Fit-for-purpose modelling of radiocaesium soil-to-plant transfer for nuclear emergencies: a review

Highly weathered mineral soils have highest transfer risk of radiocaesium contamination after a nuclear accident: A global soil-plant study

Accidental release of radiocaesium (137Cs) from nuclear power plants may result in long-term contamination of environmental and food production systems. Assessment of food chain contamination with 137Cs relies on 137Cs soil-to-plant transfer data and models mainly available for regions affected by the Chornobyl and Fukushima accidents. Similar data and models are lacking for other regions. Such information is needed given the global expansion of nuclear energy. We collected 38 soils worldwide of contrasting parent materials and weathering stages. The soils were spiked with 137Cs and sown with ryegrass in greenhouse conditions. The 137Cs grass-soil concentration ratio varied four orders of magnitude among soils. It was highest in Ferralsols due to the low 137Cs interception potential of kaolinite clay and the low exchangeable potassium in these soils. Our results demonstrate, for the first time, the high plant uptake of 137Cs in tropical soils. The most recent 137Cs transfer model, mainly calibrated to temperate soils dominated by weathered micas, poorly predicts the underlying processes in tropical soils but, due to compensatory effect, still reasonably well predicts 137Cs bioavailability across all soils (R2 = 0.8 on a log-log scale).

Decrease in radiocesium adsorption of illite induced by soil organic matter: Quantity or quality?

Organic matter blocks highly selective frayed edge sites on clay minerals and reduces radiocaesium adsorption. The effects of different soil organic matter on Cs adsorption on illite have been investigated. The quantity and quality of soil organic matter was varied by extracting from three contrasting soils and varying extraction conditions. Extracted organic matter was quantified, and analysed using UV and fluorescent spectroscopy. Cs adsorption was markedly lower in soil aqueous extracts, than in simple electrolyte solution at the same ionic strength (IS). Part of the decrease was attributed to soluble soil potassium. After correction for ionic strength and potassium, the relative distribution coefficient of Cs, KdIS,K, decreased with increasing dissolved organic carbon (DOC) concentration. The correlation between KdIS,K and DOC was largely unchanged by taking into account any of the measured spectral parameters. We find no evidence that molecular size and composition of organic coatings determine their effect on the Cs adsorption properties of illite.

Modeling long-term transfers of radiocesium in farmland under different tillage and cover crop treatments

The 2011 nuclear accident at Japan's Fukushima Daiichi Nuclear Power Plant (FDNPP) prompted inquiries about the long-term transfer of Cesium-137 (137Cs) from soil to agricultural plants. In this context, numerical modeling is particularly useful for the long-term evaluation of the consequences of agroecosystem contamination. Agricultural practices, such as tillage and cover cropping, play key roles in 137Cs recycling in agroecosystems. In this study, we used 10-year monitoring data to develop a dynamic model to predict 137Cs redistribution (via uptake, litterfall, translocation, and percolation) under different tillage (no-tillage, NT; rotary cultivation, RC; moldboard plow, MP) and cover crop (rye; hairy vetch; fallow weed) treatments. The verification exercise and assessment results indicated the model's reliability, as the temporal dynamics of predicted values agreed with observed values. Tillage significantly influenced the 137Cs distribution in soil, thereby decreasing plant uptake of 137Cs, whereas cover crop exerted a minimal effect on 137Cs cycling. Furthermore, while the 137Cs concentrations in soybean grain under RC and NT treatments were comparable 62 years after the FDNPP accident, the concentration under MP treatment remained consistently the lowest. Despite natural decay being the main cause of the decreased global 137Cs level in the agroecosystem, with minimal losses from percolation to deeper soil layers and soybean harvesting, adopting an appropriate tillage practice was shown to promote a long-term reduction of 137Cs concentration in crops. Finally, to improve the model's accuracy, further research should consider incorporating the effects of soil properties and extreme weather events on 137Cs flow into the model, as these factors are essential for realizing improved agroecosystem predictions.

