DETERMINATION OF INGESTION DOSE AND SOIL-TO-FOOD CROP TRANSFER FACTOR OF RADIONUCLIDES AND HEAVY METALS IN SOUTH-WESTERN NIGERIA.

Abstract

Cancer has been a major cause of mortality in the world and based on research, about 10% of invasive cancer is related to radiation exposure, which is known to increase over time. Till now, data base on radioactivity in food crop from Nigeria and Africa at large are still very sparse compared to other continents like Asia, Europe and North America. The activity concentration of natural radionuclides together with heavy metal concentration and soil-to-plant transfer factor have been evaluated in food crops and soil samples from agricultural farmlands in the six states of South-Western Nigeria. The samples were analyzed for radioactivity using the co-axial type Hyper-Pure Germanium (HPGe) detector (CANBERRA, USA), whiles the heavy metal analyses were performed using Atomic Absorption Spectrometer (AAS). The activity concentrations of 40K in soils of the study area ranges from 102.72 – 314.20 Bq kg-1, 105.83 – 170.50 Bq kg-1, 111.12 – 278.70 Bq kg-1, 76.08 – 219.05 Bq kg-1, 46.43 – 269.25 Bq kg-1, 62.99 – 105.50 Bq kg-1 for Ondo, Ekiti, Osun, Oyo, Ogun and Lagos state respectively. The activity concentrations of 40K in food crops in the study area ranges from 286.23 – 487.33 Bq kg-1, 197.53 – 348.46 Bq kg-1, 107.59 – 310.28 Bq kg-1, 91.14 – 213.47 Bq kg-1, 103.17 – 222.41 Bq kg-1 and 67.81 – 221.19 Bq kg-1 for Ondo, Ekiti, Osun, Oyo, Ogun and Lagos state respectively. The study revealed a high non-uniform distribution of this radionuclide in the region with activity concentrations in the soils and food crops varying significantly within the farmlands, which is due to the geological variability of the area. The activity concentrations of 226Ra in the soils ranged from 12.87 – 31.37 Bq kg-1, 21.72 – 44.51 Bq kg-1, 22.52 – 34.91 Bq kg-1, 18.95 – 28.04 Bq kg-1, 10.31 – 33.25 Bq kg-1 and 9.59 – 22.57 Bq kg-1 for Ondo, Ekiti, Osun, Oyo, Ogun and Lagos state respectively. The activity concentrations of 226Ra in food crops in the study area ranges from 7.65 – 9.56 Bq kg-1, 6.11 – 8.34 Bq kg-1, 6.93 – 11.04 Bq kg-1, 7.39 – 19.32 Bq kg-1, 6.06 – 11.25 Bq kg-1 and 10.23 – 26.95 Bq kg-1 for Ondo, Ekiti, Osun, Oyo, Ogun and Lagos state respectively. The mean value of activity concentrations of 226Ra observed in the entire sampling location of the study area compared favorably with those around the world. The activity concentrations of 232Th in the soils ranged 7.67 – 13.89 Bq kg-1, 11.41 – 29.60 Bqkg-1, 12.55 – 22.84 Bq kg-1, 11.22 – 81.33 Bq kg-1, 8.62 – 32.49 Bq kg-1, 7.36 – 18.36 Bq kg-1, for Ondo, Ekiti, Osun, Oyo, Ogun and Lagos state respectively. The activity concentrations of 232Th in the food crops ranged from 3.08 – 4.73 Bq kg-1, 2.86 – 10.35 Bq kg-1, 3.01 – 6.70 Bq kg-1, 3.13 – 13.88 Bq kg-1, 2.16 – 7.33 Bq kg-1, 7.06 – 19.49 Bq kg-1 for Ondo, Ekiti, Osun, Oyo, Ogun and Lagos state respectively. The activity