Research On Metal Emission Driven By Submarine Groundwater And Its Influencing Factors

Submarine groundwater discharge（SGD）is an important source of dissolved materials,and much evidence suggests that its contribution to nearshore metals,and can not be negligible.Compared with the studies on SGD-driven substances such as nutrients and carbon,relatively few studies have been conducted on metals transported by SGD,which play an important role in the growth,development and reproduction of organisms（e.g.,Mn,Fe,Cu,Zn,etc.）,some of which are toxic to living organisms（e.g.,Pb,Cr,U,As）,while others can be used as excellent tracers（e.g.,Ba）.The role of SGD in the study of the balance of metals in the offshore ocean can not to be ignored.However,metals are susceptible to a variety of driving forces（e.g.,season,hydrographic characteristics,human activities,etc.）,which can lead to variations in metal concentrations at various end member（especially at the porewater）,and ultimately can lead to significant changes in SGD-driven metal flux estimates.In order to investigate the above problems,this thesis selects Lianjiang,Fujian,Jintan,Guangxi and Xiangshan Bay,Zhejiang as the study areas respectively,and estimates the SGD fluxes by using the radioisotope radon(²²²Rn),initially explores the behaviors of different metals and their influencing factors in subterranean,and determines SGD-driven metal fluxes by combining with the different methods of selecting the endmember of the metals.（1）The SGD-driven study of eight metals（Mn,Fe,Ba,Cu,Zn,Cr,Pb,and U）through three seasons（July 2019,October 2019,and April 2021）in Lianjiang,Fujian,found that the fluxes of the different metals in groundwater showed significant seasonal variability.Based on this seasonal characterization,the eight metals are classified as supplying metals（Mn,Fe and Ba）,remove metal（Cu）and dynamic equilibrium metals（Zn,Cr,Pb and U）.The seasonally averaged SGD-driven Mn,Fe,and Ba fluxes were193±47,9.70±2.66,and 3.30±0.74 mg m^-2 d^-1respectively,whereas the average flux of the Cu was-0.035±0.019 mg m^-2d^-1.Both the fluxes and the concentrations of Fe were correlated with the variations in the concentrations of DOM in the groundwater and the surface water,and in combination with the literature analysis,this thesis concludes that DOM is a factor influencing the seasonal variation of Fe fluxes.Mn and Ba fluxes,on the other hand,are affected by the proportion of fresh groundwater.Based on the above findings,we conclude that single-season metal flux estimation produces a large bias when applied over longer periods of time（annual scales）,with an effect of up to 3.6 times for Fe fluxes,up to 5.5 times for Mn fluxes,and up to 15 times for Ba fluxes,whereas for some other metals（U,Zn,Cr,etc.）,they show a seasonal transformation of the source-sinks.Thus,seasonal variability strongly affects SGD-driven metal fluxes,and ignoring such variability will not be conducive to a rational assessment of metal receipts and disbursements in the offshore region,especially on long time scales.（2）In July 2019,the upper limit of SGD-driven Mn,Fe,Cr,Ba,and Zn fluxes obtained from Jintan,Guangxi was 1361,19.0,0.181,7.30,and 0.384 mg m^-2 d^-1 and the lower limit was 32.7,1.06,0.155,3.03,and 0.264 mg m^-2 d^-1,respectively.In this sandy beach,the Upper saline plume（USP）and freshwater discharge tube（FDT）were found.This hydrographic structure has significant spatial variability in the distribution of parameters such as redox potential（Eh）and salinity,which in turn affects the cycling of metals in the STE.Specifically,the low Eh of the FDT promotes the dissolution of Fe and Mn oxides to very high values(e.g.,125 ng m L^-1 for Fe and 5846 ng m L^-1 for Mn.).Ba is affected by the dissolution of Mn oxides,and its desorption is increased by the increase in salinity,so that the concentration of Ba also reaches a maximum in the FDT(34.7 ng m L^-1).In contrast to the parameter characteristics in the FDT,the ones in the USP are quite the opposite,characterized by high Eh and high salinity,where Cr and Zn concentrations reach their highest values in this region.In both FDT and USP,which are shaped by the hydrological structure,the metals reach very high concentrations,which in turn promotes their discharge through the SGD.With different metal endmember selection strategies（selected according to USP and FDT or according to Eh-salinity）,we obtain SGD-driven metal fluxes of metals under the influence of hydrological structure and without the influence of this factor.In summary,it can be obtained that FDT can strongly influence the SGD-driven metal fluxes,causing a significant increase,and despite the small extent of the structure in the beach（～4%）its contribution to metals is significant（FDT contributes 61%of the total Mn fluxes contributed by the SGD on this coast,and contributes 40%of the Fe stations）.The amount of metals contributed by the SGD is also very significant in relation to the other end-members.Thereby significantly affecting offshore metal balance and ecological security.（3）The As fluxes contributed by the SGD of Xiangshan Bay in November 2019（dry season）and August 2020（wet season）were 1310 kg As d^-1 and 5880 kg As d^-1,respectively.There was a large spatial variability of As concentrations in seawater,well water,pore water,and river water samples in the area around Xiangshan Bay.By comparing the distribution of coastal shoreline types with the As concentrations,it was found that high As concentrations may be related to local industrial activities.In addition,As concentrations in groundwater were comparable in the dry and wet seasons,but there was significant seasonal variability in As fluxes,which may indicate that rainfall plays a role in As transport in groundwater.The strong correlation between DIC,DOC,and As concentrations may indicate that dissolved carbon also plays a role in the transport of As.The industrialized shoreline,although only 14.5%of the entire bay shoreline,transported most of the As flux（75%in the dry season and 60%in the wet season）.This suggests that the coupling between human activities and groundwater may be responsible for the spatial variability of As concentrations and high As pollution,and that the SGD-driven dissolved heavy metal pollution problem,represented by high As pollution,needs more attention.The impact of dissolved metal balances in the offshore environment is significant for the whole ecosystem,where various organisms,including humans,require some trace metal elements（Fe,Mn,Zn,Cu,etc.）,while at the same time we cannot be exposed to excessive amounts of toxic and harmful metals（Pb,Cr,As,U,etc.）.This requires that we must know the sources of metals,and in particular the need to focus on SGD as a potential source of metals,which has been previously overlooked.Also,since metal fluxes may be affected by various factors thus leading to changes in SGD-driven metal fluxes,these factors need to be explored.In summary,this thesis estimated the SGD fluxes in the nearshore region using the tracer ²²²Rn and combining the box model and the one-dimensional advection-diffusion model.Based on the seasonal and spatial variations of each metal and the correlation analyses with the factors,it was elucidated that the SGD is an important source of metals in the nearshore environment,and that the concentrations of the respective metals are varied in the subterranean under the influences of the seasons,anthropogenic activities,and the subsurface freshwater.These emitted metals may ultimately affect human health and development.This thesis adds further to the study of SGD-driven metal research and provides a preliminary exploration of the factors influencing the variability of metal fluxes to provide assistance and understanding of metal research in offshore SGD sources.