Natural Radioactivity Levels in Surface Soil of Ogba/Egbema/Ndoni Oil and Gas Fields

Abstract

The activity concentrations and the gamma-absorbed dose rates of the terrestrial naturally occurring radio nuclides viz. 226Ra, 232Th and 40K were determined in soil samples collected from twelve oil fields and their host communities, using gamma ray spectrometry. The soil activity ranges from 10.10 to 41.23 Bq/kg for 226Ra, 7.42 to 30.31 Bq/kg for 232Th and 92.42 to 482.79 Bq/kg for 40K with mean values of 19.16, 21.26 and 224.29 Bq/kg, respectively for host community soil. In the field soil sample, the activity concentration ranges from 16.27 to 52.19 Bq/kg for 226Ra, 9.72 to 34.13 Bq/kg for 232Th and 134.50 to 395.15 Bq/kg for 40K with mean values of 29.61, 17.41 and 262.63 Bq/kg, respectively. The concentrations of these radionuclides are compared with the values of the control samples and the UNSCEAR, 2000 standards of 35.0, 30.0 and 400 Bq/kg for 226Ra, 232Th and 40K respectively. The measured activity concentration of 226Ra, 232Th and 40K in soil is lower than the world average except in two oil fields that 226Ra and 40K exceeded the world average. Radium equivalent activities are calculated from the results to assess the radiation hazards arising due to the use of these soil samples in the construction of dwellings. All the soil samples have radium equivalent activities lower than the limit set in the UNSCEAR report (370 Bq/kg). The overall mean outdoor Absorbed Dose rate are 32.17 and 35.45nGy•h−1 respectively for host community soil and field soil samples. The corresponding effective dose calculated has mean values less than 1.0 mSvy-1, the limit set by WHO (2008). The hazard indices calculated were all less than unity (1) showing that all the soil/sediment samples sampled are still safe for building purpose since their radiological impact is minimal. The percentage contribution of each of these radionuclide are; 64.77% for radium-226, 3.13% for thorium-232 and 32.10% for potassium-40. The entire radiation hazard indices are within the acceptable limit therefore, no immediate health risk as a result of these radionuclide but continuous exposure may result to a significant health impact.