Integrated Human Health and Radiological Risk Assessment of Heavy Metals and Naturally Occurring Radionuclides in Soil and Water from Oil

Abstract

Oil and gas exploration can both introduce and redistribute potentially toxic metals and naturally occurring radioactive material across the soils and water bodies of surrounding communities, opening exposure pathways that reach well beyond the immediate production site. This study evaluated the human-health risk from heavy metals and the radiological risk from naturally occurring radionuclides in soil and water sampled from the Apkai and Umusedege oil-field communities in Ndokwa East and Ndokwa West Local Government Areas of Delta State, Nigeria. Sixty-three soil samples and eleven water samples were analysed using atomic absorption spectrophotometry, X-ray fluorescence spectroscopy, gross alpha/beta counting and gamma-ray spectrometry, covering Ni, Cr, Cu, Co, Pb, Cd, Fe, Mn, As and Zn. Human-health risk was modelled for children and adults using USEPA exposure equations across ingestion, inhalation and dermal-contact pathways; the pathway-level components were audited directly against their own reported totals by direct summation of their reported pathway contributions. Once audited, the combined soil-and-water hazard index came to 2.53 for children and 13.60 for adults, with ingestion the dominant soil pathway for both groups and an unusually large adult water-dermal contribution driving the adult total; both hazard indices exceed the HI = 1 screening threshold. Total excess lifetime cancer risk from heavy-metal exposure was 7.18×10⁻⁶ for children and 2.40×10⁻⁶ for adults, both remaining within the USEPA-adopted acceptable range. In water, gross alpha and beta activities averaged 1184 and 2164 mBq L⁻¹ respectively, peaking at 3160 and 6145 mBq L⁻¹, with corresponding mean annual effective doses of 2.154, 1.133 and 0.566 mSv y⁻¹ (alpha) and 4.087, 2.106 and 1.053 mSv y⁻¹ (beta) for adults, children and infants respectively. Gamma spectrometry of soil gave mean activity concentrations of 36.102 Bq kg⁻¹ for ²²⁶Ra, 50.237 Bq kg⁻¹ for ²³²Th and 251.903 Bq kg⁻¹ for ⁴⁰K, with a mean absorbed dose rate of 42.22 nGy h⁻¹, annual effective dose of 0.076 mSv y⁻¹ and radium equivalent activity of 91.627 Bq kg⁻¹. Mean external hazard, internal hazard, gamma representative-level and alpha indices were 0.247, 0.335, 0.376 and 0.162 respectively, and the calculated radiological cancer risks were 3.18×10⁻⁶ for adults and 4.32×10⁻⁶ for the whole population. Taken together, the audited results point to heavy-metal exposure with elevated hazard indices for both children and adults as the primary human-health concern, while soil NORM levels imply comparatively low external radiological hazard. Water, in contrast, showed elevated gross alpha/beta activity and associated doses alongside the dominant adult dermal hazard quotient, and therefore warrants continued radiological and chemical monitoring, more targeted radionuclide-specific follow-up, and independent verification of the adult water-dermal exposure calculation.