concentrations of 232Th observed in the study area is lower than the worldwide average value of 34 Bq kg-1, reported in NCRP and 45 Bq kg-1 and 45 Bq kg-1, reported in UNSCEAR. The absorbed dose due to the primordial radionuclides ranged from 18.31 – 27.46 nGy h-1 , 24.04 – 44.89 nGy h-1 , 24.21 – 38.00 nGy h-1 , 19.05 – 70.96 nGy h-1 , 18.22 – 30.99 nGy h-1 and 12.69 – 25.03 nGy h-1 for soils of the study area, then 17.44 – 25.72 nGy h-1 , 10.34 – 21.17 nGy h-1 , 9.95 – 21.86 nGy h-1 , 5.35 – 17.35 nGy h-1 , 6.17 – 11.17 nGy h-1 and 14.05– 32.52 nGy h-1 for food crops in the study area. The overall mean value obtained is lower when compared to the worldwide range average value of 94 nGy h-1 (UNSCEAR). The mean outdoor annual effective dose due to terrestrial sources 40K, 226Ra and 232Th in this were found to range from 0.002 – 0.003 mSv y-1, 0.003 – 0.005 mSv y-1, 0.002 – 0.006 mSv y-1 , 0.002 – 0.009 mSv y-1 , 0.002 – 0.004 mSv y-1 , 0.002 – 0.003 mSv y-1 for soils of the study area and 0.002 – 0.003 mSv y-1 , 0.001 – 0.003 mSv y-1 , 0.001 – 0.003 mSv y-1 , 0.001 – 0.002 mSv y-1 , 0.001 – 0.002 mSv y-1 , 0.001 – 0.004 mSv y-1 for crops of the study area. The outdoor annual effective dose equivalent for this study is also lower than the 0.1 mSv y-1 permissible limit recommended by World Health Organization (WHO), lower than 1 mSv y-1 limit recommended by International Commission on Radiological Protection (ICRP). The soil-to-food crop transfer factor ranged from 0.4 to 4.4 for 40K, for 226Ra, ranged from 0.2 – 2.1 and ranged from 0.1 to 2.5 for 232Th. The soil-to-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 in the relationship between the activity concentration of radionuclides in soils and food crops because the crops are gotten from plant and plants receive some amount of radionuclide concentration from other sources like cosmic rays, rainwater and air. The mean concentrations of the heavy metals in the soil and food crops of the study area followed the trend Mn<Zn<Cu<Pb<Cr<Co<Cd. The enrichment factors of Cu, Pb, Cr, Zn, Co and Cd ranged from 0.06 – 37.78 mg/kg; 0.07 – 49.58 mg/kg; ND – 35.78 mg/kg; 0.21 – 101.40 mg/kg respectively. Co and Cd were not detected. The calculated values of the geo-accumulation index ranged from -6.08 to – 0.24 mg/kg for Cu, - 4.91 to – 0.74 mg/kg for Pb, ND to – 1.15 mg/kg for Cr, – 10.32 to 0.41 mg/kg for Mn and – 5.14 to – 0.26 for Zn. Co and Cd were not detected, this indicates that the study area is practically uncontaminated with cobalt and cadmium. The ranges of pollution index values were 0.06 – 0.28 mg/kg for Cu, 0.09 – 0.99 mg/kg for Pb, 0.005 – 0.99 mg/kg for Mn and 0.12 – 0.67 mg/kg for Zn. The mean pollution load index value for all investigated elements was lower than 1 which indicates that the investigated soils and food crops are unpolluted. The mean daily intake of the heavy metals was lower than the values from literatures and recommended limit by International Atomic Energy Agency (IAEA) and ICRP. There was also no degree of linearity between the activity concentration and heavy metal concentration in the samples because they have different origin. The ingestion dose of the radionuclides having classified them into vegetables, cereals, fruits, tubers and legumes. For vegetables, the estimated ingestion dose ranged from 168.98 – 295.26 mg d-1, 0.36 – 1.32 mg d-1, 0.32 -0.39 mg d-1 of 40K, 232Th and 226Ra in Ondo State. 