Use of random forest algorithm for predictive modelling of transfer factor soil-plant for radiocaesium: A feasibility study

A German dataset with soil-plant transfer factors for radiocaesium including many co-variables was analysed and prepared for the application of the Random Forest (RF) algorithm using the R libraries 'party', and 'caret'. A RF predictive model for soil-plant transfer factor was created based on 10 co-variables. These are, for example, taxonomic plant family, plant part, soil type and the exchangeable potassium concentration in the soil. The RF model results were compared with the results of two (semi-)mechanistic models. Of the more than 3000 entries in the original dataset, only about 1200 could be used, as this was the largest complete dataset with the largest number of co-variables available. The obtained RF predictive model can reproduce the experimental observations better than the two (semi)-mechanistic models, which are based on many assumptions and fixed parameter values. Model performance was quantified using the metrics of Root Mean Square Error (rmse) and Mean Absolute Error (mae). The RF model was able to reproduce the variability of the data by up to 6 orders of magnitude. The categorical co-predictors, especially taxonomic plant family and plant part, have a greater influence than the numerical co-predictors, such as pH and exchangeable soil potassium concentration. This feasibility study shows that RF is a promising tool to obtain predictive models for transfer factors. However, to build a widely applicable predictive model, a dataset is needed that contains at least thousands of entries for transfer factors and for the most important co-variables and considers a large parameter space.

Quantitative clay mineralogy predicts radiocesium bioavailability to ryegrass grown on reconstituted soils

Current radiocesium (137Cs) models to evaluate the risk of 137Cs transfer from soil to plants are based on the clay and exchangeable potassium (K) contents in soil. These models disregard the mineralogy of the clay fraction and are likely not capable of accurately predicting the 137Cs transfer factor (TF) in soils of contrasting parent rocks and weathering stages. The objectives of this study were to test that hypothesis and to identify whether quantitative information on mineralogy can improve the predictions. A pot cultivation experiment was set up with clay-sand mixtures in single and double clay doses that were fertilized, spiked with 137Cs and grown with ryegrass for 30 days. Four clays (illite, biotite, smectite and vermiculite) along with six deposits from clay-rich geological units were compared. The TF generally decreased with increasing clay dose for each of these ten different clay groups, however, the TF varied two orders of magnitude across clay groups and doses. The TF was highest for clays with little 137Cs specific sites such as bentonite and/or where the exchangeable K content was low compared to the other clays. The TF was well predicted from the soil solution 137Cs and K concentrations (R2 = 0.72 for log transformed TF), corroborating earlier findings in natural soils. The TF (log transformed) was statistically unrelated to total phyllosilicate content or 1:1 and 2:1:1 type phyllosilicate content while it significantly decreased with increasing 2:1 phyllosilicate content (R2 = 0.32). A multiple regression model with four different X-ray diffraction (XRD) based phyllosilicate groups yielded the strongest predictive power (R2 = 0.74). We conclude that XRD quantification is valuable for describing 137Cs bioavailability in plant substrates. These findings now await confirmation for natural soils.

Comprehensive analysis of a decade of cumulative radiocesium testing data for foodstuffs throughout Japan after the 2011 Fukushima Daiichi Nuclear Power Plant accident

The unexpected accident at the Fukushima Daiichi Nuclear Power Station in Japan, which occurred on March 11th, 2011, after the Great East Japan Earthquake and tsunami struck the north-eastern coast of Japan, released radionuclides into the environment. Today, because of the amounts of radionuclides released and their relatively long half-life, the levels of radiocesium contaminating foodstuffs remain a significant food safety concern. Foodstuffs in Japan have been sampled and monitored for ^134,137Cs since the accident. More than 2.5 million samples of foodstuffs have been examined with the results reported monthly during each Japanese fiscal year (FY, from April 1^st to March 31^st) from 2012 to 2021. A total of 5,695 samples of foodstuffs within the “general foodstuffs” category collected during this whole period and 13 foodstuffs within the “drinking water including soft drinks containing tea as a raw material” category sampled in FY 2012 were found to exceed the Japanese maximum permitted level (JML) set at 100 and 10 Bq/kg, respectively. No samples from the “milk and infant foodstuffs” category exceeded the JML (50 Bq/kg). The annual proportions of foodstuffs exceeding the JML in the “general foodstuffs” category varied between 0.37% and 2.57%, and were highest in FY 2012. The ^134,137Cs concentration for more than 99% of the foodstuffs monitored and reported has been low and not exceeding the JML in recent years, except for those foodstuffs that are difficult to cultivate, feed or manage, such as wild mushrooms, plants, animals and fish. The monitoring data for foodstuffs show the current status of food safety risks from ^134,137Cs contamination, particularly for cultured and aquaculture foodstuffs on the market in Japan.

