Industrial processing of mineral resources can lead to the redistribution and concentration of naturally occurring radioactive materials (NORM). In the Czech Republic, hazards associated with the release of NORM from industrial facilities, its handling, and long-term exposure are assessed by comparing activity concentrations of 10 long-living radionuclides with the associated clearance levels. Six radionuclides are determined by gamma-ray spectrometry: 238U, 226Ra, 210Pb, 228Ra, 228Th, and 40K. Activity concentrations of four radionuclides - 234U, 230Th, 210Po, and 232Th - are derived using a set of estimative equations. These equations frequently lead to overestimation of 230Th and 232Th, and underestimation of 234U. The aim of this work was to study the equilibrium in the first segments of the uranium (238U -234U - 230Th) and thorium (232Th - 228Ra - 228Th) decay series, and to propose an updated set of estimative equations for 234U, 230Th, and 232Th for materials that satisfy the equilibrium condition in the given segments. Importantly, materials originating from different industrial processes were assessed individually, and all materials included in this study were released from workplaces in the Czech Republic. In total, 48 samples from 7 workplace types were split into 8 material categories. For each sample, gamma-ray spectrometry was used to determine 228Ra, and 228Th, and alpha spectrometry to determine 238U, 234U, 230Th, and 232Th. Activity ratios for each radionuclide pair within the given decay segment were assessed using orthogonal distance regression and/or by evaluating the activity ratios. Material categories exhibiting radioactive equilibrium in both decay segments were ashes and slag from coal combustion plants, samples utilized in cement manufacturing, old building materials, drilling muds, and waste from zircon and zirconia processing. In contrast, sludges, sinters and filter materials from water treatment facilities, along with waste from oil and gas transportation and processing, were identified as materials with disturbed equilibrium in the 238U -234U - 230Th and 232Th - 228Ra - 228Th decay segments.
Rapid prediction of atmospheric radionuclide dispersion is essential for emergency decision-making and radiation protection in nuclear accidents. Although computational fluid dynamics (CFD) methods provide high accuracy, their limited efficiency makes it difficult to meet timeliness requirements. To address this limitation, this study proposed a method for rapid atmospheric radionuclide dispersion prediction and radiation dose assessment based on a physics-aware spatio-temporal neural operator (PA-STNO). Within the proposed model-based method, condition encoding represents variations in meteorological and release conditions, spatio-temporal attention enhances plume-evolution and spatial-structure features, the Fourier Neural Operator backbone models long-range transport dependencies, and a physics-aware composite loss improves the representation of high-concentration regions, plume boundaries, and overall concentration-mass consistency. Multi-condition tritium dispersion data generated with OpenFOAM were used for model training and evaluation. On the primary test set, PA-STNO achieved a Relative L2 of 0.21, an FAC2 of 0.92, and a Plume IoU of 0.82, while substantially improving prediction speed. The model also outperformed the baseline models, and a supplementary comparison with a conventional Lagrangian dispersion model showed reasonable agreement, with a mean absolute relative deviation of 8.36%. The predicted fields were further used for radiation dose assessment, linking rapid concentration prediction with dose calculation and supporting the preliminary assessment of dispersion consequences and radiological impacts.
Between 2016 and 2021, Germany conducted its first Total Diet Study to assess dietary exposure to various substances, including natural radionuclides. Activity concentrations of Pb-210, U-234, U-238, Ra-226, and Ra-228 were determined in 163 food samples of the German diet. Po-210 was additionally included using inferred values because of its known high relative contribution to ingestion dose. This study provides the first comprehensive assessment of ingestion doses from natural radionuclides in foods consumed in Germany. Two methodological approaches were applied. A conservative approach assigned 134 activity concentrations per radionuclide to the eight food groups defined in the German Radiation Protection Ordinance, resulting in a mean effective dose of 98 μSv·a-1 (median: 58 μSv·a-1). A refined approach incorporating detailed food consumption data, consistent with current dietary exposure assessment practices, yielded a markedly lower mean annual effective dose of 26 μSv·a-1 (median: 24 μSv·a-1). In both approaches, Po-210 and Ra-228 were the main contributors to the total dose, accounting for 39%/36% and 31%/34%, respectively (conservative/refined). Pb-210 is responsible for 22%/21%, while Ra-226 accounted for 7%/9%. Uranium isotopes contributed only marginally (<1%). The results demonstrate the importance of refined consumption-based assessments for realistic dose estimation and highlight the relevance of Po-210 and Ra-228 in dietary exposure. In addition, the investigation successfully shows, that the conservative approach reliably provides an upper estimate for the ingestion dose as intended. These findings provide a robust basis for radiological risk assessment and support ongoing food safety evaluations.
