## 1 Introduction
[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. [PERSON] 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. [PERSON] 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. [PERSON] 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. [PERSON] 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\(\sim\)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. [PERSON] [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. [PERSON] 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). [PERSON] 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 Booding Plain of the South-to-North Water Transfer Project under the condition of groundwater level rebound. [PERSON] and [PERSON] [41] successfully simulated the leaching of water and nitrate from two crops in the field with high accuracy using HYDRUS-1D. [PERSON] 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.