Distribution of 223Ra and 224Ra in the Bo Sea embayment in Tianjin and its implication of submarine groundwater discharge

• The distribution characters of 223/224Ra in Bo Sea embayment was analyzed.
• The mixing process was estimated with two-dimensional dispersion-advection model based on the finite difference method.
• The apparent residence ages and SGD flux were quantified with 223/224Ra data.

Submarine groundwater discharge (SGD) is now recognized as an important pathway between land and sea. In this study, in order to analyze the distribution of naturally occurring short-lived radium isotopes and the relative SGD effect in Bo Sea embayment, 223Ra and 224Ra were measured in three parts of the embayment with the radium-delayed coincidence counting (RaDeCC) system. Subsequently, the mixing process was studied by the calculation of diffusion coefficients (Kx and Kz) and advection velocities (Vx and Vz) based on the 2D advection-diffusion model. Additionally, the apparent residence ages and SGD flux were quantified based on the 224Ra and 223Ra activities. The results showed that the Ra activities exponentially decreased with the distance offshore, and both the Kx and Vx took the order of northern part > southern part > middle part. In vertical direction, there was the maximum value of Vz and minimum Kz in middle part and the maximum Kz and minimum Vz in southern part. The average ages for the northern, middle and southern parts were 4.28, 7.38 and 3.73 days, respectively. The final SGD flux yielded by 224Ra was 0.09, 0.01 and 0.03 m d−1 in the northern, middle and southern parts, respectively. The SGD flux yielded by 223Ra was 0.08, 0.01 and 0.03 m d−1 in northern, middle and southern parts, respectively. The result indicates that there is the fastest exchange rate and the biggest SGD flux in the southern part in Bo Sea embayment.