Predicting Field-Scale Solute Transport Using In Situ Measurements of Soil Hydraulic Properties

Predicting unsaturated solute transport using measured hydraulic parameters has been difficult due to the inherent variability of soil properties, and the difficulty in obtaining accurate estimates of hydraulic properties in situ. The objective of this study was to determine if in situ measurement of soil hydraulic conductivity, the α soil parameter, and the water content (θ) vs. pressure head (h) relationship could be used to predict field-scale solute transport. A series of steady-state solute transport experiments were conducted on a Fox sand (fine-loamy over sandy or sandy-skeletal, mixed, mesic Typic Hapludalf) soil in Ontario, Canada. The transport of Cl⁻ under steady-state water flux was monitored in three separate experiments using solution samplers. Steady-state water flux densities applied at the soil surface were 9.72 × 10⁻⁶, 1.53 × 10⁻⁵, and 8.68 × 10⁻⁸ m s⁻¹, respectively, for the three sites. After completion of the transport experiments at Site II, measurements of soil hydraulic conductivity and the α parameter were made using the Guelph pressure infiltrometer (GPI) beside each location and depth where solute breakthrough curves (BTCs) were measured, as well as at the soil surface. Undisturbed soil cores were taken at each location where GPI measurements were made for estimating the parameters in the θ(h) using van Genuchten's equations. The GPI- and core-measured hydraulic parameters obtained at Site II were used to predict the field-scale solute travel time probability density function (PDF) at the same site, and at Sites I and III using a stochastic-convective model. Observed solute travel time PDFs were predicted quite well at high surface water fluxes, which were close to the field saturated hydraulic conductivity, Kfₛ by both the GPI and core methods. Both methods underpredicted the variability of the observed travel time PDF.