Radium isotopes can co‑precipitate with barium in baryte (BaSO₄) scales that form on internal surfaces of offshore oil and gas infrastructure. These minerals, classified as naturally occurring radioactive materials (NORM), may pose ecological risks if released into the marine environment. Although baryte is highly insoluble in oxic seawater, reductive dissolution may occur in suboxic sediments, potentially releasing co‑precipitated radioisotopes into sediment porewaters where they may be bioaccumulated by resident organisms.
To investigate this exposure pathway, we conducted a mesocosm experiment at the National SeaSimulator. Marine sediment was amended with NORM in a concentration series between background and 1050 Bq/kg 226Ra and 190 Bq/kg 228Ra (in partial equilibrium with progeny) and acclimated for 14 days under flow‑through seawater conditions. Four marine organisms were then introduced and exposed for 28 days. After the exposure period, organisms were recovered and their tissues analysed for ²²⁶Ra, ²¹⁰Pb, and ²¹⁰Po activity concentrations. During the experiment, diffusive gradients in thin films (DGT) equipped with MnO₂ and Chelex‑100 binding layers were deployed in the sediments to quantify fluxes of ²²⁶Ra, ²¹⁰Po, and divalent transition metals from sediments and NORM into porewaters.
We found that ²²⁶Ra and ²¹⁰Po bioaccumulation occurred in some, but not all, species, while DGT‑labile fluxes of the radioisotopes remained minimal. Organism‑to‑NORM concentration ratios were derived but showed a negative relationship with sediment activity-concentration. These data will support more environmentally realistic dose assessments for marine organisms, strengthening radiological risk assessments for offshore decommissioning activities.