Evaluation of van Genuchten-Mualem relationships to estimate unsaturated hydraulic conductivity at low water contents

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, for Hanford sediments. The steady state head control method and an ultracentrifuge method were used to measure hydraulic conductivity as a function of moisture content 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 hydraulic conductivity as a function of moisture content 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 hydraulic conductivity as a function of moisture content 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 volumetric moisture content. However, use of the same moisture retention curve-fitting parameters and a single steady state head control-based hydraulic conductivity as a function of moisture content measurement near the dry regime resulted in considerable improvement. In fact, for the coarse-textured soils considered in this study, results indicate that a hydraulic conductivity as a function of moisture content measurement near the dry regime must be used to obtain reliable estimates of unsaturated K at low volumetric moisture content. The study provided important insight on application of two different experimental techniques of measuring unsaturated conductivities.