Distribution of Anthropogenic ¹²⁹I in the Western South China Sea and Its Application for Tracing the Sources and Movement of Pollution

Anthropogenic ¹²⁹I has been dispersed all over the world and could be utilized as an oceanographic tracer based on its conservative nature in the ocean. The first datasets of ¹²⁹I and ¹²⁷I were obtained by analysis of seawater of 36 water columns collected in the western South China Sea during August-September 2018. The measured ¹²⁹I concentrations decreased with depth from (0.93-1.61) × 10⁷ atoms/L in the upper 200 m to (0.04-0.14) × 10⁷ atoms/L at 1500 m, indicating a clear anthropogenic source in the upper layer, mainly originated from the global fallout. The riverine input of the deposited ¹²⁹I on the catchment area of the Mekong River is an important source besides the direct deposition in the seas. The water mass with high ¹²⁹I from the Mekong River water moves to the east at 11°N by the North Nansha Current in the surface layer (2-25 m). The exponentially decreasing ¹²⁹I level with depth indicates that the vertical dispersion of ¹²⁹I from the upper to the lower layer was mainly through slow diffusion, and the deep water at more than 1500 m was not significantly contaminated by the upper layer water at least in the past 70 years.