Study On Hydrodynamic,microbial-nitrides Cooperative Transport Mechanism In Hyporheic Zone Between River And Groundwater

The interaction zone between rivers and groundwater is a key zone of the earth with strong biogeochemical activities,which plays an important role in the global water cycle and material cycle.In recent years,the intensification of human activities has caused more and more serious nitrogen pollution in rivers and groundwater,which has brought serious harm to human health and ecological environment.The river-groundwater interaction zone has the ability to reduce nitrogen pollution due to its unique physical,chemical and biological characteristics.However,due to the limitation of in situ monitoring methods and the complex characteristics of the interaction zone,the complexity of nitrogen biogeochemical processes in the interaction zone has been the bottleneck of nitrogen pollution control.Therefore,the establishment of hydrodynamic,microbial and nitrogen cooperative transport simulation in the river-groundwater interaction zone is of great significance to describe the nitrogen biogeochemical process in the interaction zone and to prevent and repair river and groundwater pollution.Based on the research of the research group,this paper takes the typical section as the object to deeply study the hydrodynamic,microbial-nitride cooperative transport in the river-groundwater interaction zone.This study simulates the migration and transformation process of nitrogen under different hydraulic gradients by building an indoor two-dimensional seepage tank device,analyzes the environmental change characteristics of the interaction zone and the response mechanism of the microbial community,obtains important parameters by using indoor soil column seepage and adsorption experiments,construct a two-dimensional coupling model of water flow and solute transport,and explore the biogeochemical process of inorganic nitrogen when rivers recharge groundwater.Then,this study further investigates nitrogen migration and transformation mechanism under different river cut aquifers and different nutrient concentration,providing a reference for nitrogen pollution prevention and control.The main conclusions are as follows:（1）The variation characteristics of hydrodynamic field and hydrochemical field under different hydraulic gradients in the interaction zone were proved.Under infiltration modes with different hydraulic gradients,the water flow in the upper layer of the seepage channel is inclined to the lower right,and the greater the hydraulic gradient is,the more obvious the gradient is.Influenced by the bottom boundary,the flow direction tends to be stable with the increase of water depth.The larger the hydraulic gradient is,the stronger the mixing effect between water flow and medium is.The longer it takes for ORP to change from fluctuating state to stable state,the smaller the hydraulic gradient is,the more fully the reaction is carried out,and the more obvious the decline of DO and DOC values.（2）The water chemical zoning and nitrogen migration and transformation law of the interaction zone were revealed.There is no obvious acid-base zone inside the tank,but there is REDOX zone.Along the water depth,it can be divided into four types:oxidation zone,oxidation-reduction zone,weak oxidation zone and reduction zone.The higher the hydraulic gradient,the faster the oxygen supply rate,the less oxygen consumption,the smaller the reduction zone and the larger the oxygen-reduction zone.Along the water depth direction,nitrification mainly consumes NH₄⁺-N in the shallow zone.The larger the hydraulic gradient is,the lower the ammonia nitrogen interception of the interzone medium is.The intensity of denitrification and DRNA increased with the increase of water depth,which promoted the degradation rate of NO₃^--N.The larger the hydraulic gradient was,the contact time between water flow and medium decreased,and the degradation rate decreased.（3）The response characteristics of microorganisms to nitrogen migration and transformation under different hydraulic gradients were discovered.Nitrospira,Candidatus Nitrososphaera and Nitrosopumilus maritimus are the dominant bacteria controlling nitrification in the oxidation zone and the oxidation-reduction zone under the two hydraulic gradiations.Under their mediation,the ammonia nitrogen value in the layer decreases,and the nitrate nitrogen value increases.Pseudomonas and Bacillu dominate denitrification in the oxidation-reduction zone,weak reduction zone and reduction zone under the two hydraulic gradiants,while Arthrobacter and Desulfvibrio dominate DRNA dominate denitrification.Nitrate nitrogen is reduced to gaseous nitrogen or ammonia nitrogen in these processes,which reduces the nitrate content.In the reducing layer with low hydraulic gradient,the anaerobic ammonia oxidation process dominated by Aammox bacteria mainly occurs,which reduces the ammonia nitrogen content in the system.（4）A coupled model of hydrodynamic and solute transport in the interaction zone was constructed to reveal the mechanism of nitrogen biogeochemical processes under different changing conditions.The migration and transformation process of main solutes（DO,DOC,NO₃^--N,NH₄⁺-N）was simulated,and the simulated values of the model and the measured data were well fitted（R²>0.88）.When the river cuts the aquifer incompletely,the solute migration process in the interzone is affected by the water boundary.The flow carrying solute flows to the lower right.The larger the hydraulic gradient,the higher the flow velocity through the reaction zone,the shorter the reaction time,and the lower the nitrogen conversion intensity.With the increase of DO concentration in the system,the intensity of nitrification increases,the value of NH₄⁺-N decreases significantly,and the denitrification is limited,which inhibits the conversion of NO₃^--N and is not conducive to the permanent removal of inorganic nitrogen in the reaction region.When the DOC concentration in the system increases,the C/N ratio increases,the reaction in the reaction zone becomes more intense,and the degradation effect of nitrate nitrogen and ammonia nitrogen is improved.