139.08 – 526.78 mg d-1 , 0.51 – 1.37 mg d-1 , 0.29 – 0.39 mg d-1 of 40K, 232Th and 226Ra in Ekiti State. 174.64 – 286.34 mg d-1 , 0.43 – 1.45 mg d-1 , 0.20 – 0.43 mg d-1 of 40K, 232Th and 226Ra in Osun State. 73.31 – 243.80 mg d-1 , 0.29 – 1.89 mg d-1 , 0.10 – 1.54 mg d-1 of 40K, 232Th and 226Ra in Oyo State. 42.57 – 198.44 mg d-1 , 0.39 – 1.43 mg d-1 , 0.26 – 0.42 mg d-1 of 40K, 232Th and 226Ra in Ogun State. 79.18 – 178.96 mg d-1 , 2.94 – 6.04 mg d-1 , 0.48 – 1.51 mg d-1 of 40K, 232Th and 226Ra in Lagos State. People in Ekiti State have the highest radionuclide intake of vegetables while Oyo State recorded the lowest. The daily intake in fruits were found to be in the range of 41.76 – 183.60 mg d-1, 0.05 – 0.45 mg d-1, 0.06 -0.08 mg d-1 of 40K, 232Th and 226Ra in Ondo State. 14.66 – 152.42 mg d-1, 0.06 – 0.28 mg d-1 , 0.05 – 0.08 mg d-1 of 40K, 232Th and 226Ra in Ekiti State. 16.18 – 102.22 mg d-1, 0.10 – 0.44 mg d-1 , 0.06 – 0.14 mg d-1 of 40K, 232Th and 226Ra in Osun State. 22.03 – 80.34 mg d-1, 0.08– 0.44 mg d-1 , 0.04 – 0.14 mg d-1 of 40K, 232Th and 226Ra in Oyo State. 6.22 – 145.42 mg d-1 , 0.07 – 0.20 mg d-1 , 0.05 – 0.19 mg d-1 of 40K, 232Th and 226Ra in Ogun State. 16.61 – 40.61 mg d-1, 0.37 – 0.94 mg d-1, 0.10 – 0.28 mg d-1 of 40K, 232Th and 226Ra in Lagos State. People of Lagos state have the highest radionuclide intake while Ogun state has the lowest radionuclide contents in its fruits. For cereals, the ingestion dose rate varied in the range of 78.11 – 326.92 mg d-1, 0.09 – 0.26 mg d-1, 0.05 -0.07 mg d-1 of 40K, 232Th and 226Ra in Ondo State. 51.19 – 537.60 mg d-1 , 0.09 – 0.19 mg d-1 , 0.05 – 0.07 mg d-1 of 40K, 232Th and 226Ra in Ekiti State. 129.60 – 594.18 mg d-1 , 0.07 – 0.21 mg d-1 , 0.05 – 0.08 mg d-1 of 40K, 232Th and 226Ra in Osun State. 105.97 – 479.67 mg d-1 , 0.08– 1.09 mg d-1 , 0.04 – 0.32 mg d-1 of 40K, 232Th and 226Ra in Oyo State. 379.68 – 479.67 mg d-1 , 0.07 – 0.24 mg d-1 , 0.03 – 0.07 mg d-1 of 40K, 232Th and 226Ra in Ogun State. 184.33 – 514.99 mg d-1 , 0.07 – 1.13 mg d-1 , 0.06 – 0.29 mg d-1 of 40K, 232Th and 226Ra in Lagos State. Occupants of Osun State recorded the highest daily intake. Ogun State had the lowest intake. The ingestion dose rate of 40K, 232Th and 226Ra content in the studied legumes varied with concentrations ranging from 55.68 – 93.58 mg d-1, 0.14 – 0.19 mg d-1, 0.07 -0.10 mg d-1 of 40K, 232Th and 226Ra in Ondo State. 62.30 – 214.27 mg d-1 , 0.16 – 0.37 mg d-1 , 0.10 – 0.12 mg d-1 of 40K, 232Th and 226Ra in Ekiti State. 