Experimental and Numerical Investigation of Ammonium Migration Attenuation in Soils with Different Clay Contents

In recent decades, extensive use of nitrogen fertilizers has drastically increased ammonium contamination in soil and groundwater. Ammonium contamination in soil is governed by the adsorption mechanism, which depends on the different types of soil through which the contaminant migrates. Understanding the adsorption mechanism of ammonium (NH4+) ions and the factors influencing them is critical for predicting and mitigating contamination. This study attempted to thoroughly investigate the effects of soil clay content on adsorption and, ultimately, the retardation of NH4+ ion movement in porous media. Ammonium ion transport was investigated using soil and batch experiments for single, mixed, and layered soil types with varying clay proportions. The experimental results were validated by numerically simulating NH4+ ion migration with HYDRUS 2D software (version 2.05.0270). Furthermore, the linear superposition relationship between the distribution coefficient (Kd) and mass of each individual soil layer was investigated and found to be appropriate for calculating an integrated Kd of NH4+ ions in layered soil. The attenuation of infiltration of NH4+ ions into the groundwater was found to be greatly dependent on clay content and the orientation of the soil. Hence, this study can be immensely useful in suggesting mitigation measures to control ammonium contamination in groundwater.