Quantifyingseawater-groundwater Exchange Rates

As the transitional zone between sea and land,the exchange of mass and energy is more complex and rapid in the intertidal zone than other places.The groundwater flow and solute transport in the intertidal zone becomes complicated due to the influence of the inland groundwater and various marine oscillations.Based on the analysis of field observation data and numerical simulation,this paper investigated the dynamics of groundwater in asandy beach subjected to semi diurnal tides.Focusing on the vertical hydraulic gradients and pore water salinity in a sandy beach located in the southwest of Jiaozhou Bay,we found that tides provide significant forcing for flow and salt transport in the near shore shallow aquifer.The rising and falling rates of the tide directly influence the seawater and groundwater exchange rate.The spatial and temporal variations of the vertical hydraulic gradients demonstrate the complexity and dynamic nature of the processes and the extent of mixing between seawater and fresh groundwater in a subterranean estuary.During a spring and neap tidal cycle,the exchange rate between seawater and groundwater varies with the tide level.The exchange rate between seawater and groundwater reaches maximum in the upper intertidal zone and decreases in the seaward direction.The exchange rate at each well was obtained based on the generalized Darcy' law,then the results were linearlyinterpolated along the whole transect.The total submarine groundwater discharge(SGD)along the entire transect over a spring-neap tidal period was estimated to be 38.43 m3d-1m-1,which was much larger than the inflow(seawater infiltrating into the beach)of 0.63m3d-1m-1.The SGD from the seepage face was 27.56 m3 d-1 m-1,accounting for 71.7% of the total SGD.Based on the sampling and analysis of the common ions,heavy metals and nutrients of the pore water in the intertidal zone,the following hydrochemical featuresin the intertidal zone are obtained,1)Due to evaporation,the concentration of common ions in the pore water of the intertidal zone decreases in the seaward direction;2)Concentrations of Cu,Cr,were the highest among the heavy metals,belong to the third class and four class groundwater quality respectively;3)The nitrate and nitrite ions in pore water in the intertidal zonecan convert between each other due to tidal fluctuation.During rising tides,tidesbring more seawater,which is rich in nitrite ion,into the pore water in the intertidal zone,then the nitrite ion in the groundwater is converted to nitrate ion under the action of nitrate bacteria.During the ebb tide,ground-water isunder anoxic condition,nitrate ion is converted to nitrite ion under the action of denitrifying bacteria.Based on the field survey data,the two-dimensional numerical groundwater model MARUN was used to simulate the dynamic of the groundwater flow and solute transport considering terrestrial fresh groundwater discharge,tides,seepage faces,density difference between seawater and groundwater and other factors.Due tothe influence of the beach topography,the groundwater flow in the tidal flat transect is mainly composed of a large,tidal flat-induced circulation.