Modeling tritium and chloride 36 transport through an aggregated oxisol

Breakthrough curves (BTC's) of 3H2O and 36Cl simultaneously displaced through columns of various‐sized aggregates of an Ione Oxisol soil were measured under water‐saturated steady flow conditions. The data were simulated using two conceptual models. In model I, all soil water was assumed to be mobile with a physical equilibrium existing in the system. For model II, soil water was partitioned into mobile and immobile regions. Convective diffusive solute transport was limited to the mobile water region. Transfer of a tracer between the two soil water regions was assumed to occur at a rate proportional to the difference in tracer concentration between the two regions. Sorption Of 3H2O and 36Cl was considered to be an instantaneous linear and reversible process. The two unknown parameters in model I and the four unknown parameters in model II were estimated by fitting model predictions to the experimental data. Model I could only describe BTC's obtained from columns packed with small aggregates (0.5–1 mm) and for displacements run at small fluxes (0.2 cm/h), whereas model II described all the BTC's well. Peclet numbers P in model II, as measured on each separate column, were essentially constant, indicating a linear relationship between the apparent diffusion coefficient D and the mobile pore water velocity vm. The fraction of soil water that is mobile, Φ, and the mass transfer coefficient α were found to be a function of the physical and chemical properties of the porous medium (aggregate size, pore water velocity, and solution concentration).