**1. Introduction**

Due to industry [1,2], agriculture [3,4], life [5], aquaculture [6], atmospheric deposition [7] and other factors, the groundwater in many countries and regions around the world are affected by nitrogen pollution, including the United States [8], China [9], the United Kingdom [10], South Korea [11] and other countries with severe pollution. In 2005, 34.1% of 1139 groundwater samples in northern China failed to meet World Health Organization (WHO) criteria. [12]. Currently, China has become one of the countries with the highest nitrogen fertilizer used in the world [13]. Nitrate from nitrogen may rapidly leach into groundwater, affecting the ecology and human health [14]. Wild et al. concluded that even if nitrate input is significantly reduced in the future, it will take decades to significantly reduce nitrate concentrations in porous aquifers through denitrification [15]. In 2000, the Water Framework Directive to bring water bodies to good chemical and ecological status by 2027 was issued [16]. Therefore, how to scientifically understand the nitrogen migration and transformation patterns under fluctuating groundwater level conditions has become one of the hot issues in the field of environmental research.

**Citation:** Li, Y.; Wang, L.; Zou, X.; Qu, J.; Bai, G. Experimental and Simulation Research on the Process of Nitrogen Migration and Transformation in the Fluctuation Zone of Groundwater Level. *Appl. Sci.* **2022**, *12*, 3742. https://doi.org/ 10.3390/app12083742

Academic Editor: Bing Bai

Received: 1 March 2022 Accepted: 5 April 2022 Published: 8 April 2022

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During groundwater level fluctuations, soluble substances in the aquifer medium are gradually dissolved into the groundwater, thus changing the chemical composition of the groundwater [17]. Guo Huaming et al. [18] studied the influencing factors of arsenic enrichment in groundwater, and the results showed that a high pH was unfavorable to the adsorption of arsenic in the form of anions by the aquifer medium. In their study, Sorensen J et al. [19] found that chemical and biological contaminants near the surface were transported to groundwater with minimal attenuation. Water level fluctuations also have a significant effect on nitrogen. Hefting M et al. [20] selected 13 riparian sites to analyze nitrogen cycling processes and confirmed a direct positive correlation between denitrification and elevated water table levels. Heather L. Welch et al. [21] analyzed and found that nitrate-N is weakened by denitrification during downward transport using redox sensitivity metrics at the water table without a location in the vertical direction. Jurado A et al. [22] concluded that the accumulation of N2O in groundwater is also mainly due to denitrification and, to a lesser extent, nitrification. Yang L.P. et al. [23] conducted indoor soil column experiments to study the effect of pH on nitrogen transport and transformation processes and showed that pH 6.5 was the most efficient for the removal of "tri-nitrogen" in the optimal pH range for adsorption nitrification and denitrification. The authors in [24] analyzed the effect of water level fluctuation on nitrogen transformation by simulating aerobic, anoxic and anaerobic zones, and the results showed that denitrification played the greatest role in the anaerobic zone.

Groundwater level fluctuations significantly affect the migration and transformation of groundwater pollutants [25–29]. Scholars at home and abroad have conducted studies on groundwater level fluctuations. Most of their research focuses on the migration and transformation processes of soil salinity, iron, manganese, arsenic, iodin, benzene and other characteristic components when subjected to water level fluctuations, etc. [18,30,31]. Davis et al. [32] studied the changes of organic matter and oxygen at the fluctuation of water level, and the results showed that BTEX and oxygen concentration showed a relationship between this and that near the water level, and the main reason for oxygen reduction was due to microbial degradation. Kamon et al. [33] conducted an experimental and numerical study of the migration of LNAPL with water level fluctuations and showed that the entry and displacement pressures were greater for the air-water system than for the LNAPLwater system. Xiaoxi Xie et al. [34] analyzed the hysteresis relationship between saturation and the capillary pressure in a medium under the water table fluctuation conditions during alternating drying and wetting processes, and the results demonstrated that when the initial water saturation of the drying process is similar, the greater the initial water saturation of the wetting process, the degree of hysteresis gradually decreases. Xiang Li et al. [35] analyzed and verified that groundwater level fluctuations affected the physicochemical properties of soil-water bodies and further affected the movement of nitrate in soil solids.

Research on the mechanisms of solute migration and transformation in the fluctuation zone of groundwater level, mainly through technical methods, such as field investigation, indoor experiments, theoretical analysis and numerical simulation, was conducted to predict the temporal and spatial distribution characteristics of solute pollution. Chen et al. [36] found through field experiments, that the depth of shallow groundwater has more influence on the nitrogen concentration in shallow groundwater than other factors. Zhang Dan et al. [27] conducted on-site monitoring of plots at different altitudes for a year. They found that the fluctuation of shallow groundwater levels significantly affected the soil profile and the nitrogen concentration of shallow groundwater. Farnsworth et al. [37] established an indoor soil column experiment, using a set of 1.3 m quartz sand columns inoculated with microorganisms, and changing the water level in the sand column every 30~50 h to simulate the periodic production wells of production wells and groundwater caused by the cessation of mining and the influence of the level fluctuation on the oxidation of manganese—the content of manganese increases as the water level drops, and decreases as the water levels rise. Yang Yang [38] studied the influence of the groundwater level rise and fall on the migration of cadmium, and argued that the rise and fall in the water level

mainly affect the transport of cadmium ions through convection. Zhang Xuejing et al. [39] used inverse distance weighting (IDW) interpolation and the water chemistry Piper graphic method to analyze the response relationship between the groundwater chemistry characteristics and the depth of the water level in the Ejina Oasis after the ecological water transport (2001–2017). Cao Wengeng et al. [40] used the potential distribution-multi-point complexation model (CD-MUSIC) to predict the evolution of groundwater chemical composition and hydrochemical types in the Baoding Plain of the South-to-North Water Transfer Project under the condition of groundwater level rebound. Arash Tafteh and Ali Reza Sepaskhah [41] successfully simulated the leaching of water and nitrate from two crops in the field with high accuracy using HYDRUS-1D. Mo Xiaoyu et al. [42] used HYDRUS-1D to simulate the changes in nitrogen leaching under different rainfall intensities, analyzed the influencing factors, and found that high intensity would reduce the nitrogen utilization rate.

In summary, the impact of water level fluctuations on the migration and transformation of pollutants has attracted the attention of scholars. Still, the study of nitrogen migration and transformation under different media water level fluctuations is not systematic enough. In this paper, the indoor soil column is used to simulate water level fluctuations to study the temporal and spatial distribution of nitrate nitrogen, nitrite nitrogen and ammonium nitrogen in three typical soil media subjected to water level fluctuations. According to physical experimental conditions, a numerical model of nitrogen migration and transformation in the fluctuation zone of the groundwater level was established to predict the spatial distribution and temporal change of nitrogen pollutants in the fluctuation zone of the groundwater level under different scenarios. This paper provides a scientific basis for the treatment, restoration and protection of groundwater nitrogen pollution.
