Transformation Of Nitrogen And Characteristics Of Microbial Response In Riverbed Sediments During River Bank Filtration

With the increasing eutrophication and nitrogen pollution in the river,the groundwater during bank filtration is also facing the potential risk of nitrogen pollution.The riverbed sediment is the first area through which the riverbank filtration（RBF）stimulates the infiltration of the river,and it is also the area with the greatest physical,chemical and biological gradient change between the river and groundwater.With river infiltration,nitrogen biogeochemistry in riverbed sediments will affect the migration and transformation of different forms of nitrogen,and affect the quality of groundwater.Therefore,it is of great theoretical and practical significance to understand the characteristics of nitrogen migration and transformation in riverbed sediments during bank filtration,for enriching the research contents of surface-groundwater interaction,maintaining the ecological security of river and ensuring the safety of groundwater supply quality.Based on this,funded by the national natural science fund project,this study chose Liao River reach（Tieling-Shifosi reach）near a riverside well field in Shenyang as the study area,took the riverbed sediment on the typical cross section as the research object.Through field monitoring and high precision methods of in situ test,analysis of forms and distribution of nitrogen in the river,river sediment and pore water,this study revealed the migration and transformation rule of nitrogen in the riverbed sediment,calculated one dimensional convection diffusion flux of NH₄⁺-N in the infiltration process,explored the characteristics of the microbial community structure and its response to changes in environmental factors.Through this study,the following conclusions are obtained:（1）The granularity compositions of riverbed sediments were mainly silt,which was controlled by hydrodynamic conditions of the river.The compositions of riverbed sediments had obvious temporal and spatial changes,which affected the environmental characteristics of pore water in sediments.The pore water had obvious redox interface at different hydrodynamic zones in riverbed cross-section.Compared with the high-water level stage,the penetration depth of DO in riverbed sediments in the nearshore zone decreased in the low water level stage,while the change was little in the middle transition zone,while the increase was obvious in the center zone of the riverbed.（2）In the range of 30 cm depth of riverbed sediments,the content of NO₃^--N in pore water decreased gradually with the increase of depth,while the content of NH₄⁺-N increased gradually.However,due to the influence of granularity compositions of riverbed sediments,the variation ranges of different monitoring points had seasonal variations to different degrees.The nutrient load of nitrogen in the riverbed sediments was relatively high and was in the state of moderate pollution.Transformable nitrogen from sediments accounted for a higher proportion of total nitrogen,and riverbed sediments had potential risk of nitrogen release.The contents of different forms of transformable nitrogen in the sediments were from high to low as follows:SOEF-N>IEF-N>SAEF-N>WAEF-N.（3）Nitrogen migration and transformation in pore water during bank filtration mainly included adsorption,denitrification and mineralization of organic matter.As the"source"of NH₄⁺-N,riverbed sediments had strong mineralization of organic nitrogen in the sediment,and NH₄⁺-N generated by it would lead to the increase of ammonia nitrogen content in the groundwater,threatening the safety of groundwater quality.Based on the one-dimensional convective diffusion model,the NH₄⁺-N fluxes in the river infiltration process at high and low water levels were the center zone,the middle transition zone and the nearshore zone from high to low successively,in which the NH₄⁺-N fluxes in the center zone were the largest.（4）The diversity and relative abundance（phylum and genus levels）of microbial communities in surface riverbed sediments had obvious responses to the changes of total nitrogen and NO₃^--N and NH₄⁺-N contents in pore water.The relative abundance of nitrifying microorganisms decreased vertically with the increase of depth,while denitrifying microorganisms increased,indicating that nitrification was more obvious under aerobic condition in shallow layer,while denitrification was more obvious under anaerobic condition in the deep layer.