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Predicting the Solute Breakthrough Curve From Soil Hydraulic Properties

Soil hydrodynamic dispersivity and therefore inert solute transport are largely determined by soil hydraulic properties. In short travel times or long distances, diffusion is not a significant process. Thus, in these cases, only the convection of fluid in different pores determines the breakthrough curve (BTC) shape. Because convection of fluid in different pores is described by pore fluid velocity distribution and consequently pore size distribution, it is possible to connect BTC to pore size distribution or pore fluid velocity distribution. The objectives of this study were to develop a model relating soil moisture characteristic (SMC) to BTC and to evaluate model predictions through comparisons with measured values. In this article, an approach that allowed predicting inert solute transport from knowledge of the SMC, parameterized by means of the closed-form van Genuchten-Mualem model, is presented. It was assumed that only part of the total soil porosity contributes to the transport process. The immobile water content and consequently the actual relative pore volume were predicted based on the SMC. A heaviside-type miscible displacement was performed, imposing a saturated unit gradient flow. The derived model was then tested on eight different disturbed soils. Results showed that the modeled and measured BTCs were in good agreement, especially during the initial stages of the breakthrough. It was further shown that only a small part of the soil pores (∼4%-13%) contributes significantly to the convective solute transport. Results indicated that when the relative pore volume is increased, the accuracy of the model decreases. Prediction errors were attributed to estimating immobile water content, the assumption of the tortuosity factor, and neglecting of diffusion, which may become significant as the travel time increases.Abbreviations: BTC: breakthrough curve, SMC: soil moisture characteristics curve, HCC: hydraulic conductivity curve, SHC: soil hydraulic characteristics, VGM: van Genuchten-Mualem model, CDE: convection dispersion equation