Uranium is an element known for its notable chemical toxicity and relatively low radiation risk. It poses a danger to the environment and health, especially in countries such as Kazakhstan, where uranium mining is prevalent. These elements can leach into different water sources during mining, processing and waste management, which requires the use of effective disposal methods. Several treatment technologies have been developed for uranium remediation. Among these options, adsorption is particularly promising due to its high efficiency, straightforward operation, affordability and low risk of secondary pollution. Recent years have seen a growing research focus on advanced nanosorbents as a new generation of adsorbents. MXenes, metal-organic frameworks, and covalent-organic frameworks are particularly prominent due to their large surface area, adjustable pore structures, numerous functional groups, and strong affinity for U(VI). These materials have improved adsorption capabilities even in complex aqueous environments. Ongoing research is crucial to developing effective, sustainable, and scalable uranium disposal solutions that protect water resources and enhance environmental safety in uranium-rich regions. This review article discusses the environmental challenges related to uranium and highlights advanced sorbents as essential materials for removing uranium from aqueous systems.
Radon (222Rn), the only naturally occurring radioactive inert gas in the uranium decay chain, undergoes a multi-field coupled process of release and migration in porous media such as soil, rock and building materials. This paper reviews the emanation mechanisms (recoil, collision loss, particle geometry) and migration behaviours (diffusion, advection, relay transport) of radon, discusses the effects of intrinsic medium properties (nuclide abundance, pore structure, water content) and external environmental factors (temperature, wind speed, crustal stress, atmospheric pressure) on radon evolution, and summarises the progress in laboratory experiments, CFD numerical simulations, as well as the application achievements in indoor pollution control, mine safety monitoring, groundwater tracing and oil-gas exploration. Finally, the bottlenecks such as complex geological environment simulation, long-term cross-regional in-situ monitoring, and non-ideal multi-field coupled models are analysed, and future directions including cross-scale from micro to macro, development of smart sensors, and dynamic simulation of heterogeneous media are envisioned, providing scientific support for radiation protection and resource exploration.
Analytical and Monte Carlo approaches for quantitative in situ 137Cs inventory assessment were evaluated at a single field site using experimentally determined depth-profile data obtained from volcanic ash soils on Jeju Island, Korea. A relatively uniform test site was selected based on the measured horizontal distribution of 137Cs. The measured 137Cs depth profile exhibited a distinct subsurface activity maximum at a depth of approximately 10-15 cm, rather than the monotonic exponential decrease commonly assumed for fallout radionuclides. Three approaches for in situ inventory assessment were evaluated: a conventional analytical method based on an equivalent exponential distribution, a modified analytical method incorporating the measured layer-by-layer activity and bulk-density profiles with an infinite-plane source assumption, and a Monte Carlo simulation incorporating the same measured profiles within a finite source geometry. The conventional analytical method overestimated the inventory by approximately 34%, whereas the modified analytical and Monte Carlo estimates were approximately 8-9% higher than the laboratory-derived inventory and differed from each other by 1%. For the investigated site, these results indicate that the modified analytical approach can overcome limitations of the conventional analytical method in representing non-exponential depth distributions and depth-dependent bulk density while retaining the simplicity of the conventional framework for conversion-factor calculations and providing results comparable to those obtained using MCNP.
Naturally Occurring Radioactive Materials (NORM) generated during conventional mining activities represent a potential source of environmental contamination and radiological exposure to workers, the public, and ecosystems. Although international radiation protection frameworks provide risk-based approaches for managing NORM, implementation in many developing mining jurisdictions remains limited. This study presents the first systematic benchmarking of Tanzania's regulatory and institutional framework governing NORM in conventional mining against the International Atomic Energy Agency (IAEA) graded approach (SSG-60) and International Commission on Radiological Protection (ICRP) recommendations. A structured document analysis and comparative regulatory evaluation were conducted to assess national mining, environmental, and radiation protection legislation and institutional arrangements. The assessment examined legal recognition of NORM, authorization systems, institutional coordination, monitoring requirements, waste management provisions, and regulatory infrastructure. The findings reveal that, despite the existence of radiation protection legislation and recent adoption of exemption thresholds aligned with international standards, Tanzania lacks an integrated governance framework for NORM management. Radiological risk assessment is not routinely incorporated into mining authorization or environmental impact assessment processes, institutional responsibilities remain fragmented, and critical regulatory tools, including a national NORM inventory, technical guidelines, and routine monitoring systems, are absent. The study highlights the need to operationalize the IAEA graded approach through integrated legislation, coordinated institutional oversight, strengthened technical capacity, and improved environmental monitoring. The proposed benchmarking framework provides a practical model for strengthening NORM governance in Tanzania and other resource-dependent economies.
