Evaluations of submarine groundwater discharge and associated heavy metal fluxes in Bohai Bay, China

Submarine groundwater discharge (SGD) has been recognized as an important source of dissolved heavy metals to the coastal ocean. Bohai Bay, the second largest bay of Bohai Sea in China, is subjected to serious environmental problems. However, SGD and SGD-derived heavy metal fluxes in the bay are seldom reported. In this study, we present mass balance models considering the radium losses caused by recirculated seawater to estimate water age, SGD and SGD-derived heavy metal fluxes in Bohai Bay during May 2017. The water age is estimated to be 56.7–85.0 days based on tidal prism model. By combining water and salt mass balance models, submarine fresh groundwater discharge (SFGD) is estimated to be (3.5–9.3) × 10⁷ m³ d⁻¹. The SGD flux estimated by the radium mass balance models is (3.2–7.7) × 10⁸ m³ d⁻¹, an order of magnitude larger than the discharge of the Yellow River during the sampling period. SGD-derived heavy metal fluxes were estimated to be (0.2–6.0) × 10⁷ mol d⁻¹ for Fe, (1.2–2.7) × 10⁷ mol d⁻¹ for Mn, (3.0–8.2) × 10⁵ mol d⁻¹ for Zn, (2.7–7.4) × 10⁴ mol d⁻¹ for Cr and (0.6–1.8) × 10³ mol d⁻¹ for Cd, which are significantly higher than those from local rivers. This study reveals that SGD is a significant source of heavy metals (Mn, Zn and Fe) into Bohai Bay, which may have important influences on the metal budgets and ecological environments in coastal areas.