Numerical Simulation Of Groundwater Flow And Salt Transport In A Sandy Tidal Aquifer System In Jiaozhou Bay,China

Intertidal zone is an important area where the hydrodynamic conditions are the most active and complex,and the seawater-groundwater exchanges are most frequent in coastal areas.It plays a key role in the ecological environment and material circulation of coastal areas.Therefore,it is absolutely essential to study tide-induced groundwater flow and salt transport in beach aquifer systems and to systematically quantify the seawater-groundwater exchanges through the sea-land interface.The research area of this paper is located on the southwestern coast of Jiaozhou Bay.Six pair-wells are installed along the~1000 m long intertidal zone transect perpendicular to the coastline.In each well,two transducers are installed at a certain depth below the ground surface,with their vertical distance of 52.5 cm.The transducers automatically record the temperature,water pressure and conductivity of the groundwater with a frequency of once per hour.The total record time period is 437hours.According to the field observation data and the generalized Darcy's law,a large positive groundwater head difference between the upper and lower transducers were measured at the landward wells W1,W2 and W3,the surface discharge is mainly concentrated in the sandy area at W1-W3,while the inflow and outflow of the muddy area at W4-W6 is small.The total seawater-groundwater exchange through sea-land interface is dominated by discharge and the inflow is very small.The SGD(41.81 m~2/d)is more than 100 times of the inflow(0.40 m~2/d).The profile of the sandy beach aquifer system is more than 3 km long.The profile is a multi-layer aquifer system with two aquitard layers,and the sedimentary layers are inclined to the sea with a slope of about 3‰,which is substantially parallel to the beach surface.Numerical sensitivity analyses were conducted with respect to seepage surface and hydraulic conductivity of each layer.The following main conclusions can be drawn from the numerical simulation results:The exchange rate reaches its peak at the slope break of the beach surface,and the discharge rate is as high as 0.035 m/d,forming a drainage channel for groundwater.The discharge rate on the land-sea interface is much higher than inflow rate,and the discharge is mainly concentrated in the landward part at wells W1-W3,which is consistent with the results of the generalized Darcy's law.The numerical simulation results of salinity do not have the typical salt plumes under the influence of tidal pumping,the freshwater continuously drives the saline water out of the aquifer into the sea under the action of larger inland hydraulic gradient.The salinity distribution of actual beaches is the result of a combination of various factors.From the sensitivity analysis,it can be seen that the simulated head is very sensitive to the vertical hydraulic conductivity of the surface layer and the hydraulic conductivity of the bottom coarse sand gravel layer,but insensitive to the horizontal hydraulic conductivity of the surface layer and the hydraulic conductivity of the aquitard layers and the medium coarse sand layer.The seepage face has a great influence on the simulation results.When simulating an actual beach with a large tidal range,the effect of seepage face can't be neglected.In addition,due to the complexity of the evaporation process in the beach surface(including seepage face),this paper has not carried out numerical simulation studies about this.Therefore,the further research target is to explore the quasi-steady-state numerical solution considering the evaporation process.