22 soil samples.

The van Genuchten model was used to fit mathematical functions to the laboratory-measured moisture retention data. Unsaturated conductivities estimated by the van Genuchten-Mualem predictive model, using the fitted moisture retention curve and measured saturated hydraulic conductivity, Ks, were compared to those obtained by a scaled-predictive method that uses a single K(&thgr;) measurement as a match point near the dry regime. In general, the measured K values and those predicted from van Genuchten-Mualem relationships showed considerable disagreement. This suggests that the use of laboratory-measured Ks results in an inadequate characterization of K(&thgr;) for the desired range of moisture content. Deviations between the measured and predicted K were particularly severe at relatively low moisture contents; for some samples, there were differences in excess of 2 orders of magnitude at low &thgr;. However, use of the same moisture retention curve-fitting parameters and a single steady state head control-based K(&thgr;) measurement near the dry regime resulted in considerable improvement. In fact, for the coarse-textured soils considered in this study, results indicate that a K(&thgr;) measurement near the dry regime must be used to obtain reliable estimates of unsaturated K at low &thgr;. The study provided important insight on application of two different experimental techniques of measuring unsaturated conductivities. ¿ American Geophysical Union 1995 ¿ American Geophysical Union 1995