Natural Radioactivity and Radiological Hazard Assessment of Oil-Field Soils in Apkai and Umusedege Communities, Delta State, Nigeria

Abstract

Naturally occurring radioactive materials (NORM) in soil constitute an important pathway for human exposure to ionising radiation, particularly in areas affected by petroleum exploration and related environmental disturbance. This study evaluated the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, and the associated radiological hazards, in soils from the Apkai and Umusedege oil-field communities of Delta State, Nigeria. Soil samples were analysed by gamma-ray spectrometry using a NaI(Tl) detector housed in a low-background counting system at the Centre for Energy Research and Training, Ahmadu Bello University, calibrated against IAEA reference materials (RGK-1, IAEA-448 and RGTh-1) and counted for 30,000 s. Activity concentrations ranged from 22.743–59.730 Bq/kg for ²²⁶Ra, 17.760–52.860 Bq/kg for ²³²Th and 149–314.037 Bq/kg for ⁴⁰K, with corresponding mean values of 36.102, 50.237 and 251.903 Bq/kg. Relative to the control soil, the mean activities were approximately 2.49, 2.38 and 2.96 times higher for ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, with ⁴⁰K showing the strongest relative enhancement. Against the UNSCEAR worldwide average, mean ²²⁶Ra was only marginally elevated (1.03×), ²³²Th was moderately elevated (1.67×), and ⁴⁰K remained below average (0.63×). The mean absorbed dose rate, annual effective dose and radium equivalent activity were 42.22 nGy/h, 0.076 mSv/y and 91.627 Bq/kg, respectively, corresponding to approximately 72%, 7.6% and 25% of their respective reference values. The mean external hazard index, internal hazard index, representative gamma index and alpha index were 0.247, 0.335, 0.376 and 0.162, all below unity. Estimated total cancer risks were 3.18×10⁻⁶ for adults and 4.32×10⁻⁶ for the whole population, both within the USEPA-adopted acceptable range. Although several radionuclide activities were elevated relative to the control soil, and ²³²Th was elevated relative to the worldwide average, all derived radiological hazard indices remained below their recommended thresholds. The findings indicate measurable enhancement of natural radionuclide activity relative to background, without evidence of unacceptable radiological hazard under the criteria applied, and provide a baseline for continued environmental monitoring of oil-field communities in the Ndokwa region of Delta State.