Validation of stochastic flow and transport models for unsaturated soils: field study and preliminary results

The US Nuclear Regulatory Commission (NRC) is funding a field study to collect a data base specially designed to validate a stochastic unsaturated flow and transport theory. The stochastic approach is an efficient methodology for characterizing and assessing spatial variability of the hydraulic properties of soils in the unsaturated zone. At the New Mexico State University (NMSU) field experimental station, an observational trench 4.8 m x 26.5 m and 6 m deep has been excavated. Instrumentation located at various points from the trench face monitored the advancing wetting front and coincident tracers resulting from constant rate infiltration from an irrigation drip system adjacent to the trench. The measured hydraulic properties were analyzed, and directional semivariograms were developed to investigate the statistical characteristics of the hydraulic conductivity data set. The results indicated that the principal direction of anisotropy corresponds to the horizontal and vertical directions of flow. Measured soil moisture contents and tensions due to the advancing wetting showed qualitative agreement between the stochastic simulations and the observed field data, particularly for the horizontal tension distributions. Full validation has yet to be achieved. Further field experiments are being designed and will be conducted to examine transient flow and transport conditions