| In recent decades,human activities have made significant impacts on the concentrations and compositions of nutrients in bay waters.Compared with the open sea,nutrients tend to accumulate in semi-enclosed bays for a longer time,causing frequent occurrence of red tides due to eutrophication.Therefore,it is of great theoretical and practical significance to study the groundwater dynamics in nearshore aquifers under the influence of various forces.This study includes three aspects.First,typical transects around Jiaozhou Bay and Daya Bay are selected to monitor the groundwater dynamics in nearshore aquifers,and to assess the submarine groundwater discharge?SGD?and its associated nutrient fluxes.Secondly,taking a representative transect as an example,the impact of levee on the groundwater dynamics is discussed.The third is to analyze the effects of seasonal oscillations of inland groundwater levels on groundwater dynamics using a theoretical numerical model.In Jiaohzou Bay,there are four wetland types,including sandy beaches,mud flats,tidal marshes,and estuarine intertidal zones.Four typical transects representing each of the wetland types were selected to monitor the dynamics of seawater and groundwater.The SGD averaged over the observation period is estimated using generalized Darcy's law.The SGD along the four transects ranges from 3.6×10-3 to 7.6cm/d with the maximum occurring at the sandy beach.There is a positive correlation between the ratio of nitrate nitrogen to dissolved inorganic nitrogen and the vertical hydraulic conductivities.The SGD through the intertidal zone of Jiaozhou Bay and its associated fluxes of dissolved inorganic nitrogen and dissolved inorganic phosphorus are?1.9±0.45?×106 m3/d,?7.7±2.5?×105 mol/d,and?0.63±0.30?×103 mol/d,respectively.The SGD through the intertidal zone is about one-tenth of the total SGD to the entire bay estimated by the radium isotope-tracing methodField observations and numerical simulations of the groundwater dynamics in the Yanzao profile of Daya Bay were conducted.The simulation results with and without levees are compared.It is shown that with the levee,the upper saline plume contracts while the saltwater wedge moves landward.The saline groundwater discharge,the total SGD and its associated nutrient fluxes are reduced when considering the levee.Numerical simulations were conducted to investigate the groundwater dynamics caused by seasonal oscillations of inland water levels in nearshore confined aquifers.The freshwater-saltwater interface in response to the seasonal oscillations of inland water level does not always swing around the steady-state one.When K?10-4 m/s,even the maximum seawater intrusion distance caused by the fluctuation is smaller than that in the steady-state.During a seasonal cycle,the seawater intrusion time is much longer than the seawater receding time.The seawater intrusion time decreases with the fluctuation amplitude when K?10-4 m/s.The result is that seawater intrusion distance decrease with the increase of the fluctuation amplitude.Seasonal inland water level oscillations promote the seawater recirculation components of SGD. |
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