Figure 5 should be here
4 DISCUSSION
4.1 Deficiency of the model
The two-dimensional, variably saturated and multispecies reactive
transport model in this paper has many shortcomings. First of all, the
flow model was based on a series of assumptions, which could be seen for
details from the model of Liu et al. (2019). Secondly, the calibration
of the flow model was determined by mainly adjusting the hydraulic
conductivity of the aquifer. Because the hydraulic conductivity was very
sensitive to the model, and there were many deviations of the indoor
Darcy Penetration tests in obtaining the hydraulic conductivity.
Thirdly, the biogeochemical parameters of the model were set according
to the empirical values as the previous studies (e.g., Shuai et
al., 2017), which had non-ignorable influences on the model results.
Lastly, in the nitrogen cycle of the chemical model, process like
anaerobic ammoxidation (DNRA) was not taken into consideration, which
had a certain impact on the transformation of nitrogen (Zarnetke et
al., 2012). In summary, the calculations of the denitriding amount in
the riparian zone of this model under various cases might be just
estimated values, however it did not affect our exploration to the
denitriding methods and principles in the riparian zone of a regulated
river.
4.2 Implications
4.2.1 Principles of biochemical denitriding and engineering measures
In general, the influence principles of surface water and groundwater
quality and denitrifying bacteria on riparian denitrifying amount were
the same, that is to say, they all increased the denitrifying capacity
by enhancing chemical reactivity. However, there were some differences.
Increasing DOC concentration of surface water and groundwater improved
the denitriding capacity by increasing the electronic donors in the
denitrification process, while increasing denitrifying bacteria
biomass was equivalent to the catalyst function on
denitrification. Therefore, the formers had greater impacts on
the denitriding amount in the riparian zone, while the latter
mainly had a greater impact on the denitriding rate.
Hence, in the heavy nitrate contaminated riparian zone, appropriate
waste wood materials can be piled up at the interface between surface
water and groundwater according to local conditions, which can be
provided as the carbon source for DOC being infiltrated into the aquifer
from surface water, thereby promoting the removal of nitrogen.
Furthermore, some tree branches and leaves can be put or some green
plants can be planted on the surface of the riparian zone, so that the
rain will carry a certain amount of DOC into the aquifer to increase the
groundwater DOC concentration and promote the removal of nitrogen. In
addition, Mycobacterium szulgai and Pseudomonas fluorescens can be
directly put into the aquifer to increase the biomass concentration of
denitrifying bacteria. This method cannot greatly increase the
denitrifying amount in the riparian zone, but it can effectively speed
up the denitrifying process and improve the denitrifying efficiency.
4.2.2 Principles of hydrogeological denitriding and engineering measures
The influence principle of K or i on the riparian
denitriding amount was different from that of the biochemical factors.
The latters increased the riparian denitriding amount by enhancing the
chemical reactivity, while the former increased the denitriding amount
by enhancing the hyporheic exchange and increasing the total amount of
solute infiltration. Compared with the previous studies (e.g., Shuai et
al., 2017), the influences ofK and i on M in-NO3, M rem-NO3 and N rem-
NO3 are the same, which also indicates the rationality of this model.
In addition, although increasing K or i could
increase M rem-NO3 to some extent, butN rem-NO3 was decreased correspondingly. This is
because that N rem-NO3 is the ratio ofM rem-NO3 toM in-NO3.
When K or i increased, the increase extent of numerator
was smaller than denominator. For example,M in-NO3=5g and the
corresponding M rem-NO3 =2g,
then N rem-NO3 =40%. Increasing K to makeM in-NO3 be 10g, and assuming 10g is the sum of
the total infiltration in two periods. During the previous period,
NO3- infiltration amount is 5g and the
corresponding M rem-NO3 is 2g. During the latter
period, NO3- infiltration amount is
also 5g, but the corresponding M rem-NO3 is less
than 2g. This is because the concentration of DOC in the riparian zone
is lower than that of O2 after the asymmetric chemical
reaction in the previous period (nitrification consumes less
O2 while denitrification consumes more DOC),
causing O2 being relatively surplus in the latter
period, thereby inhibiting the denitrification to a certain extent.
Therefore, the total denitriding amount becomes smaller (<4g)
and finally N rem-NO3 becomes
smaller overall (<40%).
