Evaluating Equilibrium and Non-Equilibrium Bromide Transport in Forest and Rangeland Soils on a Laboratory-Scale

to better management of solute transport in porous media, it is essential to recognize their transport behavior using appropriate models. In this research, convection-dispersion equation (CDE) and mobile-immobile model (MIM), as physical equilibrium and non-equilibrium models, respectively, were used to simulate the bromide transport through saturated and unsaturated forest soil, with clay loam texture, and rangeland soil, with sandy loam texture, columns (diameter of 6 and height of 10 cm).

to obtain the BTCs, the PVC soil columns with a height of 10 and a diameter of 6 cm were prepared. The breakthrough experiment was carried out in near saturation and saturated condition under a water head of -1 and 3 cm, respectively. The soil columns were saturated from the bottom with a Ca(NO3)2 solution of 0.01 molar as the background solution. At near saturation, the CaBr2 solution with a concentration of 0.01 M equal to a pore volume was injected into the saturated columns of the background solution through the infiltration disk. A Mariotte bottle was used to establish a constant water head. After CaBr2 injection started, the effluents with a volume of 0.1 pore volume were collected at different times, and their bromide concentrations were determined using a pH-meter equipped with a bromide selector electrode. After the complete injection of CaBr2, the steady-state saturated flow of the background solution was re-established. The experiment continued until the bromide concentration in the effluent were almost zero. The measured concentrations, by dividing by the initial concentration, were converted to relative concentrations (C/Co). Then the BTCs was plotted as C/Co versus time or the number of pore volumes.

The values of mass transfer coefficient (ω<100) and mobile water fraction (β<1) as an indicator for determining the equilibrium and non-equilibrium indicated that bromide transport behavior within these columns was anomalous or non-Fickian transport. Hence, the non-equilibrium or the mobile-immobile model (MIM) is suitable and more efficient than the Fickian-based CDE model. The fitted breakthrough curves (BTCs) and the higher determination coefficient (R2) and the lower root mean square error (RMSE) values of the MIM model compared to those of the CDE confirmed the effectiveness of the MIM model in simulating bromide transport in the forest and rangeland soil columns.

Better fit of measured and estimated breakthrough curves of bromide with non-equilibrium model compared to CDE equilibrium model, especially in the tail of breakthrough curves indicates more accuracy and the should be added efficiency of the non-equilibrium model. Given that the samples were replaced in the columns as disturbed, it can be said that heterogeneity conditions were established in the columns experiments. According to Huang et al. (2005) and Berkowitz et al. (2008), heterogeneity could be one of the reasons to justify the better performance of non-equilibrium models in the present study. The high efficiency of the non-equilibrium model compared to the equilibrium model in this controlled laboratory research cannot be a reliable judgment in evaluating these models. Accurate judgment will depend on conducting research and experiments in real and field conditions, taking into account more effective parameters.