RADIONUCLIDE CONTENTS OF WATER AND SEDIMENTS FROM SELECTED RIVERS IN SOUTHERN ONDO STATE, NIGERIA.

Abstract

In this research, natural radionuclide contents of water and sediment samples collected from selected rivers in Irele and Okitipupa Local Government Areas of Ondo State, Southwestern Nigeria were determined to evaluate the radiation hazards to the populace of the study area. Forty two river sediment samples and ten river water samples were collected from river Akeun, Oha, Oluwa, Otu and Ufara in the study area. The activity concentrations of the water and sediment samples of the selected rivers were measured using a gamma ray spectrometry system with high purity germanium (HPGe) detector. The measured activity concentrations of 238U, 232Th and 40K across all the sediment samples varied from 6.25 ± 0.39 to 92.87 ± 13.93, 5.01 ± 0.75 to 78.08 ± 11.71 and 20.52 ± 3.08 to 987.30 ± 148.10 Bq kg-1 with mean values of 23.97 ± 3.20, 25.29 ± 3.07 and 206.34 ± 20.43 Bq kg-1 respectively. The measured activity concentrations of 238U, 232Th and 40K in the water samples varied from 0.12 ± 0.02 to 2.31 ± 0.35, 0.17 ± 0.03 to 2.85 ± 0.47 and 7.87 ± 1.18 to 65.51 ± 2.43 Bq l-1 with mean values of 1.28 ± 0.25, 1.44 ± 0.29 and 34.37 ± 2.59 Bq l-1 respectively. The estimated absorbed dose rates from sediment samples ranged from 8.03 to 91.75 nGy h-1 with an average value of 34.95 nGy h-1. The indoor and outdoor Annual Effective Dose Equivalent for the sediment samples varied from 9.85 to 112.52 and 39.39 to 450.08 µSv y-1 with an average value of 42.87 and 177.89 µSv y-1 respectively. The radium equivalent activity (Raeq) of all the sediment samples were lower than the accepted safety limit value of 370 Bq kg-1 as recommended by the Organisation for Economic Cooperation and Development (OECD). The calculated values of the External Hazard Index (Hex) and Internal Hazard Index (Hin) for all sediment samples are below the limit of unity, which is below the permissible limit of 1 mSv y-1 recommended by ICRP. The value of the representative gamma index (Iγ) ranged from 0.12 to 1.45 with an average value of 0.55. The Excess Lifetime Cancer Risk (ELCR) values for all sediment samples and their average in this present work are less than the world average of 0.29 × 10-3. The total annual effective dose received as a result of the combined ingestion of 238U, 40K and 232Th from each of the sampled river water to the six ICRP age groups are below 1 mSv y-1. Therefore, the water samples collected from the selected rivers in Irele and Okitipupa Local Government Areas of Ondo State, Southwestern Nigeria are radiologically safe to be used by human either as drinking water or for domestic use.