DETERMINING THE TRANSFER FACTORS FOR ESTIMATES OF THE RADIATION CONTAMINATION OF AGRICULTURAL CROPS

The study aims to provide a basis for measures reducing the consequences of a nuclear accident in its late phase, when plant contamination occurs mainly through the root system. Samples of the above-ground biomass of crops and soil were taken in 2020 in the vicinity of the Temelín and Dukovany nuclear power plants (Czech Republic). The 137Cs activities were determined using gamma spectrometry, and the 90Sr activities were measured through beta radiation. From the obtained values, the radionuclide transfer factors (TFs) from soil to crop biomass were calculated. The average area activity of 137Cs in the soil around Dukovany and Temelín was 1700 and 2400 Bq m-2, respectively. The average area activity of 90Sr around Dukovany and Temelín was 211 and 184 Bq m-2, respectively. The TF 137Cs ranged from < 6.3 × 10-6 to 7.9 × 10-3, with a mean of 3.5 × 10-4 m2 kg-1, and the TF 90Sr ranged from 2.7 × 10-4 to 6 × 10-2, with a mean of 1.7 × 10-2 m2 kg-1.

Soil to tobacco component transfer factors for natural radionuclides 40K, 226Ra, and 232Th and the risk assessment of tobacco leaf in smoking

This study determined the activity concentrations and corresponding transfer factors (TF) of ⁴⁰K, ²²⁶Ra, and ²³²Th in three tobacco components (root, stem, and leaf). The radiation hazard index parameters were assessed for the tobacco leaf. The activity concentrations in the soil were 589-762, 32-43, and 49-59 Bq kg-dw^-1 (dry weight) for ⁴⁰K, ²²⁶Ra, and ²³²Th, respectively. The average activity concentrations of ⁴⁰K, ²²⁶Ra, and ²³²Th were 447, 5.41 and 5.69 Bq/kg-dw for the root, 670, 9.64 and 7.61 Bq kg-dw^-1 for the stem, and 793, 6.79 and 6.15 Bq kg-dw^-1 for the leaf, respectively. The TF values were 0.42-1.42, 0.10-0.49 and 0.06-0.23 for ⁴⁰K, ²²⁶Ra, and ²³²Th, respectively. The stem and leaf ⁴⁰K TF values were significantly higher than the root values. The stem ²²⁶Ra TF values were significantly higher than the root values. The ²²⁶Ra and ²³²Th activity concentrations and TFs of tobacco components had a significant positive correlation. Based on the activity concentrations of the tobacco leaves, the annual inhalation effective dose to the lungs for an adult smoker was 0.32-0.81 mSv y^-1 (average 0.60 mSv y^-1). The Excess Lifetime Cancer Risk (ELCR) caused by smoking was an average of 2.39 × 10^-3.

Radiocesium distribution caused by tillage inversion affects the soil-to-crop transfer factor and translocation in agroecosystems

