Field and Undisturbed‐Column Measurements for Predicting Transport in Unsaturated Layered Soil

Abstract

AbstractTransport properties vary considerably over small distances in most soils. The stochastic streamtube model offers one approach to incorporating heterogeneity into transport predictions. This study tested the ability of the streamtube concept to predict transport in heterogeneous fields using measurements from undisturbed columns. Fifty undisturbed columns (0.15‐m diam. by 1.5 m deep) were taken every 0.4 m from a 20‐m‐long transect in a loamy sand soil with variable horizon thickness. Each core was instrumented at 0.1‐m intervals with time domain reflectometry probes to measure resident fluid concentrations of a conservative (Cl−) tracer under steady flow conditions. Large‐scale concentration curves of Cl− from solution samplers and coring were obtained from field experiments conducted on the same soil under similar boundary conditions. Differences were observed in the solute spread and mass recovery, but not in the centers of mass. Horizontal scale dependence of transport was observed in the field but not in the columns. This suggests that a higher dimensionality of transport, probably along the horizon interfaces, may be responsible for the observed scale dependence in the field. Although the stochastic streamtube model gave good predictions of the center of mass, it does not appear to be a realistic physical analogue for describing solute dispersion in soils with spatially variable layer thickness.