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 of and  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  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  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.