## 3 Results
### Temporal Variation of Inorganic Nitrogen Concentrations in the Riparian Hyporhetic Zone  
Over the study period, the daily mean NH\({}_{4}^{+}\) concentration in the groundwater increased from 0.070 to 0.106 mg/L at WB1 and rose from 0.087 to 0.114 mg/L at WB2 (Figure 7a). In contrast, it decreased from 0.337 to 0.193 mg/L at WB3 and fell from 0.110 to 0.097 mg/L at WB4. It is distinctly clear that the daily mean NH\({}_{4}^{+}\) concentration showed an upward trend in the near-shore zone and a downward one in the offshore zone. Meanwhile, the daily mean NO\({}_{3}^{-}\) concentration in the groundwater increased from 0.106 to 0.120 mg/L at WB1 and rose from 0.057 to 0.068 mg/L at WB2 (Figure 7b). However, it changed little in the offshore groundwater. The daily mean NO\({}_{2}^{-}\) concentration in the riparian groundwater showed little change (Figure 7c). This is mainly due to the NO\({}_{2}^{-}\) concentration having a very low concentration in both the river water and the riparian groundwater. The daily mean DIN concentration increased from 0.178 to 0.228 mg/L at WB1 and ascended from 0.146 to 0.185 mg/L at WB2 (Figure 7d). It decreased from 0.408 to 0.271 mg/L at WB3 and fell from 0.132 to 0.123 mg/L at WB4. It is clear that the variation of the DIN concentration was roughly the same as that of the NH\({}_{4}^{+}\) concentration in the groundwater; hence, NH\({}_{4}^{+}\) was generally the dominant form of inorganic nitrogen in the riparian hyperho  
Figure 6: Mean proportions of different forms of inorganic nitrogen in the groundwater in the riparian observation wells during the study period from 21 to 23 October 2021. Physicochemical Characteristics of Sediment and Water in the River Channel and the Riparian Hyporheic Zone  
There was a great difference between the horizontal and vertical permeability coefficients of the sediment in the riparian hyporheic zone (Table 2). For the sediment at any specific position, the horizontal permeability coefficient was 10 times higher than the vertical one, which implied that the lateral migrations of both the hyporheic flow and the solutes were dominant along the transect. According to the grain size distribution, the riparian sediment was determined as silty-clay soil. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{3}{*}{**Observation** **Well**} & \multirow{3}{*}{**Density** **(g/cm\({}^{3}\))**} & \multicolumn{3}{c}{**Coefficient of Permeability (cm/s)**} & \multicolumn{3}{c}{**Granularity Content (\%)**} & \multirow{3}{*}{**Soil Type**} \\ \cline{3-3} \cline{5-8}  & & \multicolumn{2}{c}{**Horizontal**} & \multicolumn{2}{c}{**Vertical**} & & & \\ \cline{3-8}  & & **Average** & \multicolumn{2}{c}{**Standard**} & \multicolumn{2}{c}{**Standard**} & \multicolumn{2}{c}{**(d \textless{} 0.005 mm)**} & \multicolumn{2}{c}{**Silt (0.005 mm)**} & \\ \cline{3-8}  & & **Average** & \multicolumn{2}{c}{**Deviation**} & \multicolumn{2}{c}{**Average**} & \multicolumn{2}{c}{**Standard**} & \multicolumn{2}{c}{**(d \textless{} 0.005 mm)**} & \\ \hline WB1 & 1.89–1.97 & \(4.40\times 10^{-6}\) & \(6.26\times 10^{-7}\) & \(8.71\times 10^{-8}\) & \(0.98\times 10^{-9}\) & 27–31 & 53–57 & Silty-clay \\ WB2 & 1.92–1.97 & \(4.19\times 10^{-6}\) & \(5.78\times 10^{-7}\) & \(9.14\times 10^{-8}\) & \(0.67\times 10^{-9}\) & 32–34 & 54–58 & Silty-clay \\ WB3 & 1.97–1.99 & \(3.11\times 10^{-6}\) & \(8.37\times 10^{-7}\) & \(8.13\times 10^{-8}\) & \(1.05\times 10^{-8}\) & 34–41 & 53–59 & Silty-clay \\ WB4 & 1.94–1.96 & \(3.97\times 10^{-6}\) & \(2.42\times 10^{-7}\) & \(8.44\times 10^{-8}\) & \(1.13\times 10^{-8}\) & 32–35 & 55–57 & Silty-clay \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Physical properties of the sediment in the riparian hyporheic zone.