Use of ²²²Rn to trace submarine groundwater discharge in a tidal period along the coast of Xiangshan, Zhejiang, China

²²²Rn is one of the operative tracers for submarine groundwater discharge (SGD), which plays a significant role in the land-ocean interaction of the estuarine and coastal regions. By the distribution pattern of ²²²Rn in atmosphere, groundwater and surface seawater, in a full tidal period (25 h) in March 2012, SGD was estimated along the coast of Xiangshan, Zhejiang, China. ²²²Rn activity in Xiangshan coast was in range of 2.4 × 10⁴-1.7 × 10⁵ Bq/m³ with an average of 9.6 × 10⁴ Bq/m³ for groundwater; 0.2 × 10²-2.8 × 10² Bq/m³ with an average of 1.1 × 10² Bq/m³ for surface seawater. ²²²Rn activities in groundwater were much greater than those in surface water, suggesting that the major source of radon came from coastal groundwater discharge. Rn fluxes of atmospheric emissions, sediment, and of ²²⁶Ra in situ decay can be negligible in this study, but the tidal effects play a crucial role in Rn fluxes. Using a radon inventory equilibrium model, we estimated that the average SGD was 13.2 cm/day and the average terrestrial SGD flux was 1.8 × 10⁸ m³/day. Furthermore, SGD may have a vital impact on the composition and structure of nutrients in seawater, and contribute to eutrophication events occurring in spring season along the coast of the East China Sea.