Ra-226 and Ra-228 tracer study on nutrient transport in east coastal waters of Hainan Island, China

Material fluxes (e.g., nutrients) from coastal waters to offshore areas play an important role in controlling the water quality of the adjacent sea areas not only by increasing nutrient concentration but also by changing nutrient structures. In this study, naturally occurring isotopes, Ra-226 and Ra-228, were measured with the alpha spectrometry in the Wenjiao-Wenchang and Wanquan estuaries and adjacent sea areas along the east coast of Hainan Island. The excess Ra-226 and Ra-228 activities were observed by comparison with the values derived from the conservative mixing of freshwater and seawater end-members in both estuaries. Using a one-dimensional diffusion model, the horizontal eddy diffusion coefficient of 3.16 x 10(5) cm(2)/s, for nutrients diffusing from their sources, was derived from Ra-228 activities. Consequently, the corresponding nutrient fluxes flowing into the coastal waters were assessed. The results can provide useful information for the study of the mixing and exchange processes of coastal waters as well as dissoluble pollutant transport in this sea area.