Non-Fickian transport of ammonia nitrogen in vadose zone: experiments and modeling
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ORIGINAL PAPER
Non-Fickian transport of ammonia nitrogen in vadose zone: experiments and modeling Qian Wang 1,2 & Jianmin Bian 1,2 & Hanli Wan 1,2 & Tianxue Gu 1,2 Received: 14 January 2019 / Accepted: 30 October 2019 # Saudi Society for Geosciences 2019
Abstract To investigate the anomalous migration process of ammonia nitrogen in vadose zone, laboratory and numerical experiments of chloride and ammonia nitrogen are used to the transport parameters and evaluate the physical and chemical heterogeneity. Batch adsorption experiments and column experiments of silty loam and silty clay were conducted to determine key transport parameters. BTCs of chloride and ammonia nitrogen are derived using three approaches: equilibrium advection–dispersion equation (ADE), mobile–immobile model (MIM), and continuous time random walk–truncated power law (CTRW-TPL). All the models show accepted fitness to the transport process of chloride, but the CTRW-TPL fits best. For ammonia nitrogen, the CTRW-TPL with the retardation term (Λ) can fully describe the tracer-BTC, especially for late-time tailing, while the ADE and MIM cannot. Concentration fluctuation and irregular behavior in silty clay are more violent than those in silty loam. Physical heterogeneity has little effect on anomalous trait of BTCs in homogenous media. And lower permeability and mass exchange between mobile and immobile region contribute to enhance the non-Fickian behavior. Adsorption heterogeneity is the major contributor to the nonFickian behavior. The more violent anomalous behavior can be related to the higher retardation. Our results reveal the nonFickian characteristics of ammonia nitrogen which will provide useful insights for decision-makers in the assessment and management of groundwater pollution. Keywords Ammonia nitrogen . Vadose zone . Non-Fickian dispersion . CTRW-TPL model . Adsorption heterogeneity
Introduction
Highlights • The pseudo-second order and Freundlich model shows the best fittings to the adsorption kinetic and equilibrium isotherms of silty loam and silty clay. • Of three models, the equilibrium advection–dispersion equation (ADE), mobile–immobile model (MIM), and continuous time random walk– truncated power law (CTRW-TPL), CTRW-TPL can best describe the non-Fickian behavior of ammonia nitrogen. • For ammonia nitrogen, the non-Fickian behavior is related to both physical and reactive heterogeneity. Responsible Editor: Broder J. Merkel * Jianmin Bian [email protected] 1
College of New Energy and Environment, Jilin University, Changchun 130021, China
2
Key Laboratory of Groundwater Resources and Environment, Ministry of Education, Jilin University, Changchun 130021, China
The concentration of ammonia nitrogen is one of the important indicators of groundwater quality, and the World Health Organization (WHO) and European Union (EU) have set the concentration of 0.5 mg/L as the limit (WHO 2003). When waste water moves through vadose zone, ammonia nitrogen is removed partially occurring physical and biochemical processes, such as adsorpt
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