Hydrodynamic Characteristics And Hydrochemical Evolution Of Lateral Hyporheic Zone In Xi'an Section Of Weihe River

At present,the research on the hyporheic zone at domestic and overseas mostly focuses on the vertical hyporheic exchange at the bottom of the riverbed,and there are few studies on the lateral hyporheic exchange between surfacewater and groundwater.There is a lack of systematic research on the evolution law of hydrodynamic field-hydrochemical field and the mechanism of hydrogeochemical action during the lateral interaction between surfacewater and groundwater.Based on the National Natural Science Foundation of China and the key research and development projects of Shaanxi Province,this paper takes the lateral hyporheic zone of surfacewater and groundwater in Xi'an section of the Wei River as the research object.Under the theoretical guidance of hydrogeology,environmental isotope hydrology,groundwater dynamics,hydrogeochemistry and so on.Comprehensive use of high-resolution in-situ monitoring,environmental isotope tracer technology,multivariate statistical analysis,hydrogeochemical simulation and other methods and techniques to analyze the temporal and spatial variation of hydrodynamics-hydrochemistry in the river-groundwater lateral hyporheic zone and explain the main controlling factors of hydrochemical evolution in the lateral hyporheic zone.The hydrogeochemical evolution mechanism of the lateral hyporheic zone is revealed.The main research results are as follows:(1)The spatial distribution law of the structure and material composition of the lateral hyporheic zone between surfacewater and groundwater in the Xi'an section of the Weihe River is analyzed,and the influence of the lithological structure of the hyporheic zone on the material composition is explained.The structure of the hyporheic zone is mainly composed of medium-coarse sand,fine sand and sandy soil.The riverbed sediment is mainly medium-coarse sand,the surface layer of the river floodplain sediment is mainly fine sand and sand,and the deep layer is mainly medium-coarse sand.The mineral components,oxides,organic matter and soluble salts of sediments are closely related to the particle size of sediments,and the content of various components from fine particles to coarse particles gradually decreases.For fine-grained to coarse-grained substances,the mineral quartz content has dropped by about 19.7%,the calcite content has dropped by 62.5%,and the pyrite content has dropped by 20%;the iron oxide content has dropped by 50%,and the organic matter content has dropped from about 8g/Kg to 1.58 g/Kg,Around that time,the soluble salt content dropped by 40.2%.(2)The hydrodynamics of the lateral hyporheic zone between surface waterand groundwater in Xi'an section of the Weihe River have significant changes.Surfacewater and groundwater in the lateral hyporheic zone of high,normal,and low hydrological periods show different replenishment and drainage relationships.In the dry period,high water retreat period,and normal water period,the groundwater replenishes the river water,and the surfacewater intermittently recharges the groundwater during the high water period.The seepage velocity in the lateral hyporheic zone in the wet season fluctuates greatly,the seepage velocity is between0.3m/d?0.5m/d,the average seepage velocity in the normal water period is 0.2m/d,and the average seepage velocity in the lateral hyporheic zone in the dry season It is 0.15m/d.During the flood period,the seepage velocity in the lateral hyporheic zone is the largest,reaching 0.5m/d.The mixing ratio of surface waterin the lateral hyporheic zone can reach up to 82.5%in the wet season,and the lowest in the dry season,only 29%.(3)This study explained the temporal and spatial evolution of hydrochemistry in the lateral hyporheic zone between surfacewater and groundwater in the Xi'an section of the Weihe River,and divided the water environment zones of the lateral hyporheic zone.Due to the infiltration and replenishment of low-major component content of surfacewater during wet periods,the solutes in the lateral hyporheic zone are mixed and diluted,and the concentration of major components in the lateral hyporheic zone of surfacewater and groundwater during wet periods is lower than that in normal water periods.During the wet season,the hydraulic relationship between the surfacewater and the groundwater in the lateral hyporheic zone is close,showing a rapid temperature drop zone,a slow temperature drop zone and a stable temperature zone.The type of water chemistry is mainly HCO₃·SO₄-Na·Ca·Mg in high water period,HCO₃·SO₄-Ca·Na·Mg in normal water period,and HCO₃·SO₄·Cl-Ca·Na·Mg in dry period.(4)The main controlling factors of the Hydrogeochemistry evolution of the lateral hyporheic zone were identified,and the formation mechanism of the Hydrogeochemistry of the lateral hyporheic zone was revealed.The hydrochemical evolution of the lateral hyporheic zone between surfacewater and groundwater in the Xi'an section of the Weihe River is mainly controlled by hydrogeochemical effects such as mixing,leaching,and oxidation-reduction.The mixing of surfacewater and groundwater in wet and dry periods is the main controlling factor of the Hydrogeochemistry evolution in the lateral hyporheic zone.