## 3 Results and Discussion
### Scenario Simulation  
#### 3.2.1 Increasing Water Level Fluctuation  
Scenario 1 is set to: keep the initial pollutant concentration unchanged, and increase the water level fluctuation range. That is, the initial water level is set to 30 cm, and the height is raised by 15 cm at a time, and then rises twice until the water level reaches 60 cm; then it is dropped by 15 cm each time and drops four times until the water level drops to 0 cm; finally, the water level is raised again twice, by 15 cm each time, until it reaches 30 cm, which is the initial water level. The comparison chart of water level changes is shown in Figure 7, recorded as situation A.  
Synchronously with the previous experiment scenario of groundwater level fluctuation, nitrogen migration and transformation (henceforth referred to as the fluctuation experiment), the simulation period is set to two by increasing the amplitude of the water level fluctuation, and the range of solute concentration in the water body increases. The model is built and run according to the scenario, and the simulation result is shown in Figures 8-10. Figure 8: The measured results of coarse sand and the process of dynamic change of nitrogen in situation A (**(a)** nitrate nitrogen; **(b)** nitrite nitrogen; **(c)** ammonium nitrogen). Figure 7: Comparison of water level changes. Figure 9: The measured results of medium sand and the dynamic process of nitrogen in situation A. ((a) nitrate nitrogen; (b) nitrite nitrogen; (c) ammonium nitrogen). The comparison of the standard deviation of nitrogen concentration obtained by the three media simulations is shown in Table 6. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multirow{2}{*}{\begin{tabular}{c} **Observation** \\ **Hole** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Coarse Sand** \\ **Fuctuation** \\ **Experiment** \\ \end{tabular} } & \multicolumn{2}{c}{\
\begin{tabular}{c} **Medium Sand** \\ **Fuctuation** \\ **Experiment** \\ \end{tabular} } & \multicolumn{2}{c}{
\begin{tabular}{c} **Fine Sand** \\ **Fuctuation** \\ **Experiment** \\ \end{tabular} } \\ \cline{3-8}  & 15 cm & 0.0249 & 0.0488 & 0.0348 & 0.0384 & 0.0838 & 0.0896 \\ Nitrite & 25 cm & 0.0197 & 0.0262 & 0.0141 & 0.0168 & 0.1037 & 0.0924 \\ nitrogen & 35 cm & 0.0159 & 0.0135 & 0.0261 & 0.0263 & 0.0166 & 0.0159 \\  & 45 cm & 0.0276 & 0.0047 & 0.0255 & 0.0328 & 0.0107 & 0.0072 \\ \hline \multirow{4}{*}{Nitrate} & 15 cm & 5.4152 & 6.2294 & 16.0273 & 22.4563 & 0.0921 & 0.0807 \\  & 25 cm & 4.2269 & 6.9961 & 12.8282 & 15.5376 & 0.0932 & 0.3326 \\ \cline{1-1}  & 35 cm & 5.2402 & 14.0076 & 3.8663 & 4.4202 & 24.2231 & 31.5546 \\ \cline{1-1}  & 45 cm & 9.5118 & 11.4918 & 1.9142 & 6.0021 & 0.417 & 0.4754 \\ \hline \multirow{4}{*}{Ammonium} & 15 cm & 0.7797 & 1.0168 & 0.2074 & 0.2113 & 0.0422 & 0.0441 \\ \cline{1-1}  & 25 cm & 0.5452 & 0.7755 & 0.7561 & 0.9278 & 0.027 & 0.0248 \\ \cline{1-1} Nitrogen & 35 cm & 0.6154 & 0.875 & 1.4036 & 1.8423 & 1.2271 & 1.3702 \\ \cline{1-1}  & 45 cm & 1.0365 & 1.4005 & 1.1279 & 0.8256 & 0.8747 & 0.8622 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Scenario A model standard deviation comparison. Figure 10: The measured results of fine sand and the dynamic process of nitrogen in situation A ((a) nitrate nitrogen; (b) nitrite nitrogen; (c) ammonium nitrogen). Table 6 shows that the increase in the fluctuation range of water level can effectively enlarge solute fluctuation. Water level fluctuations have different effects on the fluctuation range of nitrogen concentration in the three media. When the fluctuation range of water level increases by 5 cm, the fluctuation range of nitrogen in the coarse sand medium increases by 37.52% on average, compared to the fluctuation experiment scenario; the nitrogen concentration in the medium sand medium is increased by 37.52%. The fluctuation range increased by 31.40% on average; the fluctuation range of the nitrogen concentration in the fine sand medium increased by 21.14% on average.