EVALUATION OF METHODS FOR DETERMINING SOIL-WATER RETENTIVITY AND UNSATURATED HYDRAULIC CONDUCTIVITY

The transport of dissolved contaminants through the vadose zone is a major source of soil and groundwater contamination. Soil hydraulic properties must be determined to accurately describe water and contaminant transport and potential environmental impacts. Comparisons were made of three field and three laboratory methods for estimating soil-water retention, theta(psi), and unsaturated hydraulic conductivity functions, K(theta). Instrumentation was installed in 36 field plots, and two redistribution cycles were conducted. Field data obtained from each cycle were utilized in three outflow-based field methods: (i) instantaneous profile method, (ii) Libardi's method, and (iii) a nonlinear least squares approach. Undisturbed soil cores were extracted from 24 field plots at six depths and used in laboratory tests. Techniques consisted of (i) a multi-step outflow approach coupled with (a) ''inverse methodology'' for transient conditions and (b) a least-squares approach for equilibrium conditions and (ii) a particle size distribution model. Parametric models were coupled with the modeling efforts. The results obtained by the in situ instantaneous profile method for both soil hydraulic functions were considered to hold the greatest validity. However, the multi-step outflow methods produced feasible theta(psi) curves, and the inverse methodology was time efficient. Libardi's method for determining K(theta) relationships was accurate at deep profile depths but failed at shallow ones.