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In order to predict the transport of reacting solutes through soils, it is necessary to quantify their partitioning between the solution and surface phases. Conventional batch techniques for measuring adsorption isotherms have limitations for many surface reactions, which may be overcome by using miscible‐displacement methods. In this study, an improved miscible‐displacement technique for measuring adsorption isotherms was developed and its results compared with those from a standard batch method for cation exchange in a strongly aggregated, heavy clay soil. The new technique is based on destructive sampling of unsteady, unsaturated flow experiments and an improved method for simultaneously measuring solution‐ and exchangeable‐cation concentrations. Isotherms were obtained by the miscible‐displacement method for total solution concentrations of between 20 and 200 mmolcL−1 from experiments that were terminated after 6 to 24 h. These isotherms were identical with each other and were also not significantly different from those obtained by the batch method for both Na‐Ca and K‐Ca exchange. This confirms that exchange equilibrium was reached in the miscible‐displacement method and that this technique is suitable for measuring exchange isotherms. The results also permit some observations to be made about the rate of approach to equilibrium of ion‐exchange reactions in aggregated soils and the suitability of equilibrium exchange isotherms for application to natural flow regimes.

The ability of HYDRUS-2D (HYDRUS) to simulate water and chemical transport in large, undisturbed cores of a Vertosol and a Podosol soil was investigated. Parameters required by HYDRUS for simulating water and chemical transport, and nitrogen transformation, were obtained from previously published laboratory studies. HYDRUS simulated the measured cumulative drainage and cumulative chloride (Cl–) leaching behaviour very closely for both soil types, and also provided a very good description of coupled nitrogen transformation (conversion of ammonium to nitrate) and leaching (coefficient of model efficiency ∼1). There was little correlation between measured and predicted potassium (K+) leaching from the Podosol, suggesting that the mathematical equations governing the transport of reactive chemicals did not adequately reflect K+ behaviour in this coarse-textured soil. The reason for this discrepancy is unclear but may have been related to the use of sorption parameters obtained from batch rather than miscible displacement techniques, or mechanisms controlling K+ sorption were not well represented by the general non-linear sorption equation used by HYDRUS. The ability of HYDRUS to accurately simulate water and non-reactive chemical transport agrees with previous studies; however, more investigation into its suitability for predicting the movement reactive chemicals in soil is warranted.

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