Natural radioactivity levels in topsoil from the Pearl River Delta Zone, Guangdong, China

Concentrations of the natural radionuclides 238U, 226Ra, 232Th and 40K have been measured by γ-ray spectrometry in 796 topsoil samples from the Pearl River Delta Zone (PRDZ) of Guangdong, China. The mean concentrations for 238U, 226Ra, 232Th and 40K were found to be 140 ± 37 Bq kg −1, 134 ± 41 Bq kg −1, 187 ± 80 Bq kg −1 and 680 ± 203 Bq kg −1 dry mass, respectively. These values were all higher than the mean values in soil for China and the world. Outdoor air-absorbed dose rates, calculated from activity concentrations of 226Ra, 232Th and 40K, ranged from 86 to 237 nGy h −1, with a mean value of 165 ± 46 nGy h −1. The corresponding annual outdoor effective dose rate per person was estimated to be between 0.11 and 0.29 mSv y −1, with a mean value of 0.20 ± 0.06 mSv y −1, which was also higher than the world mean value of 0.07 mSv y −1. The radium equivalent activity (Ra eq) and the external hazard index ( I r) resulted from the natural radionuclides in soil, were also calculated and found to vary from 230 to 676 Bq kg −1 and from 0.6 to 1.8, respectively. The Ra eq and the I r in all the investigated regions were up to 75% higher than the set limits of 370 Bq kg −1 and 1.0, respectively. ► Topsoil samples are highly enriched with 238U, 226Ra and 232Th in the PRDZ. ► The 232Th and 238U distributions mainly depend on geological and geographical conditions. ► The 232Th and 226Ra were the main contributors to the higher natural radioactivity.