## 3 Results and Discussion
### Scenario Simulation
#### 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. 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 11: Comparison of Water Level Changes. Figure 12: The measured results of coarse sand and the process of dynamic change of nitrogen infusion 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. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multirow{2}{*}{**Observation**} & \multicolumn{3}{c}{**Coarse Sand**} & \multicolumn{3}{c}{**Medium Sand**} & \multicolumn{3}{c}{**Fine Sand**} \\ \cline{3-8}  & & **Hole** & **Fluctuation** & **Situation B** & **Fluctuation** & **Situation B** & **Fluctuation** & **Situation B** \\  & & **Experiment** & & **Experiment** & & **Experiment** & & \\ \hline \multirow{4}{*}{Nitrite Nitrogen} & 15 cm & 0.0249 & 0.0063 & 0.0348 & 0.0316 & 0.0838 & 0.0758 \\  & 25 cm & 0.0197 & 0.0143 & 0.0141 & 0.0099 & 0.1037 & 0.1139 \\  & 35 cm & 0.0159 & 0.0183 & 0.0261 & 0.0266 & 0.0166 & 0.0148 \\  & 45 cm & 0.0276 & 0.0206 & 0.0255 & 0.0219 & 0.0107 & 0.0112 \\ \hline \multirow{4}{*}{Nitrate} & 15 cm & 5.4152 & 2.1756 & 16.0273 & 4.0429 & 0.0921 & 0.1069 \\  & 25 cm & 4.2269 & 3.337 & 12.8282 & 8.1699 & 0.0932 & 0.0601 \\  & 35 cm & 5.2402 & 6.0552 & 3.8663 & 2.4465 & 24.2231 & 15.1679 \\  & 45 cm & 9.5118 & 4.4877 & 1.9142 & 2.658 & 0.417 & 0.3406 \\ \hline \multirow{4}{*}{Ammonium Nitrogen} & 15 cm & 0.7797 & 0.4898 & 0.2074 & 0.2042 & 0.0422 & 0.0391 \\  & 25 cm & 0.5452 & 0.4511 & 0.7561 & 0.4871 & 0.027 & 0.029 \\ \cline{1-1}  & 35 cm & 0.6154 & 0.5448 & 1.4036 & 1.4785 & 1.2271 & 1.0044 \\ \cline{1-1}  & 45 cm & 1.0365 & 0.7817 & 1.1279 & 1.3127 & 0.8747 & 0.7587 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Scenario B model standard deviation comparison. 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). 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. Figure 14: The measured results of fine sand and the dynamic process of nitrogen in situation B ((a) nitrate nitrogen; (b) nitrite nitrogen; (c) ammonium nitrogen). The impact of water level fluctuations on the three solutes is also different.