Abstract
Set in the downstream riparian zone of Xin’an River Dam, this paper established a two-dimensional coupling flow and solute transport and reaction model, and explored the denitriding methods and principles in the riparian zone from the perspective of hyporheic exchange, which provided a basis for the engineering techniques for river ecological restoration. Our studies have shown that under the condition of water level fluctuation, biological method such as adding denitrifying bacteria biomass could greatly increase the zone; chemical methods such as adding organic carbon into the surface water or groundwater could increase the total riparian nitrogen removal and its efficiency to a certain extent; hydrogeological methods such as silt cleaning of the aquifer surface or local pumping around the contaminated area could increase the total riparian nitrogen removal to some extent, but correspondingly reduce the denitriding efficiency; physical methods such as designing the bank form into gentle slope or concave shape could slightly increase the total riparian nitrogen removal and correspondingly improve the denitriding efficiency. At the application level of river ecological restoration, integratedly adopting the above methods can make the riparian denitriding effect “fast and good”.
Keywords: regulated river; water level fluctuation; riparian nitrogen removal; numerical simulation
1 INTRODUCTION
In the natural hydrologic cycle, surface water and groundwater are not independent units, but an organic whole, that is, there is a good hydrological connectivity between them. As early as 1959, Orghidan (1959) realized the ecological significance of the interface between surface water and surrounding groundwater, and first proposed the concept of the Hyporheic Zone. Up to now, scholars all over the world have carried out a mass of research on hyporheic zone in different research fields, and great progress has been made not only in the understanding of the hyporheic exchange mechanism, but also in the development of numerical models, indoor and outdoor testing techniques (Xia et al., 2013). With the maturity of hyporheic exchange theory, in recent years more and more scholars have concerned more on quantitative study of hydrochemical process in the hyporheic zone, of which the nitrogen cycle was a hot topic.
Some scholars have found that compared with flat terrain, a fluctuating riverbed structure can better improve the denitriding capacity in the hyporheic zone through numerical simulation. The typical riverbed structures are dune structure (Bardini et al., 2012), wood structure (Cardenas, 2009) and riffle-deep pool structure (Daniele and Buffington, 2007). In addition, Hu et al. (2014) further compared the influences of riverbed staircase structures with and without microtopography on nitrogen cycle in the hyporheic zone. And found that microtopography increases the hyporheic exchange intensity and produces a series of short immigration paths on the shallow layer of hyporheic zone, where has a relatively high oxygen content, thus promoting nitrification and inhibiting denitrification, furthermore resulting in a decrease in the denitriding capacity in the hyporheic zone. Compared with the terrain factor, the surface water fluctuation is generally better for the hyporheic nitrogen removal. Numerical simulation studies from Gu et al. (2012), Shuai et al. (2017) and Trauth et al. (2018) have concluded that the greater the surface water level fluctuates and the longer the water level duration is, the stronger the denitriding capacity in the hyporheic zone is. Liu (2019) found that under the assumed fixed upstream flood volume, the hyporheic denitriding capacity first increased and then decreased with the duration/amplitude ratio of water level fluctuation, while it increased logarithmically with the pulse frequency of water level fluctuation. This is of great significance for the ecological restoration of the riparian zone downstream of the reservoir. In addition, Shuai et al. (2017) further explored the impacts of surface water-groundwater hydraulic gradient, aquifer hydraulic conductivity and aquifer dispersion coefficient on the hyporheic nitrogen removal under the condition of water level fluctuation.
