Determination of the sodium retardation factor using different methods: Analysis of their characteristics and impact on the solute movement prediction in a structured Brazilian soil

The retardation factor (Rd) is one of the main important solute transport parameters. Its value can vary significantly depending on the method used for its determination. In this paper, the sodium Rd is experimentally determined using undisturbed sandy columns to compare four methods of Rd determination and assess the impact of the chosen method in the prediction of sodium movement. Column experiments in undisturbed soil columns and analytical analysis were performed. The results showed that the soil has dual-porosity and preferential pathways. The breakthrough curves were in accordance with the soil’s physical characteristics. The Rd values ranged from 1.7 to 7.77 depending on the initial concentration and on the method used. These differences arise from the conceptual model of each Rd determination method. The experimental and analytical analysis indicated that the higher the Rd, the slower the movement. The methods that best reproduced the laboratory sodium movement were Ogata and Banks’ (1961), and Langmuir and Freundlich’s isotherms. The prediction models presented smaller errors with the increase of the initial concentration. In these cases, the predicted concentrations can be overestimated up to 22.5 % when using a not suitable method. Hence these results suggest that the Rd determination method can strongly affect the prediction of the sodium movement. Because of that, it is of vital importance to evaluate each method and how they can be adequate to the soil under investigation when determining Rd.