Hydrochemical Environment And Seawater-groundwater Exchange In A Muddy Tidal Flat At The Pearl River Estuary,China

Intertidal zone is a transitional zone between land and sea,its exchange of matter and energy is very complex and intensive.Muddy tidal flat is very important to coastal protection and biodiversity protection.Intertidal zone is the intense region of seawater-groundwater exchange.The complex reactions of nutrients and pollutants occur in the intertidal zone before entering the sea.Based on the analysis of field observation data,this paper investigated the dynamics of groundwater in an intertidal zone subjected to diurnal tide with the numerical simulation software MARUN.Then we analyzed the hydrochemistry of sea water and groundwater.This paper reported the field measurements in a muddy intertidal flat with a cross-shore length of 140m and an average slope of 5%in the northeast coast of the Pearl River estuary,China,near the Shenzhen Bao'an International Airport.Five“pair-wells”were installed along the whole transect to monitor the groundwater level,electrical conductivity and temperature.The measurement frequency was once every ten minutes and the monitoring period was about 5 days.The vertical hydraulic conductivity of intertidal sediments measured in situ by the falling-head method ranges from 1.31×10^-55 to 2.63×10^-4ms^-1.Salinity measured at the 5 pair-wells ranges from 14.23 to 20.52g/L.The averaged submarine groundwater discharge?SGD?and Inflow of this transect was estimated using the generalized Darcy law to be 0.37 and 0.28m2/d,respectively.Analysis of hydrochemical data demonstrates that the concentrations of main ions increase in the seaward direction due to tidal action.The sea is nearer,the ion concentrations are higher.The ion concentrations in groundwater in the tidal flat at the place closest to the airport are high.The concentration of nitrate nitrogen is low,but the concentration of nitrite nitrogen and is high,indicating that the denitrification is strong.The concentration of Cu in the pore water of the sediments is close to groundwater quality standard of the third class.The concentrations of Zn,Cr in the pore water of the sediments are close to groundwater quality standard of the second class.Airport construction and other human factors may be the probable causes.The seawater-groundwater exchanges were calculated based on hydraulic gradient method and numerical simulation.The muddy tidal flat with gentle slope is favorable to the development of seepage face.At low tides,the groundwater in the aquifer tends to discharge into the sea,and the seawater tends to penetrate into the intertidal zone at high tides.Based on the field monitoring data,the two-dimensional numerical simulation model MARUN was used to simulate the dynamic of the groundwater flow and solute transport.Dirichlet boundary condition was used on the landward boundary.The observed data at the most seaward well W5 were matched using least-squares method and then the matched data was used as tidal head on the seaward boundary.The simulated and observed values of groundwater heads fit well.