Numerical Simulations Of Shallow Groundwater In A Muddy Tidal Flat In Daya Bay And Its Potential Influence On Hydrochemical Characteristics

The coastal intertidal mudflat is a typical zone of sea-land interactions,where the hydrodynamic conditions are the most active and complex,and the seawater-groundwater interactions are most frequent.However,the potential relationship between groundwater dynamics and hydrochemical characteristics in intertidal mudflat zone is not clear.Taking a coastal intertidal mudflat as a case study area,this paper is mainly to investigate the groundwater dynamics and the seawater-groundwater interactions,as well as the influence of the seawater-groundwater interactions on hydrochemical characteristics.It provides some scientific basis for ecological environment management in coastal areaThe study area of this paper was located in the northeast of Daya bay,which is a typical tidal mudflat with a length of about 200 m.Six observation wells were installed,including two so-called"Single-wells"and four so-called"Pair-wells"?including upper and lower sensors?,to monitor the groundwater temperature,conductivity and pressure over a spring-neap tide period with a time interval of 0.5 h.The shallow groundwater was sampled near each observation well to analyze the hydrochemical characteristics,including the major ions(Na⁺,K⁺,Ca²⁺,Mg²⁺,SO₄^2-,HCO₃^-and Cl^-),heavy metals?As and Cu?and short half-lived radium isotopes(²²³Ra and ²²⁴Ra).In the intertidal zone,the concentrations of some major ions in groundwater was gradually increasing from inland to sea and slightly higher than that in seawater,because the intertidal surface was affected by evaporation and emission.In addition,the concentrations of SO₄^2-decreased due to the sulfate reduction reaction,and the concentrations of HCO₃^-,Mg²⁺and Ca²⁺increased with the seawater infiltration;Qualitative analysis showed that As and Cu in intertidal groundwater were mainly come from inland groundwater and seawater,respectively.It was that there were slight As pollution in intertidal groundwater,while serious Cu pollution in seawater.In addition,there were a relationship between the activities of short-lived radium isotopes(²²³Ra and ²²⁴Ra)and the oxidation-deoxidation environment of groundwater.The groundwater oxidation was higher,the activities of ²²³Ra and ²²⁴Ra were lower,and vice versa.In this paper,both the generalized Darcy's law and the numerical simulations were used to evaluate the process of seawater-groundwater exchange.According to the generalized Darcy's law,the outflow was 1.20²67.94 mm/d,with an average of 88.35mm/d,while the inflow was 9.71³47.38 mm/d,with an average of 99.44 mm/d.Through the MARUN numerical simulations model,however,the inflow was simulated to be 0.02⁰.48 mm/d,with an average of 0.34 mm/d,while the outflow of groundwater was 0⁰.95 mm/d,with an average of 0.74 mm/d.There was a significant difference between the two methods.The main reason for this was that the measured the vertical hydraulic conductivity of the surface sediments for the method of generalized Darcy's law was much greater than its real value since the consolidated structure of sediments was destroyed during measurements.The numerical simulations method was used to simulate the groundwater dynamics over a spring-neap tidal period.The result showed the seawater-groundwater interactions could significantly affect the concentrations of major ions and heavy metals in intertidal groundwater,but had little effects on the activities of ²²³Ra and ²²⁴Ra.