Indoor radon accounts for 37% of population-level exposure to ionizing radiation in the United States. However, radon metrics are typically reported at coarse spatial scales, potentially obscuring meaningful local variation. We developed a high-resolution modeling framework to estimate indoor radon concentrations across Utah while explicitly quantifying predictive uncertainty. A total of 19,497 residential radon measurements collected between 2006 and 2017 were combined with environmental and housing characteristics and analyzed using a geospatial neural network that accommodates spatial dependence and nonlinear associations. Predictions were generated on a uniform hexagonal grid at 0.73 km2 resolution (H3 level 8). Out-of-sample predictions aggregated to the H3 level 8 grid showed good agreement with observed concentrations (Pearson r=0.64), while household-level predictions exhibited more moderate agreement (r=0.45). The model produced well-calibrated uncertainty estimates, with 24.1% of held-out observations exceeding the predicted 75th-percentile threshold. Maps of predicted radon concentrations and the probability of exceeding the U.S. EPA action level of 148 Bq/m3 (4 pCi/L) revealed substantial fine-scale spatial heterogeneity that was not apparent in conventional coarse-resolution summaries, with greater local variability observed in densely monitored urban counties than in sparsely sampled regions. High-resolution radon models that explicitly quantify uncertainty provide a useful framework for characterizing the spatial distribution of indoor radon and identifying areas of elevated exceedance risk. These findings highlight the value of fine-scale monitoring data and uncertainty-aware modeling approaches for radon exposure assessment, environmental risk characterization, and radon-related health research.
Radon releases from uranium-mining legacies are commonly evaluated in relation to inhalation exposure. Here, the separate question is whether the 210Pb produced from sustained legacy releases could perturb the inventories used to date and interpret soils, sediments, peat and other environmental archives. This paper bounds that possibility and tests the natural-meteoric provenance assumption underlying regional excess-210Pb interpretation. Because decay-chain mass balance conserves atoms rather than activity, conversion from a 3.8-day radionuclide to a 22.3-year radionuclide reduces the associated activity source term by λ210/λ222 ≈ 4.7 × 10-4. For a documented remediation-era source scale of 1014 Bq y-1, even complete capture of all generated 210Pb over 106 km2 would yield only about 0.05 Bq m-2 y-1, compared with a contextual natural deposition range of 50-200 Bq m-2 y-1. A complementary source-ratio diagnostic indicates that individual legacies contribute about 10-4 of the natural radon source within representative continental footprints, while the largest historical district aggregates contribute about 10-3-10-2. Documented legacy releases are therefore unlikely to perturb regional-mean 210Pb deposition or the inventories derived from it detectably. Localized source-receptor episodes cannot be excluded, but particulate and erosion-mediated inputs from 226Ra-bearing residues are the more plausible pathways at facility and catchment scales and may warrant explicit consideration and targeted, pathway-based monitoring.
This study develops an Explosive-release Particle Tracking Model (EPTM) and couples it with the Worldwide version of System for Prediction of Environmental Emergency Dose Information (WSPEEDI) to explicitly represent the explosion-induced initial condition in atmospheric dispersion simulation. The developed model is designed to resolve the particle dynamics immediately after an explosive release and to provide a physically based initial condition for subsequent atmospheric transport simulations. To demonstrate the capability and impact of EPTM, numerical experiments were conducted under the idealized meteorological field. The results show that the impact of explosion-induced initial motion on concentration and surface deposition is substantial near the explosion point but diminishes with transport distance. Immediately after explosive release, the explosive case produces a rapid vertical redistribution of radioactive materials. This vertical redistribution increases concentration near the surface and increases surface deposition near the release point compared with the non-explosive case. As a result, air concentration maxima and surface deposition patterns near the explosion point are controlled by the explosion-driven initial conditions represented by EPTM. In contrast, in the downwind area, differences in air concentration and surface deposition between the explosive and non-explosive cases become small because horizontal advection and turbulent diffusion govern the spatial distribution of the concentration and deposition, reducing the relative influence of the initial explosion as the plume evolves. Consequently, increasing explosive energy amplifies the contrast of air concentration and surface deposition near the explosion point while exerting limited influence on downwind concentration and deposition distributions.
