Using radium isotopes to estimate the residence time and the contribution of submarine groundwater discharge (SGD) in the Changjiang effluent plume, East China Sea

This paper reports the initial result of the flux of submarine groundwater discharge (SGD) into the Changjiang effluent plume (CEP). A radium mass balance model and a ²²⁴Ra/ ²²³Ra activity ratio (AR) apparent age model were applied to estimate the residence time of water above the pycnocline in the CEP. These two approaches gave similar results, yielding residence times of 5.4d and 7.0d, respectively, in the inner and outer plume zones. The nonconservative inventory of ²²⁶Ra was established in the CEP, and converted to a ²²⁶Ra flux by dividing by the water residence time and assuming steady state conditions. After subtracting the desorption of ²²⁶Ra from suspended sediment and the diffusion from the bottom sediment, the ²²⁶Ra flux from SGD was converted to a total SGD flux by dividing by the measured activity of ²²⁶Ra in local groundwater. The SGD flux in the CEP is estimated to be 0.2-1.0×10 ⁹m ³d ^-1, which is equivalent to 6-30% of the Changjiang water discharge during flood season. This large flux indicates that SGD may be another important nutrient source to the East China Sea.