*3.2. Spatial Distribution of Inorganic Nitrogen in the Riparian Hyporheic Zone*

According to the statistical analysis, the concentration of three species of inorganic nitrogen (NH<sup>+</sup> <sup>4</sup> , NO<sup>−</sup> <sup>3</sup> , and NO<sup>−</sup> <sup>2</sup> ) in the groundwater showed significant differences among different riparian wells (*p* < 0.001). The mean NH<sup>+</sup> <sup>4</sup> 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<sup>+</sup> <sup>4</sup> concentration in the river water was higher than that in the riparian groundwater (Figure 5a). The mean NH<sup>+</sup> <sup>4</sup> 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<sup>+</sup> <sup>4</sup> concentration in the groundwater in the riparian hyporheic zone was lower in the near-shore zone where WB1 and WB2 existed than in the offshore zone where WB3 and WB4 existed.

**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<sup>+</sup> <sup>4</sup> ; (**b**) nitrate nitrogen, NO<sup>−</sup> <sup>3</sup> ; (**c**) nitrite nitrogen, NO<sup>−</sup> <sup>2</sup> ; (**d**) dissolved inorganic nitrogen, DIN.

The mean NO− <sup>3</sup> 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− <sup>3</sup> concentration in the river water was higher than that in the groundwater in the riparian hyporheic zone. The mean NO− <sup>3</sup> 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− <sup>3</sup> concentrations in the riparian groundwater decreased gradually with increasing distance from the shore. The mean NO− <sup>2</sup> 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<sup>+</sup> <sup>4</sup> , NO<sup>−</sup> <sup>3</sup> , and NO<sup>−</sup> <sup>2</sup> . 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 groundwater, and the offshore groundwater had a higher DIN concentration than the near-shore groundwater.

The content of NO− <sup>3</sup> 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<sup>+</sup> <sup>4</sup> proportion gradually increased from 43% at WB1 to 81% at WB4. For any riparian well, the mean NO− <sup>2</sup> proportion kept the low value less than 5%.

**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.