This study reports the translocation of cesium-137 (¹³⁷Cs) into deep soil layers, and the ¹³⁷Cs transfer from soil to soybean in farmland under three tillage (no tillage, NT; rotary cultivation, RC; moldboard plow; MP) treatments and an undisturbed grassland (GL) at eight years after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on 11 March 2011 in Japan. Tillage influences the ¹³⁷Cs distribution in the 0-30 cm of soil; the distribution of ¹³⁷Cs in the soil was uniform under RC and MP treatments, while in the grassland, most ¹³⁷Cs was concentrated on the soil surface (0-2.5 cm). The center of vertical ¹³⁷Cs radioactivity concentration (the thickness of the soil from surface which containing half of the ¹³⁷Cs inventory) in GL was 5.5 cm, which was shallower than that in farmland (9.5 cm in NT, 13.6 cm in RC and 15.2 cm in MP). Hence, the total translocation distance of ¹³⁷Cs 8 years after FDNPP accident showed the following trend: GL (2.4 cm) < NT (7.0 cm) < RC (10.0 cm) < MP (12.3 cm). Meanwhile, a significant positive correlation was observed between ¹³⁷Cs radioactivity concentration and organic carbon and nitrogen content in the soil. However, the ¹³⁷Cs radioactivity concentration in soybean grains was negatively correlated with the center of vertical ¹³⁷Cs radioactivity concentration but positively correlated with the ratio of exchangeable ¹³⁷Cs (ExCs) and K content in the soil. The ExCs/K and ¹³⁷Cs distributions in the soil were combined into a statistical model to predict the ¹³⁷Cs radioactivity concentration in soybean grain. The results revealed the magnitude of the impact of ¹³⁷Cs distribution on the ¹³⁷Cs transfer from soil to crop. The addition of the ¹³⁷Cs distribution dramatically improved the accuracy of the prediction model of ¹³⁷Cs radioactivity concentration in soybean.

Cesium transfer to millet and mustard as a function of Cs availability in soils

¹³⁷Cs is one of the most persistent radioactive contaminants in soil after a nuclear accident. It can be taken up by plants and enter the human food chain generating a potential human health hazard. Although a large amount of literature has highlighted the role of the different processes involved in Cs uptake by plants, there is still no simple way to predict its transfer for a specific plant from a particular soil. Based on the assumption that the concentration ratio (CR) of Cs can be predicted from one plant taxon if the CR of another taxon is known and taken as reference, whatever the supporting soils, a series of plant/soil Cs transfer experiments were performed on Rhizotest during 21 days using three soils with different textures, clay and organic matter contents and two plants (millet and mustard) with potentially contrasting Cs uptake capacity based on their phylogeny. CRs of each plant varied by 2-3 orders of magnitude depending on the soil and contrary to expectations, the CRs of mustard were either higher (for clay soil), equal (for clay-loam soil) or lower (for sandy soil) than the one of millet. Considering Cs availability in soils and defining a new CR based on the amount of Cs available in the soil (CR_avail) decreased the range of variation in CR between the different soil types for a given plant by one order of magnitude. Differences in Cs (and K) translocation to shoots, possibly specific to millet within Poales, could partly explain the relative CRs of millet and mustard as a function of soils.

Impact of soil moisture on cesium uptake by plants: Model assessment

Computational models of radioisotopes soil-to-plant transfer are an essential component of decision support systems for nuclear emergency and assessment impact of nuclear accidents on human beings and the environment. The soil water regime impact on cations behavior in the «soil-plant » system. Therefore, relationships between radioisotopes accumulation by plants and soil moisture should be included in the computational models. A kinetic model for simulation of Cs⁺ root uptake under alteration of soil moisture content in soil is presented in this paper. The model simulates Cs⁺ and K⁺ diffusion from bulk soil into rhizosphere under alteration of water content. The model results show a drop of Cs⁺ and K⁺ concentration in the rhizosphere with decreasing soil moisture. The magnitude of enhanced root uptake of Cs⁺ with soil moisture alteration depends on the exchangeable potassium content and soil texture. The Cs⁺ root uptake rate has a maximum at 60%-80% of field capacity under sufficient and high exchange potassium content. A similar dependence should also be seen for the RCs soil-to-plant transfer factor. The results indicate the importance of soil water content for modification of the RCs accumulation by plants and the necessity to include this factor into an appropriate semi-mechanistic model.

