Study On The Transport Mechanism Of Radium Isotope In Coastal Aquifer And Its Tracer Seawater-groundwater Interaction Process

The interaction between seawater and groundwater is one of the main forms of sea-land interaction in the coastal zone.The environmental and ecological problems in the coastal zone are all related to groundwater and seawater.In recent years,with the impact of climate change and human activities,the typical interaction between seawater and groundwater,represented by seawater intrusion and submarine groundwater discharge,has caused large-scale underground freshwater pollution in coastal areas and the deterioration of offshore ecological environment,which has seriously hindered the sustainable social and economic development of coastal areas.Radium isotopes are natural tracers for the interaction between sea and land,which is widely used in quantifying water-rock exchange rate,water residence time,submarine groundwater discharge（SGD）and other research fields.In this paper,the southern coast of Laizhou Bay was taken as the research area,and the transport process of radium isotope in coastal aquifer was elucidated through field investigation and laboratory simulation tests.The model of radium isotope reaction transport in coastal aquifer was established,and its application in tracing seawater intrusion and submarine groundwater discharge was carried out.Geochemical characteristics of water samples indicate that the groundwater originates from modern atmospheric precipitation,and is partially supplemented by local rivers.Seawater intrusion is the main source of groundwater salinity.Due to the influence of seawater intrusion,the concentration of Cl^-in groundwater in the research area gradually increased from land to sea.And the chemical type of groundwater gradually changed from Ca·Na-HCO₃to Na·Ca-Cl from south to north,and finally to Na-Cl.Influenced by the variation of groundwater salinity gradient,the spatial activity of radium isotope in groundwater varies greatly.The salinity is the main factor controlling radium isotope activity in groundwater,and there is a positive correlation between them(²²⁴Ra:R²=0.64;²²⁶Ra:R²=0.62;²²⁸Ra:R²=0.54;²²³Ra:R²=0.477).The Ca,Sr and other elements in the aquifer system will compete with radium isotopes for adsorption points,which will affect the activity of radium isotopes in the aquifer.The core samples of WF80 on the southern coast of Laizhou Bay were selected to experiment.The high salinity non-radium groundwater sample was prepared by using local underground brine.The desorption/adsorption behaviors of ²²⁴Ra and ²²⁶Ra in solid-liquid phase under the control of salinity gradient and layers were studied through leaching experiment.The results showed that there was a significant positive correlation between the salinity and radium isotope activity.Compared with the field data,the correlation between the two was better in leaching experiment(²²⁴Ra and Cl^-:R²=0.92;²²⁶Ra and Cl^-:R²=0.82).The control mechanism of layer on radium isotopes was analyzed from the perspective of grain size.The total desorption capacity of²²⁴Ra and ²²⁶Ra in aquifer is greater than that of other aquitards.The larger the grain size of sediments,the greater the Ra desorption capacity.Based on the results of laboratory leaching experiment,a 1-D reactive transport model of radium isotope was established to simulate the transport process of radium isotope in coastal aquifer with different salinity gradients.On this basis,a mass balance model coupled with radium isotopes was used to evaluate the seawater intrusion fluxes in the southern coast of Laizhou Bay.The simulation results showed that the calculated water residence time of YW1 profile and YW2 profile were 13.62 d and 9.49 d,and the seawater intrusion fluxes were 12.39 cm/d and 17.42 cm/d,respectively.This study focused on the transport of radium isotopes in aquifers and studied the environmental effects of radium isotope enrichment under seawater intrusion conditions.By conducting a survey of radium isotopes in groundwater on the southern coast of Laizhou Bay,the sea-land interaction was traced,and the characteristics of desorption and diffusion of radium isotope were understood.This study contributes to the understanding of the geochemical behavior of radium isotopes in aquifers and provides guidance for selecting a suitable groundwater end-member in SGD estimation.Moreover,it is of great academic significance and application value.