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

The transport of dissolved contaminants through the vadose zone is a m ajor source of soil and groundwater contamination. Soil hydraulic prop erties 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 function s, K(theta). Instrumentation was installed in 36 field plots, and two redistribution cycles were conducted. Field data obtained from each cy cle were utilized in three outflow-based field methods: (i) instantane ous 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 consiste d of (i) a multi-step outflow approach coupled with (a) ''inverse meth odology'' for transient conditions and (b) a least-squares approach fo r 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 hyd raulic functions were considered to hold the greatest validity. Howeve r, 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.