**1. Introduction**

In order to ensure the security of the global food supply, a large amount of nitrogen fertilizer has been utilized in the past few decades. For example, the average nitrogen utilization has achieved the rate of 305 kg/hm2 in China [1]. Excessive bioavailable nitrogen such as nitrate nitrogen (NO− <sup>3</sup> ) and ammonia nitrogen (NH<sup>+</sup> <sup>4</sup> ) in rivers will lead to water eutrophication and, consequently, damage the ecological balance of river systems. Particularly in coastal areas, nitrogen pollution in rivers is generally serious due to dense population and active industry and agriculture production. If it cannot be maintained under effective control, the excessive nitrogen will continue to enter the ocean and further affect the health of the coastal ecosystem and the sustainable utilization of marine resources.

Hyporheic zones of river ecosystem are water-saturated sediment (known as aquifer) below the riverbed and extending to the riparian areas on both sides. It is not only a key area for water exchange and solute migration between river and groundwater, but also an important place for microbial growth and metabolism. Mass exchange and energy transfer are frequent and biogeochemical reactions are complex within a hyporheic zone, which is of great significance to the structure, function, and health of river ecosystem [2–4]. Nitrogen

**Citation:** Cai, Y.; Xing, J.; Huang, R.; Ruan, X.; Zhou, N.; Yi, D. Occurrence Characteristics of Inorganic Nitrogen in Groundwater in Silty-Clay Riparian Hyporheic Zones under Tidal Action: A Case Study of the Jingzi River in Shanghai, China. *Appl. Sci.* **2022**, *12*, 7704. https://doi.org/ 10.3390/app12157704

Academic Editor: Bing Bai

Received: 20 May 2022 Accepted: 28 July 2022 Published: 30 July 2022

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is transported between rivers and their riparian zones through hyporheic exchange and transformed between different chemical forms (e.g., NH<sup>+</sup> <sup>4</sup> , NO<sup>−</sup> <sup>3</sup> , nitrite nitrogen (NO<sup>−</sup> 2 ), and N2) through biogeochemical reactions [5]. Therefore, to realize the effective control of nitrogen pollution in rivers, it is necessary to fully comprehend nitrogen migration and transformation in groundwater in hyporheic zones.

Nitrogen in groundwater in a hyporheic zone varies geographically, and its form changes temporally, which forms a complex dynamic cycle [6–9]. Some hydrological events (e.g., rainstorm, flooding, reservoir discharge, and tidal action) can change the recharge relationship between river water and groundwater, consequently influencing nitrogen migration in hyporheic zones [10–13]. Shibata et al. carried out onsite monitoring to study the water exchange and the change in nitrogen concentration in the hyporheic zone during the rainstorm. It was found that the increase in nitrogen concentration in the hyporheic zone was mainly caused by the rainwater infiltration through the overlying unsaturated zone rather than the lateral seepage from the river [14]. The research conducted by Singh et al. indicated that the flood process extended the mixing range of surface water and groundwater and had a significant impact on the mass cycle in the riverbed hyporheic zone [15]. Sawyer et al. studied the impact of the reservoir operation on the lateral hyporheic exchange, and the results showed that the impact range could extend to 1–5 m away from the shore [16]. Unlike the hydrological events mentioned above, tides exhibit periodicity, and the tidal period is usually about 12 or 24 h. This makes the hyporheic exchange more frequent and the biogeochemical process in the hyporheic zone more complex [17]. In recent years, there has been growing concern over the hyporheic zones of tidal rivers. Musial et al. quantified the tide-driven hyporheic exchange fluxes across the bank and the bed and deduced that the tidal bank storage in the sandy hyporheic zone might remove nutrients from rivers [18]. The geochemical measurements and numerical simulation performed by Barnes et al. showed that it was possible that the hyporheic zone with permeable sediment and low organic matter content could be a source of nitrate to the tidal river [19].

Hydrogeological conditions and ecological factors could affect the nitrogen cycle in hyporheic zones [20–22]. McGarr et al. investigated the hyporheic exchange process in a sand–gravel mixed riverbed and pointed it out that the pore water flowed preferentially through the sediment of high-permeability, which affected nitrogen transport pathways [23]. The sediment heterogeneity could result in the existence of tiny anoxic blocks within the aerobic zone near the shore, thus affecting nitrogen processing [24]. The retention time of river water is usually extended in the low-permeability riverbed sediment, which is beneficial to the full reaction between nitrogen and dissolved oxygen (DO) [25]. In addition, the migration ability of NH<sup>+</sup> <sup>4</sup> in the hyporheic zone may be limited to a certain extent due to the electrostatic attraction by soil particles [26]. Eco-environmental factors mainly include the reactant concentration, redox potential (Eh), temperature (T), and pH [27–29]. The nitrogen-related biogeochemical reactions such as nitrification and denitrification are greatly governed by DO concentration and Eh in hyporheic zones [30]. Dissolved organic carbon (DOC) is another key factor controlling the nitrogen cycle, and increasing DOC concentration could promote denitrification [31]. Zarnetske et al. found that DOC and DO could be preferentially consumed in the near-shore hyporheic zone [32]. Additionally, temperature could affect the microbial activity, reaction rate, and DO concentration, consequently influencing the process of nitrogen cycle in the hyporheic zone [33,34]. Some studies demonstrated that pH showed a positive correlation with NO− <sup>3</sup> concentration and a negative one with NH<sup>+</sup> <sup>4</sup> concentration [35].

Although the nitrogen cycle in hyporheic zones has been extensively studied, there is little work aiming at the case of low-permeability riparian hyporheic zones of tidal rivers. At present, the mechanism underlying how tides drive nitrogen migration and transformation in riparian hyporheic zones with low permeability sediment is not very clear. The main objective of the study was to characterize the spatiotemporal variations of various inorganic nitrogen concentrations in groundwater in poorly permeable hyporheic zones of tidal rivers and discuss the potential reason why such variations existed. A representative transect was chosen to carry out the drilling, monitoring, and sampling. By means of onsite monitoring and lab analysis, the response of the riparian groundwater level to the tide-driven fluctuation of the river stage was studied, the spatiotemporal variations of inorganic nitrogen concentrations in the groundwater in the riparian hyporheic zone were analyzed, and the potential influencing factors were discussed. The results are expected to enrich the theory of river hyporheic zones and provide a scientific basis for nitrogen pollution treatment of tidal rivers.
