Estimating Submarine Groundwater Discharge(SGD) And Its Associated Terrestrial Material Fluxes Into Bohai Bay Using Radium And Radon Isotopes

Many studies show that submarine groundwater discharge?SGD?has been recognized as a hidden pathway for delivering terrestrial materials to the ocean.Especially,fluxes of terrestrial materials from SGD can be comparable or even higher than those from surface runoff in many coastal areas.Therefore,SGD may play an important role on coastal biogeochemical cycles and ecological environment.Bohai Bay is surrounded by many industrialized megacities and subjected to deterioration of coastal environments and degradation of ecological functions due to continuous inputs of contaminants from ports and polluted rivers.However,fluxes of terrestrial materials from SGD in Bohai Bay are seldom reported,and the importance of SGD remains poorly known.Thus,assessing the importance and effects of SGD associated nutrient and heavy metal fluxes on ecological environment is crucial for protecting and controlling ecological environment in Bohai Bay.In this study,naturally occurring geochemical tracers,radium and radon(²²²Rn)isotopes,were used to estimate SGD.The mass balance model was improved by considering the losses of radium and radon isotopes caused by recirculated submarine groundwater discharge.On the basis of this,firstly,the water age,SGD and SFGD during dry?April 2017?and wet?August2017?seasons in Bohai Bay were estimated by combining water,salt and improved radium mass balance models.Moreover,the uncertainties of the water age,SGD and SFGD induced by open sea water end-member were analyzed.By combining tidal prism model,the water age was estimated to be 64.4-98.9 days during dry season and 55.3-73.6 days during wet season,respectively.SGD fluxes during dry and wet seasons were estimated to be?3.11-7.43?×10⁸ m³ d^-1 and?4.47-12.0?×10⁸ m³ d^-1,respectively,which was one order of magnitude larger than the annual average discharge(2.45×10⁷ m³ d^-1)of the Yellow River in 2017.SFGD fluxes during dry and wet seasons were estimated to be?2.70-8.00?×10⁷ m³ d^-1 and?1.85-9.35?×10⁷ m³ d^-1,respectively.Then,based on an improved²²²Rn mass balance model,SGD flux in the western part of Bohai Bay was estimated to be?5.9±3.9?×10⁸ m³ d^-1.In order to enhance accuracy in estimating SGD,both radium and radon mass balance models were performed and compared to quantify SGD in this study.The results show that the estimated fluxes of dissolved inorganic nitrogen?DIN?,phosphorus?DIP?and silicate?DSi?from SGD were 1-3 orders of magnitude larger than those from local rivers,benthic sediments and atmospheric deposition,suggesting that SGD was a major source of nutrients in Bohai Bay.SGD associated nutrient fluxes are likely the causative factor of red tide outbreaks in Bohai Bay.Moreover,the estimated fluxes of heavy metals?Fe,Mn,Zn,Cr and Cd?from SGD were 1-2 orders of magnitude larger than those from local rivers.By comparing with the excess metal inventories in the bay,SGD was found to be a major source of Fe,Mn and Zn into the bay.Therefore,SGD associated nutrient and heavy metal fluxes have significant influences on coastal biogeochemical cycles,and should be carefully considered in the future studying and assessment of marine pollutions.