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).