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

Abstract

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.