| Intertidal zone,commonly known as the beach,is located at the center of the land-sea intersection and is a key input place into the sea for submarine groundwater discharge(SGD)and SGD-associated pollutants such as nutrients and heavy metals.The slope of muddy and silty tidal flat is small(<1%),the permeability of the sediments is low,leading to the forming of a large ranges of seepage faces.The effects of evaporation on seepage faces on the groundwater flow field and salinity in the aquifer system under the intertidal zone have not been studied quantitatively.This leads to significant salinity differences between the numerical simulation results and field observations.Therefore,quantifying the effects of evaporation on groundwater flow field and solute transport is essential for the proper assessment and management of coastal aquifer ecosystems.This paper conducted a case study at the silt tidal flat in the Haimiao area of Laizhou Bay.By using the groundwater head and salinity data observed in the field,the numerical simulation code MARUN is used to describe the interaction between groundwater and seawater in the tidal flat.Based on the drilling data,a multi-layered aquifer system was considered,including a surface low-permeable beach sediment layer(about 1 m thick),an unconfined aquifer,a confined aquifer,and a semi-permeable layer between the unconfined and confined aquifers.The temporal and spatial variation and distribution of groundwater flow field and salinity were numerically simulated by considering the combined effects of factors such as beach topography(slope break points),anisotropy,spring-neap tide fluctuations,seepage face,,leakage,and surface evaporation including on seepage faces.Numerical simulation results show that,if evaporation is neglected,the simulated groundwater salinity does not exceed the seawater salinity of 24.33 g/L.When evaporation is considered,the simulated groundwater salinity in the surface layer increases and becomes higher than the seawater salinity in some zone.The maximum evaporation occurs in the supratidal zone,where the maximum salinity is 34.1g/L.At all six observation wells,both the simulated groundwater head and salinity match their observed values very well,respectively.The average standard deviation and relative error are 0.052 m and 2.6% for the head fitting,respectively,and0.39 g/L and 1.6% for salinity fitting,respectivelty.The results also show that a wide range of seepage face developed on the tidal flat,with a maximum length of 580 m,and the maximum height of the seepage face is 1.52 m when negelcting evaporation.When evaporation is considered,the height of the seepage face decreases slightly,and the maximum height decreases to 1.48 m.The maximum evaporation rate on the seepage face is 60 mm/d.Under the effect of evaporation,a new small range of clockwise circulating flow appeared under the surface near the low tide line,which was more significant at the time of low tide.Under the effects of evaporation,the exchange of groundwater and seawater occurs in the entire intertidal zone,and the amount of groundwater discharged(including evaporation)is much greater than the amount of seawater infiltration,which is five times that of the non-evaporation model.The sensitivity analysis shows that the salinity of the tidal flat surface layer is sensitive to the change of the values of hydraulic conductivity in the surface sediments and the unconfined aquifer,while the inland boundary water table has significant influence on the salinity near the "fresh water discharge channel". |
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