## 3 Results
### Spatial Distribution of Inorganic Nitrogen in the Riparian Hyporheic Zone  
According to the statistical analysis, the concentration of three species of inorganic nitrogen (NH\({}_{4}^{+}\), NO\({}_{3}^{-}\), and NO\({}_{2}^{-}\)) in the groundwater showed significant differences among  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Mean Water Level Fluctuation Range (cm)**} & \multicolumn{3}{c}{**Response to the River Stage**} \\ \cline{2-7}
**Observation Well** & **21 October** & **22 October** & **23 October** & **Time Period (h)** & **Delayed Time (min)** & **Correlation Coefficient** \\ \hline WR0 & 22.7 & 17.9 & 12.9 & 12.2 & — & — \\ WB1 & 6.4 & 5.8 & 5.2 & 12.2 & 90 & 0.75 \\ WB2 & 5.6 & 4.9 & 5.2 & 12.2 & 135 & 0.66 \\ WB3 & 4.3 & 4.0 & 4.4 & 12.2 & 175 & 0.56 \\ WB4 & 3.7 & 3.5 & 4.3 & 12.2 & 180 & 0.52 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Fluctuations and the response relationship of the river stage and the groundwater level during the monitoring period from 21 to 23 October 2021.  
Figure 4: Variations in (**a**) daily mean water level and (**b**) daily mean temperature of the river water and the groundwater over the monitoring period from 21 to 23 October 2021.  
different riparian wells (\(p<0.001\)). The mean NH\({}_{4}^{+}\) concentration was 0.540 mg/L in the river channel, while those in the riparian observation wells ranged from 0.085 to 0.247 mg/L. This indicated that the NH\({}_{4}^{+}\) concentration in the river water was higher than that in the riparian groundwater (Figure 5a). The mean NH\({}_{4}^{+}\) concentrations in the groundwater in WB1, WB2, WB3, and WB4 were 0.085, 0.099, 0.247, and 0.105 mg/L, respectively. It is, thus, clear that the NH\({}_{4}^{+}\) concentration in the groundwater in the riparian borroheic zone was lower in the near-shore zone where WB1 and WB2 existed than in the offshore zone where WB3 and WB4 existed.  
The mean NO\({}_{3}^{-}\) concentration was 0.265 mg/L in the river water and varied from 0.022 to 0.112 mg/L in the groundwater in the riparian wells (Figure 5b). This implies that the NO\({}_{3}^{-}\) concentration in the river water was higher than that in the groundwater in the riparian borroheic zone. The mean NO\({}_{3}^{-}\) concentrations in the groundwater were 0.112, 0.062, 0.060, and 0.022 mg/L in WB1, WB2, WB3, and WB4, respectively. The mean NO\({}_{3}^{-}\) concentrations in the riparian groundwater decreased gradually with increasing distance from the shore. The mean NO\({}_{2}^{-}\) concentrations in both the river water and the groundwater in the four wells were below 0.02 mg/L (Figure 5c). This was mainly due to its chemical instability.  
Dissolved inorganic nitrogen (DIN) was approximate to the sum of the concentrations of NH\({}_{4}^{+}\), NO\({}_{3}^{-}\), and NO\({}_{2}^{-}\). The mean DIN concentration was 0.782 mg/L in the river water and varied from 0.129 to 0.327 mg/L in the groundwater in the riparian wells (Figure 5d). The DIN concentration in the river water was higher than that in the riparian  
Figure 5: Boxplots of the observed concentrations of inorganic nitrogen in the river water and the riparian groundwater during the study period from 21 to 23 October 2021: (**a**) ammonia nitrogen, NH\({}_{4}^{+}\); (**b**) nitrate nitrogen, NO\({}_{3}^{-}\); (**c**) nitrite nitrogen, NO\({}_{2}^{-}\); (**d**) dissolved inorganic nitrogen, DIN.  
groundwater, and the offshore groundwater had a higher DIN concentration than the near-shore groundwater.  
The content of NO\({}_{3}^{-}\) in the groundwater accounted for about 56% of the total DIN content at WB1 on average and decreased with increasing distance from the shore (Figure 6). In contrast, the mean NH\({}_{4}^{+}\) proportion gradually increased from 43% at WB1 to 81% at WB4. For any riparian well, the mean NO\({}_{2}^{-}\) proportion kept the low value less than 5%.