Uranium bioremediation with U(VI)-reducing bacteria

Uranium (U) pollution is an environmental hazard caused by the development of the nuclear industry. Microbial reduction of hexavalent uranium (U(VI)) to tetravalent uranium (U(IV)) reduces U solubility and mobility and has been proposed as an effective method to remediate uranium contamination. In this review, U(VI) remediation with respect to U(VI)-reducing bacteria, mechanisms, influencing factors, products, and reoxidation are systematically summarized. Reportedly, some metal- and sulfate-reducing bacteria possess excellent U(VI) reduction capability through mechanisms involving c-type cytochromes, extracellular pili, electron shuttle, or thioredoxin reduction. In situ remediation has been demonstrated as an ideal strategy for large-scale degradation of uranium contaminants than ex situ. However, U(VI) reduction efficiency can be affected by various factors, including pH, temperature, bicarbonate, electron donors, and coexisting metal ions. Furthermore, it is noteworthy that the reduction products could be reoxidized when exposed to oxygen and nitrate, inevitably compromising the remediation effects, especially for non-crystalline U(IV) with weak stability.

An updated strategic research agenda for the integration of radioecology in the european radiation protection research

The ALLIANCE Strategic Research Agenda (SRA) for radioecology is a living document that defines a long-term vision (20 years) of the needs for, and implementation of, research in radioecology in Europe. The initial SRA, published in 2012, included consultation with a wide range of stakeholders (Hinton et al., 2013). This revised version is an update of the research strategy for identified research challenges, and includes a strategy to maintain and develop the associated required capacities for workforce (education and training) and research infrastructures and capabilities. Beyond radioecology, this SRA update constitutes a contribution to the implementation of a Joint Roadmap for radiation protection research in Europe (CONCERT, 2019a). This roadmap, established under the H2020 European Joint Programme CONCERT, provides a common and shared vision for radiation protection research, priority areas and strategic objectives for collaboration within a European radiation protection research programme to 2030 and beyond. Considering the advances made since the first SRA, this updated version presents research challenges and priorities including identified scientific issues that, when successfully resolved, have the potential to impact substantially and strengthen the system and/or practice of the overall radiation protection (game changers) in radioecology with regard to their integration into the global vision of European research in radiation protection. An additional aim of this paper is to encourage contribution from research communities, end users, decision makers and other stakeholders in the evaluation, further advancement and accomplishment of the identified priorities.

Contamination of Slovak Bilberry (Vaccinium Myrtillus L.) with Radiocaesium 137Cs in Selected Slovak Locations

The aim of this study was to determine the activity of post-Chernobyl 137Cs in Slovak bilberry (Vaccinium myrtillus L.) from selected locations of Slovakia depending upon: the season, location and the soil-plant transfer factor (TF). The 137Cs activity was determined in samples of forest soil and bilberry bushes (stems, leaves and fruits) growing on the soil in three locations: Arboretum Mlyňany (1), Hodruša-Hámre (2) and Javorníky (3). Altogether we collected 35 samples; namely 15 samples from Arboretum Mlyňany, 10 samples from Hodruša-Hámre and 10 from Javorníky. The samples of soil were examined also for the activity of 40K. The samples were collected in the spring, summer and autumn in the years 2019 and 2020 and were analysed by a gamma-ray spectrometry method. The highest levels of 137Cs activity concentrations were determined in Hodruša-Hámre in September where they reached 161 ± 13 Bq.kg−1 in the soil, 3.95 ± 1.07 Bq.kg−1 in the stems and 14.7 ± 4.26 Bq.kg−1 in the leaves. The lowest levels were determined in the Arboretum Mlyňany in October where the 137Cs activity in the soil reached 13.1 ± 1 15 Bq.kg−1, while in the stems, leaves and fruits it was lower than the minimum detectable level. In the latter location, we also determined the highest activity of 40K, the radioactive isotope physiologically and metabolically similar to 137Cs. The mass activity of 137Cs in all samples of bilberries were lower than the minimum detectable activity (MDA). The comparison of the results obtained in this study with the limit for total mass activity of radiocaesium (134Cs + 137Cs) in the food (600 Bq.kg−1) allowed us to conclude that the fruits, stems and leaves of Slovak bil-berries present neither health nor environmental risk.

