Study On Submarine Groundwater Discharge (SGD) And Its Driven Nutrient Fluxes From Typical Area In Coastal Sea Of China

Submarine groundwater discharge(SGD)has been recognized as an important pathway for the transport of terrestrial chemical components(i.e.,nutrients,trace elements and contaminant)to the ocean over the past few decades.In most reported study areas,SGD-derived chemical components fluxes could have a magnitude which is comparable to or even exceeding that of river-water driven fluxes.Thus,SGD has been found to play an important role in the biogeochemical processes and the ecosystem of coastal oceans.Utilizing the temporal and spatial variation of naturally occurring radium isotopes activities as well as radium mass balance box-model,SGD fluxes and its driven nutrient fluxes into the Sanggou Bay,which is a semi-closed bay with aquaculture,lagoons along eastern Hainan Island(Laoye Lagoon and Xiaohai Lagoon),which are relatively closed bays with aquaculture and the East China Sea(ECS),which is one of the world largest river-dominated ocean margins,were estimated in this study.The data was based on samples collected during June 2012,October 2012 and August 2013,respectively.Sources and sinks of the nutrient in each specific area were analyzed and nutrient budgets were also reassessed by including SGD input in these areas.Semi-closed bay-Sanggou Bay.The activities of 223Ra,224Ra,226Ra and 228Ra were measured in seawater,groundwater and river water in the Sanggou Bay during June 2012.By constructing radium mass balance model,SGD was found to be the main source of the excess radium into the Sanggou Bay and the total flux was estimated to be(2.59-3.07)×107 m3 d-1,that is 0.13-0.15 m3 m-2 d-1.At the same time,SGD fluxes driven by tidal pumping were estimated to be(0.16-1.97)×107 m3 d-1 and(0.33?1.00)×107 m3 d-1 based on 226Ra and 228Ra from the time series observation,with averages of 1.00×107 m3 d-1 and 0.75×107 m3 d-1,respectively.Tidal driven SGD can account for up to 76%of the total SGD flux.In contrast,tidal pumping plays an important role in this semi-closed bay with aquaculture,although a variety of driving forces can contribute to SGD emissions.In addition,SGD-derived nutrient fluxes into the Sanggou Bay was estimated.Results showed that compared to the nutrient inputs from other sources like river,excretion by scallop,bottom sediment exchanging and sediment resuspension,etc.SGD-derived nutrient is the primary sources of dissolved inorganic nitrogen(DIN)and dissolved inorganic phosphorus(DIP)input,which is one to two orders of magnitude larger than any other sources.Fisheries extract DIN equivalent to the SGD input.Fisheries extract DIP is one order of magnitude larger than SGD input.Therefore,SGD plays an important role in the nutrient budgets of the Sanggou Bay and cannot be ignored.Relatively closed bay-Lagoons along eastern Hainan Island.Based on the sampling at Laoye(LY)Lagoon and Xiaohai(XH)Lagoon in October 2012,the radium mass balance model provided evidences that groundwater accounts for a significant proportion of the discharge to the LY Lagoon and the XH Lagoon with average SGD fluxes of 1.7×106(0.094 m3 m-2 d-1)and 1.8×106(0.041 m3 m-2 d-1)m3 d-1,respectively.The estimated SGD rates in the LY Lagoon and the XH Lagoon fell into the range of other lagoons that have been reported around the world.The SGD rate in the LY Lagoon was larger than that in the XH Lagoon,which is most likely due to the high permeability of regional sandy soils and the larger shoreline-to-volume ratio around the LY lagoon than those around the XH lagoon.In addition,tidal pumping was an important for driving force for SGD fluxes in the LY Lagoon and the XH Lagoon,with average fluxes of 1.7×105 and 1.4×105 m3 d-1,respectively,in the tidal channels.Tidal-driven SGD into the tidal channels of both lagoons can account for approximately 10%of the total SGD flux into the lagoons.In addition,the dissolved inorganic nutrient budgets were reassessed in the LY Lagoon and the XH Lagoon,which showed that SGD was the major source of nutrients entering the LY Lagoon and that the LY Lagoon behaved as a source for DIN and DIP and as a sink for dissolved silicate(DSi).Nutrient loads in the XH Lagoon were mainly derived from riverine inputs and SGD,and the XH Lagoon behaved as a source for DIP,but a sink for DIN and DSi.Open sea-the East China Sea(ECS).The ECS is one of the world famous river-dominated ocean margins.The activities of223Ra,224Ra,226Ra and 228Ra were measured in the seawater and groundwater of the ECS to study the SGD flux into the ECS in August 2013.Results showed that SGD plays an important role in the continental shelf of the ECS.According to the locations and features of water masses in the ECS,radium isotopes mass balance model was built based on the concept of the conservation of water and salt masses.Then the conservative and non-conservative(excess)radium isotopes were estimated.And the inventories of excess 228Ra and 226Ra in the ECS were used to estimate the residence time of water in the ECS,which was averaged 1.30±0.27 a.Then,both the conservative and non-conservative radium fluxes were discussed.By correcting for radium due to the diffusion of the bottom sediment from the non-conservative radium flux,the SGDRa flux was figured out.Then the SGD flux was estimated to be 4.17×1011 m3 a-1(227Ra model)and 5.42×1011 m3 a-1(226Ra model)in summer,which were 36-47%of the total river discharge into the ECS along the coast in 2013,In the ECS,nutrients fluxes driven by SGD were estimated to be(5.63-7.32)×1010,(1.38-1.79)× 109 and(1.21-1.59)×1011 mol a-1,respectively,which were equivalent to the nutrient inputs from riverine discharge.Therefore,SGD is an important nutrient source that cannot be ignored in the ECS.In summary,the naturally occurring radium isotopes were used to study SGD into the typical semi-closed bay and relatively closed bay under the influence of humanistic activities and the river-dominated ocean margin in China.Results showed that SGD was the main source of nutrient in the semi-closed and relatively closed study area with little river discharge and small areas.And SGD-derived nutrient fluxes can account for more than 40%,even up to 98%,of the total nutrient input fluxes.While,the nutrient input contributed by SGD could be comparable to that of riverine input when the river discharge is large and the study area is large and open.Where SGD is also an important input source that cannot be ignored.This study will help people to fully understand the importance of SGD and provide basic data for the development of typical humanistic activities in coastal areas-(such as marine aquaculture).Meanwhile,this study will be a complement to the SGD research in the coastal areas of China and provide a reference for the specific study of biological geochemical changes,such as nutrients,trace gases and carbon,in the subterranean estuaries of coastal seas.