EVALUATION OF NATURAL RADIOACTIVITY, CAESIUM CONCENTRATION, CHEMICAL PARAMETERS AND SOIL-TO-PLANT TRANSFER FACTOR IN SOUTHERN NIGERIA.

Abstract

Cancer has become a major cause of mortality in the recent times. Up to 10% of invasive cancers are related to ionizing radiation exposure, which is known to be capable of causing increase in future incidence of cancer cases. Up till now, database on radioactivity from Africa, and Nigeria especially are still very sparse compared to information from several other environments of North America, Europe and Asia. Activity concentration of natural radionuclides and artificial radiocaesium together with heavy metals concentration and soil-to-plant transfer factor have been evaluated in soil and plant samples from seventeen states in Southern Nigeria. The samples were analysed for radioactivity using the co-axial type Hyper-Pure Germanium (HPGe) detector (CANBERRA, USA), while heavy metals analyses were performed using Atomic Absorption Spectrometer (AAS). The activity concentration of 40K, 226Ra, 232Th and 137Cs in the soils ranges from 13.06 to 2723.22 Bq kg-1, 8.53 to 160.37 Bq kg-1, 11.09 to 295.24 Bq kg-1 and 0.53 to 3.53 Bq kg-1 with the mean value of 276.40, 37.30, 56.58 and 1.26 Bq kg-1 respectively. The activity concentration of 40K, 226Ra, 232Th and 137Cs in plants ranges from 419.79 to 9221.05 Bq kg-1, 3.67 to 56.40 Bq kg-1, 1.77 to 64.88 Bq kg-1 and 0.53 to 3.53 Bq kg-1 with a mean values of 2536.33, 19.36, 22.00 and 1.64 Bq kg-1 respectively. The radium equivalent concentration, the external and internal hazard indices were estimated to range from 28.47 to 701.53 Bq kg-1, 0.08 to 1.89 and 0.10 to 2.33 with mean values of 139.50 Bq kg-1, 0.38 and 0.48 respectively. The mean values of Zn, Cd, Mn, Co, Ni and Pb concentrations in the soils were 82.70, 0.07, 183.98, 2.81, 5.71, and 27.67 mg kg-1 respectively. The mean values of Zn, Cd, Mn, Co, Ni and Pb concentrations in the plants were found to be 225.31, 0.49, 198.36, 2.06, 7.63, and 7.59 mg kg-1, respectively. The mean daily intake of heavy metals through vegetable consumption in the study areas was estimated at 80.25, 0.18, 70.65, 0.73, 2.72 and 2.70 mg d-1 for Zn, Cd, MN, Co, Ni and Pb respectively. The mean absorbed dose and the annual outdoor effective dose in soil samples were higher in the South- West (95.40 nGy h-1 and 0.12 mSv y-1) than in the South-South (32.30 nGyh-1 and 0.04 mSv y-1) and South-East (45.53 nGy h-1 and 0.06 mSv y-1), the world average (0.07 mSv y-1) and international permissible standards of 0.1 mSv y-1 and 1.0 mSv y-1 for World Health Organization (WHO) and International Commission on Radiological Protection (ICRP) respectively. The soilto- plant transfer factor estimation showed that 40K has very high value compared to other radionuclides in the samples. Regression analysis showed that there was no linearity (R ranges from -0.83 to -0.34) in the relationship between activity concentration of radionuclides in soils and in plants because plant receives some amount of radionuclide concentration from other sources like air, rainwater, and cosmic rays. The mean daily intake of heavy metal was higher than the values from literatures and recommended limit by International Atomic Energy Agency (IAEA) and ICRP. There were some degree of linearity in the relationship between the activity concentration of radionuclides and concentration of heavy metals in the soil with few cases of weak or insignificant correlation (R ranges from -0.24 to 0.50). This could be because radionuclides and heavy metals have different origin. The mean probability of developing cancer in a lifetime by inhabitants of the study areas through residential soil channels and vegetables is 5.55 x 10-5 and 1.94 x 10-3 respectively. This implied that from every one million humans, 56 and 1,940 persons respectively for residential soil and vegetable consumption pathways, are liable to developing cancer during a life time. This value constitutes an important fraction of the 1% of global population that was estimated by Biological Effect of Ionizing Radiation (BEIR) report of developing cancer as a result of natural background radiation at some points in their lifetime.