VARIABILITY OF AN UNSATURATED SAND UNIT UNDERLYING A RADIOACTIVE-WASTE TRENCH

Properties of soils vary considerably within any field. This study was conducted to investigate the variability in properties of an unsaturated sand unit that lies at a depth of 13 m below land surface. Four-hundred-forty soil core samples, obtained from a 1.75 by 18-m horizontal plane within a sand unit underlying a waste trench, were used to describe the variability of moisture content, tritium concentration, and several physical properties. A simple model based on unit-gradient theory was used to calculate fluxes of water and tritium through the study plane. The effects of including spatial variability of properties in model calculations were examined. The sand was uniform in terms of median particle size, bulk density, and porosity, but several variables, including saturated hydraulic conductivity (K(s)) and tritium concentration, showed considerable variability. In general, correlation coefficients calculated between variables were low. Spatial correlation structures were identified for moisture content, tritium concentration, and a few particle-size variables, but were absent for K(s) and most other variables. Variances of K(s) and field-measured volumetric moisture content (theta) were lower than published values for soils. Model-calculated fluxes of water and tritium that accounted for spatial variability of properties were considerably higher than those that did not account for the variability. Model results were more sensitive to changes in theta and residual moisture content (each parameters of the relative hydraulic conductivity function) than to changes in K(s).