Study Of Submarine Groundwater Discharge By Ra And Its Associated Nutrient Fluxes Into The Qinzhou Bay,China

Submarine groundwater discharge?SGD?is an important way for nutrient,carbon and trace metals to enter the nearshore water.In particular,the nutrient fluxes by SGD have been attracted more and more scientific attention in recent years.SGD-derived nutrients have contributed equal or even more than that from the riverine inputs.Therefore,SGD plays an important role in the cycle of water and nutrient in the subterranean estuary and its adjacent areas.This study selected the Qinzhou bay,a subtropical strong tidal region,as a case study area.The desorption behaviors of radium?Ra?isotopes in estuarine area were investigated.Based on the Ra desorption experiment of river sediments in the Qinzhou bay,it can be found that the desorption behaviors of Ra isotopes in sediments was under the influence of the sediments'grain size and the salinity of seawater.The results showed that,within the average grain size range of 0.9?m¹36?m,the desorption amount of radium isotopes from the sediment to seawater?when the salinity was 33.9?decreased as the grain size increased,and the desorption amounts kept almost constant when the grain size was larger than 44?m.Within the salinity range of 4.9³3.9,the desorption amounts of radium isotopes from sediments gradually increased as the salinity increased until the salinity reached 24.9.By establishing creatively the radium desorption model using the sediment surface fractal structure theory,the maximum exchangeable activities of ²²⁴Ra,²²⁶Ra and ²²⁸Ra from the river sediment of the Qinzhou bay were estimated as 1.1,0.17 and 0.85 dpm g^-1,respectively;and the concerned desorption ratios were 30%,7.0%and 18%.The theoretical model of Ra desorption has been proved by data from relevant literatures.Compared with those from other estuarine/coastal regions,the maximum exchangeable activities of ²²⁴Ra and ²²⁶Ra from the river sediments in the Qinzhou bay were in the middle or low ranges,while the maximum desorption ratios were in the higher or lower levels,respectively.The results of this study are helpful to understand the desorption behavior of radium isotopes better and can be used in estimating the SGD flux of the Qinzhou Bay.Moreover,the temporal and spatial distribution characteristics of Ra isotopes were analyzed based on the samples collected during the flood season in 2016 and the dry season in 2017.In combination with the parameters obtained from the desorption experiment,the flux of SGD and its associated nutrients to the Qinzhou bay were estimated,and the differences between the two seasons were compared.Based on the hydrological characteristics and Ra isotopes distribution characteristics of the Qinzhou bay,the flushing time of the flood season and the dry season were calculated as 2.7 d and 5.8 d,respectively.Besides,the mass balance model of Ra was established,and the SGD flux of the flood season and the dry season were estimated to be?2.5±1.3?×10⁷ m³ d^-1 and?3.7±1.3?×10⁷ m³ d^-1,respectively,and the corresponding SGD rate was 8.9±4.8 cm d^-1 and 13±4.6 cm d^-1,respectively.The SGD flux accounted for 13%and 70%of the riverine runoff,respectively in the flood season and the dry season.The dissolved inorganic nitrogen?DIN?,dissolved inorganic phosphorus?DIP?and dissolved silicon?DSi?fluxes associated with SGD were 5.4×10⁶ mol d^-1,6.9×10⁴ mol d^-11 and 7.8×10⁶ mol d^-11 in the flood season,which account for 28%,10%and 30%of the fluvial fluxes,respectively.While in the dry season,the estimated DIN,DIP and DSi fluxes were 3.9×10⁶ mol d^-1,6.7×10³ mol d^-1and 4.5×10⁶ mol d^-1,accounting for 38%,3.8%and 69%of the fluvial fluxes,respectively.It can be seen that the SGD flux during the flood season was a bight less than that during the dry season,but the nutrient fluxes were clearly greater than that in the dry season.Though the recirculated saline groundwater discharge by tidal didn't contribute a“net”discharge water,it provided a great deal of nutrient to the coast,which highlights the importance of the SGD in nutrient fluxes in both seasons.This means the SGD-derived nutrients must be considered when the nutrient balance is built in the Qinzhou bay.The results of this study are expected to provide important basic data for the Qinzhou bay's eco-environment assessment.