3.2.2. Scenario of Reduced Water Level Fluctuation

Scenario 2 is set to: keep the initial pollutant concentration unchanged, and reduce the fluctuation of the water level. That is, the initial water level is set to 30 cm, and the height is raised by 5 cm each time, and then rises two times until the water level reaches 40 cm; it then starts to drop by 5 cm each time, and drops four times until the water level drops to 20 cm; finally, the water level rises again two times, by 5 cm each time, until it reaches 30 cm, which is the initial water level. The comparison chart of the water level changes is shown in Figure 11. It is recorded as situation B.

**Figure 11.** Comparison of Water Level Changes.

Similarly, the simulation period is set to 2. By increasing the fluctuation range of the water level, the variation range of the solute concentration in the water body is increased. The model is built and run according to the scenario, and the simulation result is shown in Figures 12–14.

**Figure 12.** The measured results of coarse sand and the process of dynamic change of nitrogen in situation B ((**a**) nitrate nitrogen; (**b**) nitrite nitrogen; (**c**) ammonium nitrogen).

**Figure 13.** The measured results of medium sand and the dynamic process of nitrogen in situation B ((**a**) nitrate nitrogen; (**b**) nitrite nitrogen; (**c**) ammonium nitrogen).

The comparison of the standard deviation of the nitrogen concentration obtained by the three media simulations is shown in Table 7.


**Table 7.** Scenario B model standard deviation comparison.

Table 7 shows that the decrease in water level fluctuation can effectively reduce solute fluctuation.

Water level fluctuations have different effects on the fluctuation range of nitrogen concentration in the three media. When the fluctuation range of the water level decreases by 5 cm, the fluctuation range of nitrogen in the coarse sand medium is reduced by 36.74% on average, compared to the fluctuation experiment scenario. In the medium sand medium, the fluctuation range of the concentration of nitrogen decreased by 14.70% on average, while the fluctuation range of the nitrogen concentration in the fine sand medium decreased by 9.39% on average.

Table 7 shows that in the coarse sand, the fluctuation of the nitrogen concentration changes most significantly with the decrease in the fluctuation range of the water level, followed by the medium sand, and the fine sand demonstrates the most minor change.

The impact of water level fluctuations on the three solutes is also different. When the fluctuation range of the water level is reduced by 5 cm, the fluctuation ranges of nitrite nitrogen, nitrate nitrogen and ammonium nitrogen in the coarse sand medium are reduced by 28.07%, 29.55%, and 22.62%, respectively. In the medium sand medium, the average fluctuation range is reduced by 13.00%, 27.23%, and 3.85%, respectively. In the fine sand model, the fluctuation range is reduced by an average of 1.51%, 18.79%, and 7.86%, respectively.

Table 7 shows that among the three media, water level fluctuations have the most significant impact on the fluctuations of nitrate nitrogen, have less impact on the fluctuations of nitrite nitrogen and ammonium nitrogen, and the difference is the most obvious in the fine sand medium, followed by medium sand. This difference is not great in the coarse sand.

## **4. Conclusions**

In this paper, through the experiment examining nitrogen migration and transformation in the groundwater fluctuating zone, we analyzed the nitrogen migration and transformation process. A numerical model of nitrogen migration and transformation in the groundwater level fluctuating zone was established with the help of the HYDRUS-1D model. The paper obtained the following main conclusions:


**Author Contributions:** Conceptualization, Y.L. and J.Q.; methodology, Y.L., L.W. and X.Z.; software, L.W. and X.Z.; validation, Y.L., L.W. and J.Q.; writing—original draft preparation, Y.L. and L.W.; writing—review and editing, Y.L. and J.Q.; visualization, G.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Doctoral Research Fund of North China University of Water Resources and Electric Power (40651) and the Key Project of Science and Technology Research of Henan Education Department (14A170006), and by the Key R&D and Promotion Projects in Henan Province (202102310012).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.
