Natural radioactivity levels of 238U, 232Th and 40K and radiological risk assessment in paddy soil of Ezillo rice fields in Ebonyi State, Nigeria

Abstract

The activity levels of 40K, 238U, and 232Th radionuclides in soil and associated radiological risk in rice field 1 and field 2 located in Ezillo, Ebonyi State, Nigeria were investigated in this study. Topsoil soil samples at depths 0–4 cm and 4–8 cm were collected before planting (BP), after planting (AP) and during harvesting (DH) stages. The activities of the radionuclides in the soil were measured by gamma spectrometric technique employing NaI(TI) detector. The results indicate that the activity concentrations of the radionuclides in the paddy fields, in all stages and depths, are higher than those obtained from the control site. The activity concentrations of the radionuclides across the farming stages indicated this order of DH > AP > BP for 238U in both fields at both depths; DH > BP > AP at both depths in field 1 and at depth 4–8 cm in field 2 and DH > AP > BP at depth 0–4 cm in field 2 for 232Th; while for 40K, the orders are DH > BP > AP at depth 0–4 and DH > AP > BP at depth 4–8 cm in field 1 and AP > DH > BP at both depths in field 2. The radionuclides activities in both fields indicate this order 40K > 238U > 232Th in all the stages and depths. The mean activities of the radionuclides determined before and after planting stages in both fields and at both depths were below UNSCEAR world average levels of 420 (40K), 33 (238U), and 45 (232Th) Bqkg−1. Only 40k has a value below the world average during harvesting. The values of the examined radiological risk indices in the paddy fields in all the stages and depths were all higher than those of the control site. All the risk indices across the farming stages indicate that DH > AP > BP in both fields. Only the risk indices estimated during harvesting were higher than the respective permissible limit. The risk assessment shows that while the radiation doses from 232Th, 238U and 40K before and after planting stages do not constitute any threat to the farmers at this present moment, those during the harvesting stages present some levels of threat.