Study On Hydrochemical Environment And Seawater-groundwater Exchange In A Reed-swamp Tidal Flat At Yang River Estuary Of Jiaozhou Bay

Reed-swamp is an important wetland ecosystem in the intertidal zone.The study area is located in the muddy tidal flat at the Yanghe River estuary of Jiaozhou Bay.There are a lot of reeds growing in the study area.At low tide,the surface and groundwater samples were collected from the bare tidal flat and reed distribution area for comparison study of the hydrochemical characteristics between the bare tidal flat and reed distribution area.The observation wells were installed to observe the groundwater heads in the tidal flat,which are used to quantify the water exchange between seawater and groundwater.The results can be summarized as follows.(1)Study on hydrochemical characteristics of the bare tidal flat.Due to evaporation and the dilution effects of tidal submergence,the concentrations of main ions are proportional to the distance from the tidal channel.The mixing of surface water with groundwater makes HCO3-/Cl-?K+/Cl-?Ca2+/Cl-ratio obviously highin groundwater in the tidal flat.The concentration of nitrate nitrogen is low,and the concentration of nitrite nitrogen and ammonium nitrogen is high.Ammonium nitrogen is significantly higher than that of nitrite nitrogen,indicating that denitrification is strong.The concentrations of Cu,Cr and Cd in the pore water of the sediments meet or close to groundwater quality standard of the third class.Correlation analysis reveals that Zn,Cr,Pb,and Cu may come from the same source.The main parameters(PH,ORP,etc.)also show certain regularity.(2)Comparative study on water chemistry of the bare tidal flat and reed distribution area.Due to the difference of the evaporation intensity between the reed area and the bare tidal flat area,the concentrations of main ions and heavy metals(except HCO3-and Hg)in the shallow groundwater are greater than those in the reedarea.The effect of plant respiration and decomposition of organic matter results in a larger amount of HCO3-in the groundwater of the reed area.Growth of reed hasenrichment effect on Hg.Because of the anaerobic environment and growth of aquatic plants,the concentration of inorganic phosphorus is higher in the reed area than in the bare tidal flat.Under the action of the concentration gradient,the migration direction of the main ions in the reed area is opposite of that in the bare tidal flat area,and the migration direction of heavy metals in the bare tidal flat and reed area is generally the same.Compared with the bare tidal flat,the fluctuation of ORP in reed area is small with a strong reducing environment.(3)Study on the seawater-groundwater based on hydraulic gradient method.The muddy tidal flat with gentle slope is favorable to the development of seepage face.At high tides,the seawater tends to penetrate into the intertidal zone,and the groundwater in the aquifer tends to discharge into the sea at low tides.The hydraulic gradient method is used to estimate the seawater-groundwater exchange rate.It was found that submarine groundwater discharge in the northern side of the tidal channel is greater than seawater infiltration,but they have the same order of magnitude.(4)Numerical simulation of seawater-groundwater exchange.The numerical model MARUN is used to simulate groundwater-seawater exchange along the north side transect of the tidal channel.The observed and simulated values of groundwater heads at the observation wells fit well.On this basis,preliminary analysis of the characteristics of groundwater flow and salinity distributions in the transect are conducted.