Field Solute Transport across a Soil Horizon Boundary

Abstract

AbstractModels describing solute transport in a field soil need to account for transport through soil horizons and across horizon boundaries. This study investigated the effects of soil horizons on the transport of a conserving tracer and the validity of treating soil horizons as independent (uncorrelated) layers. The occurrence or absence of correlated solute travel times with depth was tested by comparing predicted solute travel time variances with variances measured from solute breakthrough curves. Steady‐state transport experiments were conducted on a Fox sand (fine‐loamy over sandy or sandy‐skeletal, mixed, mesic Typic Hapludalf) in Ontario, Canada. The transport of Cl through the Ap and the Ap + B horizons was measured at constant surface flux densities of 3.5 and 5.5 cm h−1. The upper 20 cm of soil was then excavated and the transport of Cl through the B horizon was measured independently at a flux density of 5.5 cm h−1. The horizons exhibited differences in solute velocity, spatial variations in velocity and in equilibrium water contents. At the transect scale, successful predictions of travel time variance were made at both flux densities when it was assumed that solute travel times were correlated with depth. At the local scale, the successful prediction of variance assuming correlated travel times occurred only at the lower flux density. At this site a correlated model is appropriate for describing solute transport at the transect scale. Independent estimates of transport properties from each horizon is not sufficient information to predict field transport. The nature of the boundary between horizons must be evaluated.