FIELD AND UNDISTURBED-COLUMN MEASUREMENTS FOR PREDICTING TRANSPORT INUNSATURATED LAYERED SOIL

Transport properties vary considerably over small distances in most so ils. The stochastic streamtube model offers one approach to incorporat ing heterogeneity into transport predictions. This study tested the ab ility 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 reflect ometry probes to measure resident fluid concentrations of a conservati ve (Cl-) tracer under steady flow conditions. Large-scale concentratio n curves of Cl- from solution samplers and coring were obtained from h eld experiments conducted on the same soil under similar boundary cond itions. Differences were observed in the solute spread and mass recove ry, but not in the centers of mass. Horizontal scale dependence of tra nsport 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 t he field. Although the stochastic streamtube model gave good predictio ns of the center of mass, it does not appear to be a realistic physica l analogue for describing solute dispersion in soils with spatially va riable layer thickness.