Polonium-210 (210Po) is a naturally occurring radioisotope that can bioaccumulate in aquatic organisms, contributing substantially to internal radiation dose. However, empirical transfer parameters for freshwater ecosystems remain limited, constraining accurate dose assessments. This study presents a comprehensive assessment of 210Po transfer and internal dose rates across 38 freshwater systems spanning non-mine and mine-impacted environments in Canada. Site-specific 210Po activity concentrations in water, sediment, whole organisms and fish tissues were used to derive empirical sediment-water partition coefficients, bioaccumulation factors, and internal dose rates. Bioaccumulation factors followed clear organism-level patterns, with benthic invertebrates exhibiting the highest uptake and macrophytes the lowest. Among fish, northern pike had the greatest whole-body exposure, and viscera consistently received the highest tissue-specific dose. Bioaccumulation factors for fish exceeded international reference values and measured internal dose rates across organisms were frequently higher than published background benchmarks for naturally occurring radionuclides. Together, these findings improve current understanding of 210Po transfer and internal dose in Canadian freshwater systems and provide empirically derived parameters that can support ecological risk assessments where site-specific data are unavailable. The results also highlight the importance of organism group, tissue type and local site conditions when applying transfer parameters to freshwater dose assessments.
Depleted uranium (DU) sits at the intersection of radioactive-waste management, resource policy and environmental radioactivity. Its initial specific activity is low, and it may be retained as a potential resource or managed within low-level, low-activity or material-management categories. These categories are useful for regulation and storage, but they do not resolve the final-disposal problem for large concentrated inventories. DU is persistent on geological timescales, chemically toxic, potentially mobile under some groundwater conditions, and radiologically evolving through ingrowth of Th-230, Ra-226, Rn-222, Pb-210, Po-210 and other daughters. This review article examines DU disposal-route ambiguity and its environmental-radioactivity consequences. It synthesises institutional and technical evidence from the United States, France, the United Kingdom, Germany, the Netherlands, IAEA materials, European classification studies and near-biosphere uranium/DU stewardship examples. The record is fragmented but consistent on one point: DU disposal is not settled by classification alone. Some systems use LLW-type disposition, others preserve resource status, and others connect non-reused uranium to long-lived LLW, intermediate-depth or geological-disposal planning. The paper does not argue that near-surface disposal is generically unacceptable, nor that every DU inventory requires a high-level-waste-type geological repository. Its claim is narrower: the adequacy of surface/shallow, intermediate-depth or geological disposal must be demonstrated for the specific inventory, waste form, site conditions and assessment timescale. For large DU inventories, a credible safety case should address uranium mobility, daughter-controlled pathways, radon, groundwater transport, intrusion, erosion, chemical toxicity and long-term uncertainty.
Reliable chronological frameworks are fundamental for reconstructing Quaternary environmental change, paleoclimate evolution, landscape development, and human history. Recent advances in radioisotope geochronology have substantially improved the accuracy, precision, and temporal resolution of dating across diverse geological and environmental archives. This review synthesizes developments in major Quaternary dating methods radiocarbon (14C), lead-210 (210 Pb), cesium-137 (137Cs), uranium-series, potassium-argon (K-Ar), argon-argon (40Ar/39Ar), and cosmogenic radionuclide techniques (10Be, 26Al, 36Cl, 81Kr) and highlights emerging analytical technologies including accelerator mass spectrometry, multi-collector ICP-MS, atom trap trace analysis, Bayesian age-depth modelling, and integrated multi-proxy approaches. Applications spanning terrestrial, marine, lacustrine, archaeological, and glacial settings demonstrate the value of combining complementary methods to build robust chronologies for sedimentation, tectonics, landscape evolution, paleoclimate reconstruction, and groundwater studies. Improvements in calibration datasets, uncertainty quantification, computational modelling, and high-resolution analytics have expanded the applicability of radioisotope geochronology to more complex systems. Persistent challenges contamination, open-system behaviour, reservoir effects, inheritance, and post-depositional alteration necessitate rigorous sample selection and methodological validation. We recommend prioritizing standardized analytical protocols, enhanced calibration frameworks, interdisciplinary integration, and adoption of machine-learning-assisted chronological modelling to further improve precision and reproducibility. Continued innovation in radioisotope geochronology promises more reliable chronological frameworks for investigating Quaternary environmental change and for reconstructing Earth's climatic, geological, and ecological conservation.
