## 4 Discussion  
During the study period, the pH and the DO concentrations in the groundwater in the four riparian wells varied from 6.27 to 6.59 (neutral environment) and from 5.2 to 6.8 mg/L, respectively. It is generally considered that an environment is aerobic if DO concentration is greater than 3 mg/L [41]. Thus, the riparian hyporheic zone within 4 m from the bank was an aerobic and neutral environment in favor of nitrification. In past studies, denitrification was regarded as an anaerobic process, and it was considered that denitrification would no longer occur if DO concentration was greater than 4.5 mg/L [42; 43]. However, recent research results showed that denitrification could also occur in aerobic zones in soils due to the existence of aerobic denitriying bacteria which could live in the environment with a DO concentration of 3-9 mg/L and a neutral pH [44; 45; 46]. It was, therefore, deduced that aerobic denitrification and nitrification might coexist in the riparian hyporheic zone studied, and the combination of these two effects was conducive to the removal of nitrogen from the riparian zone. In the investigation, the NH\({}_{4}^{+}\) concentration in the near-shore groundwater was lower than that in the offshore groundwater, and the lowest value found at WB1 closest to the bank. It was reported by [PERSON] et al. that the NH\({}_{4}^{+}\) concentration in the riparian groundwater decreased with increasing the distance from the bank of the nitrogen-rich river [47]. This is inconsistent with the results of our research. By comparison, it was found that there were significant differences in NH\({}_{4}^{+}\) concentration in the river water and type of the sediment. The NH\({}_{4}^{+}\) concentration in the river water was 0.9-1.5 mg/L, and the sediment mainly consisted of coarse sand in their research, while the former was 0.4-0.7 mg/L and the latter was silty-clay soil in our research. Some studies indicated that a low permeability of the sediment could reduce flow velocity of the pore water and, consequently, prolong the solute residence time, which provided the sufficient reaction time for nitrogen in the hyporheic zone [48; 49]. Thus, the lower-permeability sediment and the river water with the lower nitrogen concentration contributed to more consumption and less recharge of the NH\({}_{4}^{+}\) in the near-shore groundwater, respectively. This could explain why groundwater with less NH\({}_{4}^{+}\) was observed in the near-shore hyporheic zone. However, upon increasing  
Figure 9: Spatial variations of eco-environmental factors along the transect: (**a**) dissolved oxygen, DO; (**b**) dissolved organic carbon, DOC; (**c**) pH; (**d**) temperature. the distance from the bank, the DO concentration in the groundwater deceased gradually. The decline in DO concentration could lower the intensity of nitrification [50]. Furthermore, the mean TN content in the sediment at WB3 was highest among the four observation wells. The higher TN content could promote ammonation, which is an important pathway for recharging the NH\({}_{4}^{+}\) in the groundwater in hyperboic zones [51, 52]. The greater recharge and lower consumption resulted in more NH\({}_{4}^{+}\) existing in the groundwater in the offshore hyperboic zone. The NO\({}_{3}^{-}\) concentration in the riparian groundwater showed a downward trend as the distance from the bank increased along the test transect. This matches with the work of [PERSON] et al. [53]. Due to the strong nitrification reactions and the recharge from the river water, the NO\({}_{3}^{-}\) was enriched in the near-shore groundwater. However, in the offshore hyperboic zone with the poorly permeable sediment, the recharge of NO\({}_{3}^{-}\) from the river water was very slow and could take several hours or even more according to the delayed response time of the groundwater level to the river stage in the study. Moreover, due to the decline in DO concentration, a lower amount of the NO\({}_{3}^{-}\) in the groundwater was produced by nitrification.