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

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, Ra-223 and Ra-224 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 (K-x and K-z) and advection velocities (V-x and V-z) based on the 2D advection-diffusion model. Additionally, the apparent residence ages and SGD flux were quantified based on the Ra-224 and Ra-223 activities. The results showed that the Ra activities exponentially decreased with the distance offshore, and both the K-x and V-x took the order of northern part > southern part > middle part. In vertical direction, there was the maximum value of Vz and minimum K-z in middle part and the maximum K-z and minimum V-z 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 Ra-224 was 0.09, 0.01 and 0.03 m d(-1) in the northern, middle and southern parts, respectively. The SGD flux yielded by Ra-223 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 (C) 2015 Elsevier Ltd. All rights reserved.