Estimating coastal mixing rates and submarine groundwater discharge (SGD) in Guangdong-HongKong-Macau Greater Bay Area, China using radium isotopes

As a major component of the hydrological cycle, submarine groundwater discharge (SGD) has been widely recognized as a significant source of water and an important pathway for dissolved material transport from land to ocean. Natural radium isotopes are recognized as ideal tracers for effective and efficient assessment of SGD in local scales and global scales since they are conservative chemically and widely enriched in SGD. Here we report the estimates of coastal mixing rates and SGD in Guangdong-HongKong-Macau Greater Bay Area, China using radium isotopes. The distributions of short-lived ²²³Ra, ²²⁴Ra and long-lived ²²⁸Ra in seawater and coastal groundwater were investigated. Based on the horizontal distribution of short-lived Ra and a mixing model, the horizontal eddy diffusion coefficient in the region was estimated to be 230-1085 m²/s. The offshore fluxes of ²²⁸Ra can be derived from their across-shelf activity gradients and the eddy horizontal diffusion coefficient. Such ²²⁸Ra fluxes require a substantial volume of groundwater discharge to balance Ra removal, and thus SGD can be estimated via radium mass balance model.