Using radium isotopes to estimate SGD flux in Gaochang bay, Shengsi

Submarine groundwater discharge(SGD) has been recognized as an important pathway for nutrients into ocean and plays an important role in global water cycle.In this study, seawater and groundwater samples were collected from the coast of Shengsi Island, Zhejiang Province and radium isotopes (²²³Ra, ²²⁴ Ra, ²²⁸ Ra) and nutrients were analyzed in the lab.Then SGD and its derived nutrients fluxes were estimated.The results showed that Radium activities in groundwater (²²³Ra:16.7 ±2.9 dpm/100 L, ²²⁴Ra:479 ±18 dpm/100 L, ²²⁸Ra:97 ±4 dpm/100 L) were much higher than those in seawater (²²³Ra:3.5 ±0.7 dpm/100 L, ²²⁴Ra:80 ±4 dpm/100 L, ²²⁸Ra:31±2 dpm/100 L).Moreover, the tidal-drived SGD rate were calculated to be 7.6 and 4.1 cm/d based on ²²³Ra and ²²⁴Ra, respectively.However, according to the decay of ²²³Ra with distance offshore, a SGD flux of 22 cm/d was derived.Based on SGD flux, the flux of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorous (DIP) and dissolved silicon (DSi) were calculated to be 593, 4.16 and 241 μmol/(m²·s), respectively.