Evaluation of submarine groundwater discharge into the northern Bohai Bay, China using ²²⁶Ra

Submarine groundwater discharge (SGD) has been widely recognized as a significant source of water and dissolved material transport from land to ocean. To quantify SGD into the northern Bohai Bay, China, naturally occurring radium isotope (²²⁶Ra) was measured in water samples collected along two transects in September 2012. Based on a tidal prism model, two different flushing times of the coastal water were determined to be 9.1 d and 11.5 d with respect to the different return flow factor (b) obtained from a physical model and a mass balance model of ²²⁶Ra and salinity, respectively. Using the derived flushing time, we developed a ²²⁶Ra mass balance model to estimate the SGD into the bay, which includes mixing, sedimentary input and SGD. The ²²⁶Ra budget indicated the ²²⁶Ra input from SGD accounted for 99% of the total tracer input to the northern Bohai Bay. We arrived at an average flux from SGD of 4.83 × 10⁷ m³/d. The large volume of SGD confirms its importance in supplying a considerable quantity of nutrients to the bay.