Radiological impact from NORM discharges in the North Sea - A comparative study of aquatic modelling approaches and their implications for environmental protection.

This study examines the radiological impact of naturally occurring radionuclides discharged with produced water from offshore oil and gas platforms in the North Sea. To assess the impact, two modelling approaches were compared: the compartmental model POSEIDON-R and the hydrodynamic model THREETOX. POSEIDON-R, traditionally used for large-scale assessments, divides the marine environment into interconnected "boxes" governed by mass-balance equations. In this study, its resolution was enhanced by introducing micro-compartments to capture near-field behaviour. THREETOX, in contrast, solves the Reynolds-averaged Navier-Stokes equations, providing a detailed three-dimensional simulation of plume dispersion, salinity and temperature stratification, and sediment interactions. Results show that while concentrations and doses are small, near-field hydrodynamics have a major influence. Within 3 km of the discharge point, estimated dose rates to non-human biota varied by up to six orders of magnitude depending on the model and the physical properties of the discharge. The hydrodynamic model captured localized areas of elevated concentrations of naturally occurring radionuclides missed by standard compartment models, although the refined POSEIDON-R with micro-compartments yielded improved agreement. Long-term simulations (40 years) showed sediment-dwelling species such as polychaete worms receiving the highest dose - up to about 1.8 μGy h-1 from radium-226, or 4.0 μGy h-1 when including radium-228, lead-210 and polonium-210 - but remain well below ICRP's derived consideration reference level of 400 μGy h-1 for worms. The study concludes that simplified compartmental models remain practical for prospective assessments. Nevertheless, accurate near-field evaluation requires refined or nested models, guided by hydrodynamic data. Furthermore, assessments should include long-term modelling of ecological exposure scenarios, due to radionuclide accumulation in the sediment.