In practical measures, K can be increased by clearing the
sedimentary silt along the interface between river and bank. In general,
the hydraulic conductivity of the silt in aquifer surface is about two
orders of magnitude lower than that of the aquifer. Hence, silt cleaning
will greatly increase the hyporheic exchange during the water level
fluctuation, and improve the riparian denitriding capacity
correspondingly. As for the increase of i , it can be achieved by
local pumping measures in the bank. In order to reduce the workload,
local underground pumping can be carried out near the seriously
contaminated riparian zone.
4.2.3 Principles of topography denitriding and engineering measures
The influence principle of the bank form on the riparian denitriding
capacity during the water level fluctuation was basically the same asK or i , all of them increasedM in-NO3 and M rem-NO3 by
enhancing the hyporheic exchange. However, the bank form was a factor
that influenced the hyporheic exchange by influencing the exchange
scope, while K or i was a factor that influenced the
hyporheic exchange by influencing the exchange intensity. This result
has been proved by the previous studies (e.g., Siergieie et
al., 2015). The influence principles of the bank forms such as bank
slope, concave and convex shape on the riparian denitriding capacity
were similar. The convex bank essentially had the same effect on the
riparian denitriding capacity as the increase of the bank slope, both of
which reduced the length of the river-bank interface, and thus reduced
the scope of hyporheic exchange, thereby resulting in the corresponding
decrease of Q max, M in-NO3
and M rem-NO3. Similarly, when the bank was
concave, it had the same effect as the decrease of the bank slope,
resulting in the increase of M rem-NO3. Compared
the impacts of bank forms with that of the above biochemical and
hydrogeological factors on riparian denitriding capability, it could be
found that the influence degree of changing bank form on the riparian
denitriding capability was relatively much smaller. This is due to the
little effect of changing bank form on the hydrodynamic exchange between
surface water groundwater, which results in a small amount of the solute
infiltration.
Comparing to the concave or convex shape, the bank slope has a
relatively greater impact on the riparian denitriding capacity, but it
has some limitations in the application of engineering measures. It is
impossible to make the bank slope unrestrictedly small, so in practical
applications the bank can be designed as a gentle slope with a concave
shape, which will improve the riparian denitriding capacity in a good
way. The previous studies
(e.g., Bardini et
al., 2012) have shown that riverbed dune morphology has a
positive impact on the vertical hyporheic exchange and hypoeheic
denitriding effect. Therefore, this paper has also calculated and
compared the denitriding capacity in the riapqian zone with flat and
undulated bank (calculation process omitted). However, the undulating
shape of the bank had no obvious effect on the riparian denitriding
capacity. The possible reason is that a bank form with a certain
undulating shape is equivalent to the combination of a series of concave
and convex shapes. As mentioned above, the effect of the concave and
convex shape of the bank on the riparian denitriding capacity was
opposite, which made the undulating shape of the bank had a
mutual offset on riparian denitriding capacity.
5 CONCLUSIONS
The main conclusions were summarized as follows:
(1) Increasing the DOC concentration of surface water and
groundwater could largely increase the denitriding amount in the
riparian zone and accordingly increase the denitriding efficiency. By
comparison, adding denitrifying bacteria biomass had a smaller impact on
the denitriding amount, but it could improve the denitriding rate to a
great extent. The combined applications of these methods can make the
denitriding effect in the riparian zone “fast and good”.
(2) Enhancing the hydrological connectivity of the aquifer surface could
increase the denitriding amount in the riparian zone to a certain
extent, but the denitriding efficiency was reduced correspondingly. By
comparison, increasing the surface-groundwater hydraulic gradient had a
much greater impact on the denitriding amount, with the denitriding
efficiency reducing too. In practical applications, through pumping the
groundwater in the heavily polluted reach and cleaning the surface
sedimentary sludge can effectively improve the denitriding capacity in
the riparian zone.
(3) Designing the bank form into a concave shape could slightly increase
the denitriding amount in the riparian zone, and correspondingly improve
the denitriding efficiency. By comparison, reducing the bank slope could
largely increase the denitriding amount, and also improve
the denitriding efficiency. In practical applications, designing the
bank form into a gentle slope with concave shape can improve the
denitriding capacity in the riparian zone to a certain extent.