## 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]. [PERSON] 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. 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]. [PERSON] 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, [PERSON] 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. [PERSON] 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. [PERSON] 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. [PERSON] et al. [22] concluded that the accumulation of N\({}_{2}\)O in groundwater is also mainly due to denitrification and, to a lesser extent, nitrification. [PERSON] 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; 26; 27; 28; 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, iodine, benzene and other characteristic components when subjected to water level fluctuations, etc. [18; 30; 31]. [PERSON] 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. [PERSON] 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 LNAPL-water system. [PERSON] et al.