Vertical Hyporheic Exchange And Nitrogen Transport And Transformation Of Straight And Meandering Reaches In Prairie River

Hyporheic exchange is a key link of river water cycling,and the hyporheic nitrogen cycling is an important part of river biogeochemical cycling.It is of great significance to explore the vertical hyporheic exchange and nitrogen transport and transformation process of prairie river in semi-arid region for improving river water environment and maintaining water ecosystem stability.In this study,we selected the straight and meandering reaches of the Xilin River as the research object.Combined with hydrodynamic and hydrochemical methods,the hyporheic exchange pattern and nitrogen transport and transformation characteristics together with their influencing factors in vertical hyporheic zone of the Xilin River were explored.This study can provide scientific basis and practical suggestions for the control and management of nitrogen pollution in prairie rivers.The main results are as follows:（1）The vertical hyporheic exchange process of the straight reach and meandering reach of the Xilin River was active during the summer flooding season in 2020 and2021.The vertical hyporheic exchange of the two reaches was dominated by downwelling in the flooding season of 2020,while upwelling in 2021.Flood and rainfall storm could increase the intensity of vertical hyporheic exchange and even reversed the exchange direction.The vertical hydraulic gradient gradually decreased with the increase of depth.The vertical hydraulic gradient and the exchange capacity at the depth of 100cm were both close to 0,which could be seen as the lower boundary of the vertical hyporheic exchange.The vertical exchange capacity was affected by bed topography,river sinuosity,hydraulic conductivity coefficient and other factors.The exchange capacity of the straight reach could reach 0.59m/d,and the relatively high value was found at the position where channel width and bed topography changed.The relatively high exchange capacity of the meandering reach was found at the outflow point with relatively high sinuosity and the value could reach 2.34m/d.（2）The nitrogen transport and transformation processes in the vertical hyporheic zone of the straight and meandering reaches of the Xilin River were determined by mixing and dilution effects and biogeochemical reactions of nitrogen.The types of nitrogen transformation reactions in the vertical hyporheic zone were spatially variable.Ammonification and nitrification reactions mainly occurred in the shallow sediments.The shallow sediments between riverbed surface and the depth of 50cm was dominated by NO₃^-production,which could be regarded as the source of NO₃^-.The deep sediments between the depth of 50cm and 100cm was dominated by NO₃^-removal,which could be regarded as the sink of NO₃^-.The hydraulic residence time of the two reaches increased with the increase of depth.The NO₃^-removal rates could achieve 84%and 98%when the residence time was 14d and 17d,respectively.The removal rate of NO₃^-was stronger in the upwelling than in the downwelling,reaching 27%and 44%respectively in the upwelling of straight and meandering reaches.（3）In the straight reach,the riffle was dominated by the upwelling,while the pool was dominated by the downwelling.The highest NO₃^-removal rates in riffle and pool were 67%and 55%,respectively.The inflow point and outflow point of the meandering reach were dominated by the downwelling.The apex of meander was dominated by the upwelling,with the NO₃^-removal rate reaching 46%.The nitrogen purification effect was relatively strong in the riffle-pool sequence and the meandering unit,so the nitrogen removal capacity of the vertical hyporheic zone could be further strengthened by protecting and constructing the diversity of river topography.（4）The vertical hyporheic zone of the Xilin River showed nitrogen removal capacity,but flooding and rainfall events increased the nitrogen source in the river and hyporheic zone,which could be further transported into deep sediments by hyporheic exchange.The intense hydrological fluctuation during flooding season might increase the risk of nitrogen pollution.Therefore,it is necessary to strengthen the management of river corridors in the flooding season,ensure the hydrological connectivity and flood discharge capacity,and promote hyporheic exchange,nitrogen retention and purification performance.