The Effects Of Heterogeneity Of Hyporheic Zone On Nitrogen Migration And Transformation Under Fluctuation Of River Level

Nitrogen pollutions of soil,groundwater and surface water are global environmental problems.Irrational uses of a large amount of nitrogen fertilizers lead to severe pollutions in the soil,groundwater and surface water.The nitrogen percolates into the groundwater through the soil along with recharge processes,then the nitrogen in the groundwater is discharged to the river,which further aggravates the nitrogen pollution in the river.The hyporheic zone is an important part of the river ecosystem,which plays an important role in removing nitrate from rivers and reducing the nitrate transport downstream.The water flow in the hyporheic zone is the driving force of multiple biogeochemical processes.Precipitations,floods,and dam releases can cause periodic fluctuations of the river stages.Under natural conditions,the hyporheic zone is usually heterogenous,which results in the complicated flow field in the zone and significantly affects the biogeochemical reaction process of nitrogen.In this study,a two-dimensional numerical model,which couples variable saturated flow and reactive transport of the nitrogen,was proposed using PFLOTRAN.In this model,the effects of the river stage fluctuation and the heterogeneity of the hyporheic zone on the nitrogen transport and reaction were studied.First,the dynamic characteristics of the groundwater levels,groundwater flow velocities,exchanges of the river-aquifer,and nitrogen concentrations were investigated.Then,the nitrate removal efficiency and nitrate removal rate in the hyporheic zone were calculated.Finally,the influences of the amplitude of the river stage fluctuation and the heterogeneity of the hyporheic zone on the biogeochemical reaction were analyzed.The results indicate that the hyporheic zone has the ability to remove nitrate.As the amplitude of river stage fluctuations increased from 0.3 m to 0.7 m,nitrate removal efficiency in the hyporheic zone increased from 7.3% to 9.8%.In a period of fluctuation of river stage,the nitrate removal efficiency and nitrate removal rate in the homogeneous hyporheic zone are 8.6% and 4.09 g/m/d,respectively.By contrast,the nitrate removal efficiency and nitrate removal rate in the heterogeneous hyporheic zone are 6.4% and 2.62 g/m/d,respectively.The heterogeneity of the hyporheic zone reduces the nitrate removal efficiency and nitrate removal rate.The results of this study can be used to further understand the transport and reaction of nitrogen in hyporheic zones.