The Influences Of Hyporheic Water Exchange On Nitrogen Concentrations Of Pore Water In Sediment Of The Weihe River

In the river system,the hyporheic zone is an important place for the removal of nitrogen-containing pollutants.Hyporheic exchange can cause the heterogeneity of the oxygen environment in the river bed sediments,which in turn affects the conversion of nitrogen-containing pollutants.In this study,the Xi'an section of Weihe River was used as a research site to analyze the effects of subsurface flow and water exchange on the nitrogen-containing pollutants(NH4+,NO3-,and NO2-)in interstitial water.In August 2016 and January 2017,respectively,water exchange tests were conducted in the south bank of the Weihe River.0-45 cm sediments and interstitial water were collected,and analyze the concentration of nitrogen-containing pollutants in surface water,groundwater,and interstitial water,as well as various basic physical and chemical indicators.The results indicated that:Firstly,in summer,the NH4+ concentrations of the pore water in test points in the left test area were relatively low but the NH4+ concentrations were relatively high in the central area.The concentration distribution of NO3-was opposite.The phenomenon indicated that nitrification in the left test area was relatively powerfully.Also,the major of nitrogen pollutions was NH4+ pollutants.In winter,the NH4+,NO3-and NO2-concentrations of pore water in the left test area were relatively high.The phenomenon indicated that pollution level of the left test area was relatively high.The major of nitrogen pollutions in the test area was NO3-in winter.Secondly,the pattern of the water exchange was upwelling in the summer,and the average upwelling fluxes was 360mm/d.The major pattern of the water exchange was downwelling in the winter,the average downwelling fluxes was 80mm/d.Water exchange in summer was powerfully relative to the process in winter.Thirdly,the magnitudes of the water exchange in hyporheic zone were influence by the riverbed topography and particle size of sediment.There was a negative correlation between the upwelling fluxes and the water depth in summer.In test points with deeper water depths,the hydraulic pressure is greater,which may hinder the process of upwelling.And there was a significant negative correlation between the downwelling fluxes and the water depth in the winter.This may be raised by sediment particle size.Fourthly,in summer,the upwelling has a greater influence on the concentrations of NH4+ in the pore water,a non-linear relationship was observed between NH4+ concentrations and upwelling fluxes.Notably,increasing upwelling fluxes less than 400 mm/d resulted in high NH4+ concentrations,whereas fluxes exceeding 400 mm/d led to low NH4+ concentrations.There was a non-linear correlation between the downwelling fluxes and NH4+ and NO3-concentrations in pore water during winter test period.Meanwhile,increasing downwelling fluxes less than 100 mm/d resulted in low NH4+ and NO3-concentrations,whereas fluxes exceeding 100 mm/d led to high NH4+ and NO3-concentrations.These relationships were derived by analyzing the effect of water exchange on biogeochemical activity and nitrogen transformation.