Radiochemical Assessment and Dose Estimation from Naturally Occurring Radioactive Material in Oil Field Brines and Produced Water in Offshore Nigerian Fields

Naturally occurring radioactive material in produced water and oil field brines from Nigerian offshore production facilities represents a significant radiological occupational hazard and marine pollution concern, yet no comprehensive study has characterised the isotopic composition, activity concentrations, radioactive scale deposition mechanisms, and associated dose estimates for workers and the marine environment in Nigerian offshore fields using modern radiochemical methods. This study conducts a comprehensive radiochemical assessment and develops original dose estimation models for naturally occurring radioactive material in produced water, scale deposits, and sludge from six Nigerian offshore field platforms in the Bonga, Agbami, and Egina oilfield clusters. Radionuclide analysis employs high-purity germanium gamma spectrometry for thorium-232 and uranium-238 decay series members, radiochemical separation followed by alpha spectrometry for polonium-210 and radium-226 individual quantification, and inductively coupled plasma mass spectrometry for uranium and thorium concentration measurement. Scale deposits from production tubing and water treatment equipment are characterised mineralogically by X-ray diffraction and Raman spectroscopy, and the thermodynamic conditions governing radium sulphate and barium sulphate co-precipitation are modelled using PHREEQC thermodynamic speciation coupled with a surface complexation model calibrated to measured partition coefficients. Probabilistic Monte Carlo dose assessment models are developed for external irradiation, inhalation, and ingestion exposure pathways for offshore platform workers, and for marine biota dose assessment using the International Atomic Energy Agency ERICA framework. An original radiological risk index for Nigerian offshore naturally occurring radioactive material is proposed as a regulatory tool for the Department of Petroleum Resources. Keywords: naturally occurring radioactive material, produced water, offshore oil fields, radiochemical assessment, dose estimation Nigeria