11.05 – 194.16 mg d-1 , 0.16 – 1.12 mg d-1 , 0.08 – 0.29 mg d-1 of 40K, 232Th and 226Ra in Osun State. 13.42 – 109.12 mg d-1 , 0.15– 0.26 mg d-1 , 0.07 – 0.51 mg d-1 of 40K, 232Th and 226Ra in Oyo State. 7.21 – 93.12 mg d-1 , 0.16 – 0.85 mg d-1 , 0.11 – 0.30 mg d-1 of 40K, 232Th and 226Ra in Ogun State. 12.60 – 160.51 mg d-1 , 0.30 – 1.50 mg d-1 , 0.09 – 0.40 mg d-1 of 40K, 232Th and 226Ra in Lagos State. Osun State recorded the highest daily intake. Ingestion dose intake from the study area via tuber like food crop consumption range from 27.97 – 77.95 mg d-1, 0.08 – 0.13 mg d-1, 0.05 -0.08 mg d-1 of 40K, 232Th and 226Ra in Ondo State. 13.12 – 72.90 mg d-1 , 0.06 – 0.14 mg d-1 , 0.01 – 0.06 mg d-1 of 40K, 232Th and 226Ra in Ekiti State. 19.90 – 55.67 mg d-1 , 0.06 – 0.16 mg d-1 , 0.04– 0.06 mg d-1 of 40K, 232Th and 226Ra in Osun State. 5.60 – 31.50 mg d-1 , 0.06– 0.31 mg d-1 , 0.04 – 0.09 mg d-1 of 40K, 232Th and 226Ra in Oyo State. 20.08 – 25.79 mg d-1 , 0.05 – 0.95 mg d-1 , 0.02 – 0.06 mg d-1 of 40K, 232Th and 226Ra in Ogun State. 11.98 – 63.21 mg d-1 , 0.06 – 0.44 mg d-1 , 0.03 – 0.19 mg d-1 of 40K, 232Th and 226Ra in Lagos State. The values obtained from the ingestion rate of the vegetables, cereals, fruits, legumes and tubers of the study area are below the 1 mSvy-1 limit set by the International Commission on Radiological Protection (ICRP). The internal and external hazard exposure to carcinogenic radon in the studied samples were calculated. The external hazard index (Hex) for soils ranged from 0.10 – 0.16, 0.41 – 0.65, 0.14 – 0.22, 0.10 – 0.43, 0.11 – 0.19, 0.07 – 0.15 and 0.09 – 0.13, 0.07 – 0.13, 0.005 – 0.12, 0.07 – 0.12, 0.05 – 0.09, 0.08 – 0.19 for crops of the study area. The internal hazard index (Hin) ranged from 0.09 – 0.13, 0.07 – 0.13, 0.05 – 0.12, 0.07 – 0.12, 0.05 – 0.09, 0.08 – 0.19 and 0.002 – 0.004, 0.003 – 0.006, 0.003 – 0.005, 0.002 – 0.009, 0.002 – 0.004, 0.002 – 0.003 for soils and food crops of the study area. The value of this index is less than 1 mSvy-1 and this indicates that the radiation hazard has negligible hazardous effects to the respiratory organs of the occupants of the study area. The results of the excess lifetime cancer risk in this study ranged from 0.08 – 0.12 mSv y-1, 0.11 – 0.21 mSv y-1, 0.11 – 0.18 mSv y-1 , 0.09 – 0.34 mSv y-1 , 0.09 – 0.15 mSv y-1 , 0.06 – 0.12 mSv y-1 for soils of the study area and 0.08 – 0.12 mSv y-1 , 0.05 – 0.09 mSv y-1 , 0.005 – 0.10 mSv y-1 , 0.03 – 0.08 mSv y-1 , 0.03 – 0.05 mSv y-1 , 0.07 – 0.15 mSv y-1 for crops of the study area. The mean probability of developing cancer in a lifetime by occupants of the study area through consumption of the food crop is 1.12 x 10-5 and that means for every one million humans 11 persons are liable to developing cancer during a lifetime. This value constitutes a fraction of the 1% of the population that was estimated by Biological Effect of Ionizing Radiation (BEIR) report of developing cancer as a result of exposure to radiation at some points in their entire lifetime