*3.3. Temporal Variation of Inorganic Nitrogen Concentrations in the Riparian Hyporheic Zone*

Over the study period, the daily mean NH<sup>+</sup> <sup>4</sup> 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<sup>+</sup> <sup>4</sup> concentration showed an upward trend in the near-shore zone and a downward one in the offshore zone.

Meanwhile, the daily mean NO− <sup>3</sup> 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− <sup>2</sup> 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<sup>+</sup> <sup>4</sup> concentration in the groundwater; hence, NH<sup>+</sup> <sup>4</sup> was generally the dominant form of inorganic nitrogen in the riparian hyporheic zone within 4 m from the shore.

**Figure 7.** Variations in daily mean inorganic nitrogen concentrations in the river water and the riparian groundwater over the study period from 21 to 23 October 2021: (**a**) ammonia nitrogen, NH<sup>+</sup> <sup>4</sup> ; (**b**) nitrate nitrogen, NO− <sup>3</sup> ; (**c**) nitrite nitrogen, NO<sup>−</sup> <sup>2</sup> ; (**d**) dissolved inorganic nitrogen, DIN.
