Uptake and Distribution of Natural Radionuclides in Cassava Crops from Nigerian Government Farms

Abstract

Radioactivity distribution and transfer factor (TF) in plants are crucial parameters used to assess radioactive contamination in the environment, impact of soil radioactivity on agricultural crops and its risks to humans.  The root crop cassava (Manihot esculenta) provides about 50 percent of the calories consumed in Nigeria. Gamma - ray spectroscopy was used to measure activity concentrations of 226Ra, 232Th and 40K in cassava root and soil. The average activity concentration of 40K, 226Ra and 232Th in cassava was 565.31± 13.17, 21.89±5.94 and 817.28±2.52 Bqkg-1 respectively. The mean activity concentration   40K, 226Ra and 232Th in soil range from 92.07±35.08 to 689.28±14.35 Bqkg-1with a mean value of 413.64±21.22 Bqkg-1, 5.37 ± 8.90 to 64.93 ± 7.23 Bqkg-1 with a mean value of 54.43 ± 3.22 and BDL to 928.15 ± 2.36 Bqkg-1 with a mean value of 561.67 ± 2.21 Bqkg-1. The transfer values for 226Ra, 232Th and 40K were in the range of 0 to 1.81, 0 to 3.41 and 0.68 to 4.5 respectively. The high value of transfer factor for 40k may be due to its importance in plant growth, fertilization and adaptability of plant to environmental pressures. It may have also been enhanced by the application of NPK fertilizers in those farms. Thorium showed the highest mean transfer factor which may be due to its higher accumulation in soil and higher uptake by plants (Figure 3). The average transfer factors of 226Ra (0.99) < 40K (1.55) < 232Th (1.66) show that although activity concentration of the natural radioisotopes in the area under study are high, the rate at which they are transferred to cassava are still moderate.  The average values of radium equivalent activity (Raeq), absorbed dose rate (D), annual effective dose rate (AEDE), internal hazard index and excess life cancer risk (ELCR) are 1009.27 Bqk-1, 346.50 nGyh-1, 1.51 mSvy-1, 2.78 and 3.92 x 10-3 for respectively. These values were higher than their corresponding permissible values of 370Bqk-1, 55nGyh-1, 1.0 mSvy-1, 1.0 and 0.29 x 10-3 respectively. The mean values of Hex and Hin are greater than unity and may, therefore, constitute a significant radiological health risk. The mean annual gonad dose estimated value of 2943.90 mSvy-1  was above the world acceptable value of 300 mSvy-1 and the annual effective dose in all the samples except in few locations as shown in Figure 2, exceeded the safe value of 1.0 mSvy-1. The use of soil from these farms and the crops may constitute a threat to the bone marrow and general health conditions of the inhabitants.