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Natural particle dominance of radionuclide reservoirs in the presence of microplastics: A quantitative cross-compartment analysis for aquatic systems
Summary
This review of over 180 studies finds that when radioactive particles end up in oceans, rivers, and lakes, they overwhelmingly stick to natural particles like sediment and algae, not microplastics, which hold less than 2% of the total. That's actually reassuring news: while microplastic pollution is a real concern, this research suggests they aren't a major carrier of radioactive contamination in water, including near sites like Fukushima where treated wastewater has been released.
We introduce the Relative Reservoir Dominance Index (RRDI), a dimensionless metric integrating substrate-specific distribution coefficients (K, L/kg) with field-measured particle mass concentrations to quantify what fraction of particle-associated radionuclide inventory is held by natural particles relative to microplastics (MP) in aquatic systems. Applying this framework across six aquatic compartments - open ocean, coastal marine, oligotrophic gyre, river flood, river baseflow, and lake - with Monte Carlo uncertainty propagation (10,000 iterations), we demonstrate that natural particles retain ≥98.0% of particle-associated radionuclide inventory in all scenarios evaluated. This critical review synthesizes over 180 peer-reviewed studies. The RRDI values approaching 1.00 indicate near-total natural particle dominance. A three-component extension is presented for radionuclides with high aqueous mobility (°Sr, Tc, H). Our synthesis reveals a consistent K hierarchy spanning five orders of magnitude, from ∼10 L/kg for pristine MP to ∼10 L/kg for Fe-(hydr)oxide-associated Pu. Monte Carlo uncertainty propagation yields RRDI ≥ 0.980 across all compartments (95% CI: 0.990-0.9999). The oligotrophic gyre represents the global minimum (RRDI = 0.980 under bounding conditions; salinity-corrected K applied); gyre RRDI is sensitive to the local C/C ratio rather than absolute C, rising to ≥ 0.999 under central concentration estimates. For non-sorbing radionuclides (H, C, Tc, I), dissolved-phase transport dominates; biofilm-coated MP may represent a novel I retention pathway. Post-2023 ALPS-treated water discharge from Fukushima Daiichi introduces monitoring imperatives for H, C, °Sr, and Tc in coastal Pacific environments. We introduce the Microplastic Radionuclide Vector (MRV) framework, formally define and experimentally specify the Biofilm Substrate Convergence Hypothesis (BSCH), and provide six evidence-ranked research priorities structured as a predictive modelling and experimental roadmap for the radioecology community.