Migration And Transformation Process Of Iron And Manganese During River Water Infiltration Affected By Riverbed Erosion And Deposition

Riverbed sediments are the key zone and important interface for the interaction between river water and groundwater.Due to the changes in composition,structure and thickness of the riverbed sediments caused by the erosion and deposition of the riverbed,the hydrodynamic conditions,acid-base conditions and redox conditions of river water infiltration are strongly affected.This impact has led to the transformation of river-groundwater hydraulic linkages and the complexity of iron-manganese biogeochemical processes.At present,the coupling mechanism of river water infiltration hydrodynamics and iron-manganese migration and transformation processes under the influence of riverbed erosion and deposition has not been fully investigated.This research takes Huangjia groundwater source area in Shenyang as the research area,and applies technologies such as water drilling,dynamic observation,indoor simulation and numerical simulation.On the basis of identifying the lithological composition and thickness changes of Liaohe riverbed sediments,to identify the temporal and spatial distribution of iron and manganese,the change law of redox zoning and the characteristics of microbial response in the process of river water infiltration.Explore the mechanisms of iron and manganese migration and transformation under the influence of riverbed sediment thickness,and further reveal the hydrodynamics-iron and manganese migration and transformation coupling features in the river water infiltration zone under the influence of riverbed erosion and deposition.This research can provide a scientific basis for enriching the research content of the surface water-groundwater interaction zone,deepening the understanding of the biogeochemical cycle mechanism in the riverbed sediments under the conditions of riverbed erosion and deposition,and ensuring water quality and ecological security.The following main conclusions and achievements have been obtained through this research:(1)During the investigation period(September to November 2020),the riverbed sediments in the river reach were generally in a state of scouring in the low water stage,and the scour depth was 4?9cm.Affected by the erosion of the riverbed,the permeability of the riverbed sediments had obvious temporal and spatial variability.the riverbed sediment permeability in the weak scour area is lower than the medium-strong scour area,and the riverbed sediment permeability in the low water stage is higher than the high water stage.(2)Affected by the scour of the riverbed,the lithological grains of the riverbed sediments became coarser,the infiltration rate of the river water increased,and the nutrient flux increased.The potential geochemical activity of iron and manganese and the activity of organic carbon in the riverbed sediments gradually increased,and the correlation between carbon and iron-manganese species is stronger.(3)The erosion and deposition of the riverbed significantly affects the redox zoning in the process of river water infiltration.The redox zone in the nearshore zone moved up by 5.5-23 cm,and moved down by 0.1-10 cm in the intermediate transition zone and the central zone of the riverbed.The spatial distribution and redox zoning of dominant bacteria in riverbed sediments have co-evolutionary characteristics.(4)The reductive dissolution of iron and manganese oxides and the oxidation of organic matter-bound iron and manganese are the main biogeochemical reactions that control the migration and transformation of iron and manganese in the process of river water infiltration.During the process,the river water infiltration environment changed from anoxic to relatively oxygen-rich,the redox zone became narrower,and the reduction of iron and manganese oxides weakened.(5)The coupled simulation results of hydrodynamics and iron and manganese migration and transformation in the river infiltration zone show that,driven by riverbed deposition and groundwater exploitation,the iron and manganese migration and transformation processes have distinct characteristics in the process of river water-groundwater connection from saturated connection to complete disconnection.In the saturated connection stage of river water and groundwater,anoxic conditions drive the reduction of iron and manganese oxides.In the transitional disconnection stage,DO intrusion dominates the oxidation process of Mn²⁺and Fe²⁺.In the complete disconnection stage,iron and manganese become the electron acceptor utilized by microorganisms,and the reduction of iron and manganese oxides will gradually take the leading role.