Numerical Simulation Of Seawater-Groundwater Exchange In Qingxiang Profile Of Laizhou Bay,China

Laizhou Bay,located in the south of Bohai Sea,possesses abundant resources and occupies an extremely important position in the economic development system of the Shandong province.However,the waters of Laizhou Bay are seriously polluted in recent years,which affect the sustainable development of industry in the coastal Numerical simulations of the groundwater in the aquifer system in this area are helpful for us to learn and understand the temporal-spatial distributions of submarine groundwater discharges.The study can provide base information for the protection and management of coastal ecological environment in this area.A typical transect perpendicular to the coastline was chosen in the south coast of Laizhou Bay near Qingxiang village.The tidal flat of the transect has an average slope of 0.4‰.Fourteen pair-wells are arranged evenly along the vertical coastline.There are two transducers in each pair-well,which were used to monitor pressure,temperature and conductivity of seawater or groundwater once per hour for 33 days.The vertical distance between the upper and lower transducers of each pair-well is fixed to be 52.8 cm.According to geological data,there is a large amount of underground brine in this study area.This article summarizes the formation and distribution characteristics of the underground brine,and combining with the measured data,the salinity of the aquifer system of the study area has three typical values:the surface of the tidal flat is a relatively low salinity zone with a salinity of 40g/L,under the surface there is a transition zone with the salinity ranging from 40 g/L to 120 g/L,and the salinity of the other deeper zones is 120 g/L.An observation period of 360 hours is chosen to simulate the groundwater flow in the multi-layered aquifer system,taking into account the effects of tides,terrain,brine's variable density and seepage surface,by using the numerical model MARUN.The submarine groundwater discharge,the inland fresh groundwater recharge and the recirculation of seawater were calculated to be 10.05 m³d^-1m^-1,9.49 m³d^-1m^-1and 0.56m³d^-1m^-1,respectively.The inland fresh groundwater recharge accounts for 94.43%of the total SGD.The recirculated seawater caused by the tides accounts for 5.57%of the total SGD.It is found that 63.7%of the total SGD occurred in the area of high-permeability,which indicates that the permeability of surface sediments has great influence on groundwater discharge.A tidal flat with a small slope is beneficial to the development of seepage face.We find that the longest seepage face length in the Qingxiang profile is about 3800 m,which occurred at the lowest tide level.The exit point located in the high intertidal zone.The location is almost insensitive to tidal level fluctuations.According to the analysis of how the exchange of seawater and groundwater varies with the tide fluctuation in a solar day,we can find that the greater the tidal range,the greater the exchange of seawater and groundwater.The results show that the inflow quantity of seawater is far less than submarine groundwater discharge,indicating that the groundwater with high salinity may hinder the process of infiltration of seawater.