In addition to the aforementioned topography and hydrogeological factors, there are many biochemical factors influencing nitrogen cycle in the hyporheic zone, for example: 1) denitrifying bacteria. Adding denitrifying bacteria, like Thiobacillus denitrificans and Micrococcus denitrificans, can effectively accelerate the denitriding process (Hou et al., 2015). 2) Dissolved oxygen. Dissolved oxygen has an inhibitory effect on denitrification, and it is generally controlled at 1mg/L (Zhang et al., 2014). Duff and Triska (2011) studied the effect of dissolved oxygen concentration on the nitrogen cycle, and confirmed that the nitrogen cycle in the hyporheic zone is mainly based on the redox process of biological effect. 3) Organic carbon source. The electron donors (hydrogen donors) in the denitrification process are a variety of organic substrates (carbon sources). For example, if methanol is taken as an organic carbon source, not only can NO2-N and NO3-N be reduced, but also oxidative decomposition of organics can be promoted. In consideration of an additional consumption of dissolved oxygen for organic carbon source, the dosage of organic carbon is generally 3 times of NO3-N. Hu et al. (2014) pointed out that the increase of organic carbon concentration in surface water can effectively promote aerobic respiration and cause the attenuation of nitrification for the consumption of dissolved oxygen. However, due to the existence of microtopography, denitrification is basically unaffected.
In summary, the researches on nitrogen cycle in the hyporheic zone have made great achievements, but there are still some deficiencies. Firstly, although there have been some studies on the effects of chemical factors on surface-subsurface nitrogen flux (e.g., Hu et al., 2014), quantitative studies involving nitrogen transformation in the hyporheic zone are not sufficient, especially under the condition of water level fluctuation. Secondly, the researches on nitrogen cycle driven by the bank form are still insufficient, such as the effects of bank slope, concave and convex shapes on the riparian nitrogen cycle. Finally, there is still a lack of comparison among the impacts of the above mentioned hydrological, chemical and physical factors on the hyporheic denitriding capacity.
In addition, although the hyporheic zone plays an unneglectable role in the maintenance of river ecological health, which has been gradually proved and accepted by the global community of scholars, the practice of incorporating the hyporheic zone into designing schemes and engineering measures for the ecological environment protection and restoration of the whole river is still lagging behind. The focus of many projects such as reservoir operation, aeration, aquatic plant restoration, sediment dredging, ecological revetment, constructed wetlands, chemical remediation is only limited to the surface water, while the hydrodynamic exchange process and ecological significance between surface water and nearby shallow groundwater are not considered, which makes it impossible for the river to maintain effective long-term self-purification capacity. Therefore, to coordinate and consider the basic elements of the river system, and to carry out river ecological restoration from the level of hyporheic exchange should be one of the important contents of river ecological restoration and management.
Therefore, this paper discussed the influence principles of various factors on the hyporheic nitrogen removal from the perspective of biochemistry, hydrogeology and topography. Under the premise of surface water fluctuation, the following aspects are discussed: 1) the impacts of denitrifying bacteria and dissolved organic carbon on the hyporheic denitriding capacity. 2) The impacts of hydrological connectivity and surface water-groundwater hydraulic gradient on the hyporheic denitriding capacity. 3) The impacts of river bank slope and convex and convex forms on the hyporheic denitriding capacity. Furthermore, this paper narrated the corresponding feasible engineering measures, aiming at providing technical support for the current river restoration.
2 METHODS
2.1 Study site
The field site is located in the riparian zone (29°24’N, 119°21’E) downstream of the Xin’an River Dam, Jiande, China (see Liu et al. (2018) for details). The river water level at the site has been often affected by the upstream reservoir discharge for years, with the amplitude up to 1m. Three water-level monitoring wells were arranged along the cross section of the riparian zone. The horizontal line 699.3cm below the bottom of well #1 (Liu et al., 2018) was set as the baseline (i.e., 0m). In each well an HM21input liquid level transmitter that is accurate up to 0.1cm was installed. The data were automatically recorded every 5 minutes through a real-time automatic acquisition system to the computer via a remote terminal. There were large amount of gravels in the riparian zone, of which the maximum gravel size exceeded 10cm. Hence, it was inconvenient to conduct slug tests. By sampling the soil at different depths around the monitoring wells, and conducting particle diameter analyses and indoor Darcy Penetration tests, the effective porosity of the aquifer was measured to be 0.4 and the average saturated hydraulic conductivity (K ) was measured to be 137.2m/d (Table 1).