Natural radionuclides in soil produce an outdoor photon field, but an organ absorbed dose also depends on source depth, body geometry, scored mass, emission data and charged-particle transport settings. This work calculates absorbed-dose quantities in defined scoring regions for 40K, the 238U decay series and the 232Th decay series with Geant4 and a simplified block phantom. The Geant4 source uses the full nuclear gamma-emission library recovered from the adopted RadioactiveDecay6.1.2 and PhotonEvaporation6.1.2 data. The production calculation consisted of 20 independent replicas and 4.0×109 source histories. Source-specific results are absorbed-dose-rate coefficients, while the adopted-mixture results are scored-region dose rates for the stated activity mixture. The outdoor photon field was compared with a matched FLUKA calculation. The median FLUKA/Geant4 spectral ratio was 1.001 for the silicon-reference case and 0.993 for the realistic-soil case, with most matched nonzero spectral bins within a factor of two. The Geant4 scored-region results give 8 strong, 11 acceptable, 7 low-precision and 2 unresolved scored-region/source quantities by independent-replica uncertainty. Low-precision and unresolved quantities are reported with their precision labels and are not used as stable coefficients. A complementary calculation with the ICRP 110 adult male and female reference phantoms compared the lungs, stomach and colon regions; the ICRP 110/simplified-phantom reconstructed-dose ratios ranged from 0.98 to 1.22. The main coefficient set remains specific to the simplified geometry and is not presented as a set of ICRP reference-phantom coefficients. The added ICRP 110 calculation is limited to the comparison of corresponding lung, stomach and colon regions.
The various sources from the natural world and the human world like uranium and thorium series decay, 40K, radon gas, nuclear power operations, uranium mining, industries, medicine, nuclear weapon tests, and accidental spills, contribute to the formation of radionuclides in the environment. Radionuclides have drawn concern regarding the persistence, redistribution, and potential ecological impacts of these contaminants because of their increasing presence in the environment. It is thus essential to forecast the behavior of these contaminants, determine the vulnerability of the ecosystems and develop effective methods for managing these contaminants in the environment based on our knowledge of the movement and behavior of radionuclides. The environmental fate of a substance is determined by the physicochemical processes of adsorption-desorption, speciation, complex formation, oxidation-reduction reactions, and radioactive decay; radionuclide transport occurs by means of atmospheric transport, hydrological transport, sediment transport, and soil transport processes. Radionuclide transport and availability are also affected by environmental conditions, including pH, organic content, mineral composition, and climatic factors. Radionuclides can influence microorganism populations, plant life, aquatic life, and wild animal life, with possible repercussions on biodiversity and interspecies interactions. Chronic exposure scenarios at low doses, which continue to pose challenges due to complex ecosystem responses and the long-term effects that remain uncertain, are given priority. Knowledge gaps regarding ecosystem resilience, species sensitivity, pathways of radionuclide transfers, and effects of climate change on contaminant dynamics remain prevalent despite advances in monitoring and radioecological modeling. Solving these problems necessitates more extensive monitoring systems, coupled with transport-fate modeling and adaptive management approaches.
The cosmogenic radionuclide 81Kr (half-life 229 ka) is widely used to date groundwater and ice on 104-106 year timescales. Anthropogenic nuclear activities can lead to an increase of 81Kr in the atmosphere, but this contribution has not been tightly constrained experimentally. We report a precise measurement of the anthropogenic contribution using Atom Trap Trace Analysis (ATTA). A pre-nuclear atmospheric krypton sample collected in the 1940s was measured alongside a present-day atmospheric sample, acquiring over one million 81Kr atom counts for each sample. Given the measured difference of δ = (0.09 ± 0.14)% and after correcting for the cross-sample contamination, we obtain an upper limit of 0.43% (90% confidence level) for the anthropogenic contribution, validating the stability of the atmospheric 81Kr baseline for high-precision dating applications.
