Quantifying Exchange Rates Of Seawater And Groundwater In Mudflat Estuary

Submarine groundwater discharge(SGD)is an important component of global water cycle.It is also one of the important sources of coastal ocean,gulf water,and various chemical substances.At the same time,SGD is an important and hidden channel which transports material from land to ocean.Therefore,the impact of SGD on coastal marine ecosystems cannot be ignored.Jiaozhou Bay is a typical semi-enclosed bay with small area,small average water depth and slow water exchange rate with open sea,leading to the occurrence of red tides.According to previous studies,researchers pay attention to the water and chemicals flowing into the sea via rivers,while ignore SGD whichmay also deliver large quantity of water and the chemical substances.Therefore,studying SGD and exploring the impacts on ecological environment in coastal areas can bring significance not only to theoretical studies,but also to real world practicesto protect the water recourses and to prevent red tides.The case study site of this study is the mudflat section at the Dagu River estuary of Jiaozhou Bay.The value of SGD was estimated usingeither the vertical hydraulic gradient method or numerical simulation.The results of SGD are0.91m3/d/m and 0.94m3/d/m,respectively.They are very close to each other.The value of inflow is 0.45 m3/d/mand 0.41m3/d/m,respectively.The nitrogen existedprimarily in the form of ammonia nitrogen and nitrate nitrogen.NH4-N and NO3-N in groundwater took up 56% and 42% of the total N,while the amount of NH4-N and NO3-N in seawater took up 30% and 68% of the total N.The average concentration of phosphate contained in groundwater samples was 40.53?g/L,but the average concentration of phosphate in seawater samples was 25.70?g/L.The nutrient concentration in groundwater wasabout ten times more than that in seawater.Due to the interaction between groundwater and seawater,the fluxes ofthe nutrients N and P into the sea weremeasured to be 1.48mol/m/d and 0.82mmol/m/d,respectively.The concentrationsof Cu,Pb and Zn werelowerin groundwater thanin river and seawater.The concentrationsof Cd,Cr and As were higher in groundwater than in the river and seawater.SGD may be one of the important sources ofCd,Cr and As in seawater.Heavy metal Hg was not detected.The concentration of heavy metals Cu in seawater exceeded the standards(50?g/L)of fourth class water quality.The concentration of Cu was lower in groundwater and river water than in seawater.The excessive amount of Cu in seawater was not fromriver water or groundwater input due to low concentration in river or groundwater,but was likely from factory waste discharging into rivers and bay.There was also another possibility that some of the Cu was transported into the seawater along with factory ships.