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The simultaneous retention of both cations and anions due to ion-pair adsorption (IPA) has been identified in several soils. The mechanism controlling IPA is not yet understood, however, and the mathematical description of this phenomenon is only incipient. In this study, a parameterization analysis was performed using a recently proposed adsorption model that considers the occurrence of IPA. The model, which has two alternative approaches to describe IPA, has been shown in a previous study to describe experimental data reasonably well. That analysis was incomplete, however, as the parameter uncertainties were not estimated. In this study, with the parameterization procedure using the maximum-likelihood method, a robust estimation of the model parameters, including the analysis of their uncertainties, was developed. The model was fitted to a data set obtained in batch experiments using the variable-charge Egmont soil, from New Zealand, and a comprehensive combination of [SO.sub.4] and Ca concentrations. This procedure allowed the analysis of some of the underlying model characteristics and also some inferences about the IPA phenomenon. Overall, it was shown that the proposed adsorption model is a useful tool for the modeling of solutes in soils where IPA occurs. Abbreviations: IPA, ion-pair adsorption; PGA, particle governed approach; SGA, solution governed approach.

Ion retention in variable-charge soils can be enhanced by the presence of certain ions with opposite charge, thereby influencing the movement of these ions through the soil profile. Studies examining these interactions are still incipient, however, especially regarding its modeling. We present results from batch and miscible displacement experiments describing [Ca.sup.2+] and S[O.sub.4.sup.2-] movement in a variable-charge soil from New Zealand. Evidence was found for ion-pair adsorption (IPA) of both [Ca.sup.2+]and S[O.sub.4.sup.2-] The results were modeled using the convection-dispersion equation (CDE), coupled with two different mathematical approaches proposed to account for IPA. The first approach related IPA to the single soil adsorption capacity, which is governed by particle-surface phenomena. For the second approach, IPA was related solely to the soil solution concentration. Both these approaches described the adsorption data from the batch experiment reasonably well, as well as the breakthrough curves from the miscible displacement experiments. The first approach showed better overall agreement. Significant differences were found, however, when the adsorption parameters were identified by fitting models to data from either batch or miscible displacement experiments. Although more studies are needed to better understand IPA, our results showed that the extent of IPA can be large and it should not be ignored when predicting S[O.sub.4.sup.2-] and [Ca.sup.2+] movement in variable-charge soils. Abbreviations: BTC, breakthrough curve; CDE, convection dispersion equation; IPA, ion-pair adsorption; PGA, particle governed approach; SGA, solution governed approach.

To investigate a rapid, nondestructive way of characterizing solute transport properties, time domain reflectometry (TDR) and disk permeametry have been used in combination. Calibration measurements had previously related TDR measurements to both the volumetric water content and the pore water concentration of [Cl.sup.-]. Laboratory measurements from a horizontal TDR probe were used to estimate transport parameters in a soil column by applying a one-dimensional numerical model in an inverse sense. A vertical TDR probe was used to provide independent verification of these parameters. A repacked column of Ramiha silt loam (an Andic Dystrochrept) was used under unsaturated, transient flow conditions. The disk permeameter, set to a pressure head of - 50 mm and containing a solution of 0.032 M KCl, was placed straight onto the repacked soil column, which had an initial water content of 0.32 [m.sup.3] [m.sup.-3]. The soil wet to 0.60 [m.sup.3] [m.sup.-3] However, in the columns only an envelope of [Cl.sup.-] concentration could be obtained, due to exchange between the initially resident [Ca.sup.2+] and the invading [K.sup.+]. This illustrates why cation exchange needs to be considered when TDR is used to infer solute movement. From the numerical simulations, values for the solute dispersivity and the retardation were found to be 2.3 mm and 1.2, respectively. The retardation is shown to be due to the anion-exchange capacity varying with the concentration of the invading soil solution.

Time domain reflectometry (TDR) possesses potential for determining solute-transport parameters, such as dispersion coefficients and retardation factors for reactive solutes. We developed a simple method based on peak-to-peak measurements of water and solute velocities through the soil using TDR. The method was tested by carrying out unsaturated leaching experiments in the laboratory on two soil columns packed with a South Pacific soil from Mare, which is a ferrasol with variable surface charge. One column was left bare and the other was planted with mustard. Pulses of Ca[Br.sub.2] and Ca[([NO.sub.3]).sub.2] were applied to the surface of either wet or dry soil and then leached by water from a rainfall simulator applied at a steady rate of between 30 and 45 mm [h.sup.-1]. Water and solute transport were monitored by collecting the effluent. Contemporaneous in situ measurements of the water content and electrical conductivity were made using TDR. Transport parameters for the convection-dispersion equation, with a linear adsorption isotherm, were obtained from the flux concentration and the solute resident concentrations measured by TDR. Anion retardations between 1.2 and 1.7, and dispersivities between 1 and 9 mm, were found. Retardations also were calculated using our simple approach based on TDR-measured water and solute front velocities. These used TDR measurements of soil water content and bulk soil electrical conductivity with time, and were similar to those obtained from the effluent. The agreement suggests TDR could be a valuable in situ technique for obtaining the parameters relating to reactive solute transport through soil.