Variation of naturally occurring radionuclides, dose rate and mineral characteristics with particle size and altitude in bottom sediments of a river originating from Anamalai hills in the Western Ghats of India

Abstract

The natural radioactivity levels in sediment samples were carried out along the second longest river in Kerala between March and June 2012. For this purpose, a high-resolution gamma-ray spectrometry with 20 % relative efficiency was used to determine 226Ra, 232Th and 40K in the collected samples from 33 locations along the Bharathapuzha river basin. The sediment samples were then classified according to their particle size (category I, II and III sediments) and altitude (physiographic regions: the highland, the midland and the lowland). The mean activity concentration for category II sediments were 7.55, 0.18 and 28.05 % lower than category I sediments for 226Ra, 232Th and 40K, and they vary greatly with particle size from location to location. It is also found that the mean values of 226Ra, 232Th and 40K were 34.78, 50.21 and 644.94 Bq/kg, respectively, in the highland region; 43.45, 50.29 and 337.32 Bq/kg, respectively, in the midlands region and 53.38, 71.99 and 428.06 Bq/kg, respectively, in the lowland region. The air-absorbed (DAA) and indoor gamma (DIN) dose rates were calculated from the activity concentration of natural radionuclides in the sediment samples and it is found to be lower by 9.64 % and 10.02 % in category II sediments when compared to that of category I sediments. Similar calculations were carried in samples along the various physiographic regions. It is noted that the activity concentration and the calculated dose rates were lower in category II sediments than that of category I sediments, but higher than the world average values, except for 40K. Mineral characteristics have been carried out by FTIR spectroscopy in category I and III sediments and a total of 19 minerals were identified in the sediment samples. Also, the crystallinity index of quartz and the extension coefficient of some major minerals was calculated and presented. Statistical analyses were performed to study the relation between the natural radionuclides, dose rates and mineralogical characteristics. The results obtained in the present study reveals that the higher concentration of the natural radionuclides is due to the presence of the clay mineral kaolinite.