Numerical Simulations Of Tidal Loading Effects On Groundwater Head Fluctuations In A Coastal Aquifer System At The Bailang Estuary Of Southern Coast Of Laizhou Bay

The periodic tidal level fluctuations will result in the periodic variations in the overlying load on the aquifer system,thereby leading to the corresponding periodic fluctuations of groundwater head in the aquifer system,namely,the tidal loading effect.In an aquifer system of coastal zone,the interaction between groundwater and seawater will exert remarkable influences on the hydrodynamic characteristics and solute transport process of groundwater in the aquifer,of which the tidal loading effect is an important factor in the interaction between seawater and groundwater.Researchers have conducted numerous analytical studies on the fluctuations of groundwater levels caused by the tides,in which the tidal loading effect has been taken into consideration.Nevertheless,such effect has not included in some common commercial numerical software,such as the MODFLOW and FEFLOW.In this paper,we took the coastal aquifer system at the Bailang Estuary of Southern Coast of Laizhou Bay as the case study area which includes the lower confined aquifer,middle aquitard,and an unconfined aquifer—according to the data at two hydrogeological boreholes near the research field,namely,ZK03(monitoring well for phreatic water level)and ZK08(monitoring well for confined level),and then determined the key hydrogeological parameters based on the aquifer's lithology,such as permeability coefficient,specific storage,porosity,tidal loading efficiency,etc.We conducted numerical simulations on the groundwater regime in the coastal multilayered aquifer system under the tidal tiding effect using the numerical simulation,code MARUN,and identified the best hydrogeological parameters of the aquifer system using the trial and error method.On this basis of the identified best model parameters,we further analyzed the groundwater heads,groundwater flow fields in the aquifer system at mid-rising tide,high tide,mid-falling tide,and low tide during one tidal cycle,and found that the average water level and phase difference decreased and the amplitude increased in the seaward direction.The magnitude of the groundwater flow velocity in the aquifer of inland region was significantly higher than that in oceanic region.Then,we conducted sensitivity analysis with respect to the permeability coefficient,tidal loading efficiency,as well as water table of the inland boundary.When the permeability coefficient increased by 1.44 times,the confined water head amplitude would increase by 1.36 times.When the permeability coefficient decreases to 29%,its amplitude would decreases to 33%.The tidal loading efficiency mainly controlled the change of the phase of groundwater head fluctuation in the confined aquifer.On the basis of the boundary flow analysis,we made a quantitative evaluation of submarine groundwater discharge(SGD)of the aquifer system,and found that the groundwater discharged into the sea mainly through the beach surface and the terminal area of aquifer.The SGD through beach surface was 5.3 m~2/d,accounting for 67.9%of the total SGD.The SGD through the terminal area of aquifer was 2.5 m~2/d,accounting for 32.1%of the total SGD.The inland fresh water recharge was 0.24 m~2/d,accounting for 3%of the total SGD.