We introduce the Relative Reservoir Dominance Index (RRDInat), a dimensionless metric integrating substrate-specific distribution coefficients (Kd, 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 RRDInat values approaching 1.00 indicate near-total natural particle dominance. A three-component extension is presented for radionuclides with high aqueous mobility (9°Sr, 99Tc, 3H). Our synthesis reveals a consistent Kd hierarchy spanning five orders of magnitude, from ∼10 L/kg for pristine MP to ∼106 L/kg for Fe-(hydr)oxide-associated Pu. Monte Carlo uncertainty propagation yields RRDInat ≥ 0.980 across all compartments (95% CI: 0.990-0.9999). The oligotrophic gyre represents the global minimum (RRDInat = 0.980 under bounding conditions; salinity-corrected Kd,MP applied); gyre RRDI is sensitive to the local Cp,nat/Cp,MP ratio rather than absolute Cp,MP, rising to ≥ 0.999 under central concentration estimates. For non-sorbing radionuclides (3H, 14C, 99Tc, 129I), dissolved-phase transport dominates; biofilm-coated MP may represent a novel 129I retention pathway. Post-2023 ALPS-treated water discharge from Fukushima Daiichi introduces monitoring imperatives for 3H, 14C, 9°Sr, and 99Tc 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.
Many households in gold-mining regions of northern Tanzania have limited access to safe and reliable drinking water and rely on surface water, groundwater, and locally caught fish as key drinking-water and dietary sources, yet the radiological safety of these sources remains uncertain. This study evaluated age-dependent radiological risks from ingestion of naturally occurring radionuclides (234U, 238U, 226Ra, 210Po, 210Pb) in multiple water sources and in fish tissues (bones, gills and flesh) of African lungfish (Protopterus aethiopicus), Nile tilapia (Oreochromis niloticus) and African Catfish (Clarias gariepinus). Activity concentrations were determined by radiochemical separation followed by alpha spectrometry and gas-flow proportional counting. Annual effective dose (AED) and excess lifetime cancer risk (ELCR) were estimated using age-specific water and fish consumption rates. Groundwater, particularly from shallow wells, produced the highest exposures, with a maximum AED for a 5-year-old child, dominated by 234U, exceeding both the World Health Organization (WHO) screening level (0.1 mSv/y) and the International Commission on Radiological Protection (ICRP) public dose reference level (1 mSv/y). Fish consumption was the dominant pathway, with African lungfish consistently yielding the highest AEDs, followed by Tilapia and Catfish. The dose contributions were dominated by 210Pb (75% -96%) and 210Po (4% - 25%), while 234U, 238U, and 226Ra contributed <1%. The estimated ELCRs associated with consumption of all fish species, exceeded the USEPA acceptable risk range (10-6 - 10-4). Elevated 234U/238U activity ratios (>1) indicated preferential mobilisation of 234U via alpha-recoil, while low 210Po/210Pb ratios (<1) confirmed 210Pb predominance. These findings underscore the need for continued radiological monitoring and targeted mitigation to protect vulnerable communities.
The contamination of forest ecosystems by radiocesium (137Cs) released from the Fukushima Dai-ichi Nuclear Power Plant accident has seriously damaged forestry activities in Fukushima. Previous field-based descriptive studies have reported that a considerable proportion of 137Cs remains in the soil O horizon, where the presence of various types of organic matter at different stages of decomposition contributes to both strong retention of 137Cs by decomposing litter and a heterogeneous spatial distribution. To investigate the dynamics and mobility of 137Cs during litter decomposition and to interpret our observations, we conducted litterbag experiments over approximately two years using fallen leaves of konara oak. Litterbags were deployed either suspended above the ground or placed on the forest floor. Leaching experiments were also performed on retrieved litterbag samples. In the suspended litterbags, we observed a decline of 137Cs, mainly in the water-soluble form, likely due to washout associated with rain/snowfall and subsequent spring snowmelt. In contrast, the incorporation of soil particles by forest-floor litterbags resulted in an increase of 137Cs relative to initial conditions, mainly in forms that were scarcely extractable, even by ammonium acetate. The large variability of residual 137Cs revealed by comparisons with previous litterbag and equivalent experimental studies highlighted the difficulty of predicting 137Cs activity concentrations and distributions on the forest floor. Furthermore, comparison with potassium (K), a competing alkali metal, demonstrated that 137Cs is less mobile than K, regardless of soil particle incorporation. This low mobility likely contributes to the prolonged, strong retention of 137Cs in the O horizon.
