Abstract: The activity concentrations of six naturally occurring radionuclides (U, Th, ²²⁶Ra, ⁴⁰K, ²¹⁰Po, ²¹⁰Pb) were investigated in four major tributaries in the Hengyang section of the Xiangjiang river (Chong Ling river, Zheng river, Lei river, Mi river) after sampling in dry season (November—December) and abundant season (April—June) in order to find out the level and source of naturally occurring radionulides . The results showed that the concentrations of ²²⁶Ra, ²¹⁰Po and ²¹⁰Pb in the Xiangjiang river (Hengyang section) were significantly higher than average level of the Yangtze River system and the Dongting Lake system. The concentrations of radionuclides in the Chongling and the Zhengshui river were abnormal and significantly higher than those for the other two tributaries and the mean values of the Yangtze River system and the Dongting Lake system over the years. Thus, radioactive pollution sources might exist adjacent to these two tributaries, and further work need to be conducted.

Key words: Xiangjiang river, Hengyang section, naturally occurring radionuclides, U, Th, ²²⁶Ra, ⁴⁰K, ²¹⁰Po, ²¹⁰Pb