The Hyporheic Exchange And Nitrogen Transport In Upstream And Downstream Of Xilin River Reservoir

Surface water and groundwater closed relate to each other and frequently interact in semiarid areas,which is a vital driving force of the ecosystem in the semiarid areas,and a vital part of the terrestrial hydrological cycle.Study of surface-groundwater interaction can provide scientific reference for the effective utilization of water resources,restoration of aquatic ecosystem and pollution management.Two typical river sections in the upstream and downstream of the Xilin River reservoir were selected,monthly measurement and sampling were conducted from July 2016to June 2017,daily measurement and sampling were conducted during Apr.6^thh and Apr.25^th,2017.To explore the hyporheic exchange,both hydrodynamic and hydrochemistry methods were applied.Hyporheic exchange was estimated by Darcy’s law.The migration characteristics and source/sink effect of nitrite in hyporheic zone were further explored.The main conclusions are as follows:1.In the upstream of the reservoir,groundwater is mainly discharged into river,and the flow pattern of the HZ zone is upwelling.The vertical exchange flux between the river and groundwater in the upstream reach was greater in 2016 than 2017.During the spring snowmelt period,the interaction direction between river water and groundwater unchanged.The vertical exchange flux is between 0.02×10^-33 m³·s^-11 and 0.13×10^-33 m³·s^-1.Affected by damming,the downstream river of the reservoir mainly recharged into groundwater,and the flow pattern in the HZ zone is downwelling.The river water also recharged to the groundwater on the river bank.The vertical exchange flux of the downstream reach in 2016 is greater than that in 2017 and the transverse seepage is higher in2017.During the spring snowmelt period,the river section is also dominated by river water recharge,with the vertical seepage flux between 0.02×10^-33 m³·s^-11 and 0.29×10^-33 m³·s^-1.The amount of groundwater recharge to the river in RB1 was small and change steadily,with an average value of 0.27×10^-33 m·s^-1.The amount of groundwater recharge to the river in RB2 fluctuated between 1.31¹.99×10^-33 m·s^-1.2.Hydrochemical parameters in upstream and downstream of the reservoir varied during the sampling period.In the upstream,the pH of the water samplings was higher than 7 and the pH of the HZ water was lower than 7.During the spring flood,the pH of river water was higher than the HZ water.The EC value of river water was lower than the HZ water.The EC value of river water is similar to that of the HZ water.During the spring flood,the pH of the river water slightly increased,with an average value of 7.5.The EC value of river water is relatively stable and higher than HZ water during spring flood.The hydrochemical types of both the upstream and downstream of the reservoir are Ca-Cl type,with no obvious seasonal variation.The concentration of various ions in the downstream of the reservoir is higher than that in the upstream.Ca²⁺and Cl^-are the two dominant ions.4.The concentration distribution of NO₃^-in water bodies varied in the upstream and downstream of the reservoir.The concentration of NO₃^-in the upstream river and HZ was higher than that in the downstream,while the concentration of NO₃^-in the downstream river bank was higher.Upwelling is dominant in the upstream HZ,with nitrification occurrence in the HZ.HZ played a role of source to river water and a sink to the groundwater.The river morphology of the downstream showed a downwelling.Downwelling is dominant in the downstream HZ,with denitrification occurrence in the HZ.HZ played a role of sink to river water and a source to the groundwater.