Numerical Simulation Of Groundwater Flow And Salt Transport In A Multilayered Gentle Tidal Aquifer System

Field studies have shown that muddy and silty tidal flats usually have a small slope?<1%?and a long intertidal zone width.Due to the low permeability of the sediments and large intertidal zone width,a large-scale seepage face is formed,the numerical simulation of the dynamic change of groundwater in the aquifer under this type of tidal flat has a slow calculation speed.On the other hand,such gentle-slope tidal flats are widespread along the coastline of the Bohai and Huanghai Sea.Therefore,the study of the seawater-groundwater interaction in such sea-land interaction zones has important practical significance.This paper uses the numerical simulation software MARUN to carry out two-dimensional numerical simulations of groundwater-seawater interaction on the plumb profile of the gentle tidal flat.The aquifer system for the basic model comprises four layers,including a surface low-permeability beach sediment layer in the gentel-slope intertidal zone,an unconfined aquifer,a semi-permeable layer and a confined aquifer.The width of the intertidal zone is about 600m with a slope break.The intertidal zone is divided into a steep upper part with a slope of about 6%,and a gentle lower part with a slope of about 0.1%.This paper explored the temporal and spatial distribution of groundwater flow field and salinity in the multi-layered aquifer system with gentle-slope tidal flat under tidal forcing,and performed sensitivity analyses on relevant model parameters such as hydraulic conductivities and anisotropy ratios of the four layers).The numerical simulation results show that the groundwater-seawater exchange mainly occurs in the landward direction of the slope break,seawater infiltration mainly occurs on the upper part of the steep intertidal zone,and the discharge of groundwater to the sea mainly occurs on the lower part of the steep intertidal zone.Due to the existence of slope break and large-scale gentle-slope tidal flats,the groundwater flow forms three circulation flows,including a leakage circulation,a seawater-groundwater circulation below the slope break,and a new type of counterclockwise circulation in the seaward direction of freshwater discharge tube.This counterclockwise circulation flow is more obvious and regular in the presence of low permeability beach surface sediments.Due to the impact of low-permeability beach surface sediments,the freshwater drainage tube become wider,and the intersection of the traditional bottom salt wedge and the lower confining bed has moved significantly to the seaward direction.Affected by the leakage in the semi-permeable layer,the leakage effect between the unconfined aquifer and the confined aquifer causes water exchange.The leakage flow direction in semi-permeable layer is time-dependent.The leakage plays a certain role in the submarine groundwater discharge.The total groundwater discharge?SGD?is3.22 m²d^-1,the leakage circulation is 0.40m²d^-1,the inland freshwater recharge is 0.49m²d^-1,and the recirculated seawater is 2.73 m²d^-1.Among them,the amount of groundwater discharged from the seepage face accounted for 51%of the total;the amount of groundwater discharged in the large-scale gentle tidal flat on the seaward side of the slope break accounted for 49%of the total.The existence of the low-permeability beach surface sediments increases the groundwater discharge and seawater infiltration rate near the break point,and also reduce the total groundwater discharge due to the low permeability.The larger the hydraulic conductivity of the unconfined aquifer,the greater the seawater-groundwater exchange.