Microbial interaction with and tolerance of radionuclides: underlying mechanisms and biotechnological applications

Radionuclides (RNs) generated by nuclear and civil industries are released in natural ecosystems and may have a hazardous impact on human health and the environment. RN-polluted environments harbour different microbial species that become highly tolerant of these elements through mechanisms including biosorption, biotransformation, biomineralization and intracellular accumulation. Such microbial-RN interaction processes hold biotechnological potential for the design of bioremediation strategies to deal with several contamination problems. This paper, with its multidisciplinary approach, provides a state-of-the-art review of most research endeavours aimed to elucidate how microbes deal with radionuclides and how they tolerate ionizing radiations. In addition, the most recent findings related to new biotechnological applications of microbes in the bioremediation of radionuclides and in the long-term disposal of nuclear wastes are described and discussed.

Distribution and transfer of naturally occurring radionuclides and 137Cs in the freshwater system of the Plitvice Lakes, Croatia, and related dose assessment to wildlife by ERICA Tool

The aim of this study was to investigate the natural radioactivity of Plitvice Lakes, under the assumption that due to its status as a National Park, the area can be considered an example of a natural freshwater system. Also, considering the transfer parameter data as the largest source of uncertainty in radiological risk assessments, the impact of site-specific data on dose rate assessment, as opposed to currently available data, was investigated. The study included gamma and alpha spectrometric measurements of ²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²²⁸Ra, and ⁴⁰K in water, sediment, and fish samples, as well as ¹³⁷Cs due to the coinciding of the study with the Fukushima accident. The content of naturally occurring radionuclides significantly varied in sediments of different Lakes, probably as a reflection of the different underlying geology of the area. Also, the ²¹⁰Pb distribution in sediments indicated an up to 312 Bq kg^-1 of the allochthonous contribution of this radionuclide at the beginning of the Lake's watercourse, which probably entered into the lake system by the major inlet river with its steady decrease along downstream lakes. Low ⁴⁰K activity concentrations (27.5 ± 20.1 mBq L^-1) in the Lake's waters might be one of the causes of increased ¹³⁷Cs activity concentrations in fish samples (1.5 ± 0.4 Bq kg^-1), which was found to be an order of magnitude higher than average values for different fish species from other Croatian freshwater systems (0.2 ± 0.1 Bq kg^-1). A temporary increase of ¹³⁷Cs activity concentrations was measured in water samples collected immediately after the Fukushima accident. Calculated site-specific sediment/water distribution coefficients and fish/water concentration ratios for radium and caesium were on average lower than generic ones found in the literature. Background dose rate assessments performed by the ERICA Tool indicated a profound impact of different input data on assessment results with water activity concentrations resulting in significantly higher dose rates (0.1-67 μGy h^-1) in comparison to sediment activity concentrations (0.03-9 μGy h^-1). An incremental dose rate due to ¹³⁷Cs was found to be in the range of < 0.001-0.023 μGy h^-1 which, in comparison to background dose rates, can be considered negligible.

Evaluation of semi-mechanistic models to predict soil to grass transfer factor of 137Cs based on long term observations in French pastures

The aim of this study was to evaluate and improve the accuracy of the semi-mechanistic models used in regulatory exposure assessment tools, to describe the transfer factors of ¹³⁷Cs from pasture soils to grass observed in different grazing areas of France between 2004 and 2017. This involved a preliminary parameterization step of the dynamic factor describing the ageing of radiocesium in the root zone using a Bayesian approach. A data set with mid-term (10 years about) and long term (more than 20 years) field and literature data from 4 European countries was used. A double kinetics of the bioavailability decay was evidenced with two half-life periods equal to 0.46 ± 0.11 yr and 9.57 ± 1.12 yr for the fast and slow declining rates respectively. We, then, tested a few existing alternative models proposed in literature. The comparison with field data showed that these models always underestimated the observations by one to two orders of magnitude, suggesting that the solid-liquid partition coefficient (Kd) was overestimated by models. The results suggest that semi mechanistic models might fail in the long-term prediction of the radionuclide transfer from soil-to-plant in the food chain. They highlight the need to calculate Kd using easily exchangeable ¹³⁷Cs (i.e. labile fraction) rather than total soil ¹³⁷Cs.

Role of modelling in monitoring soil and food during different stages of a nuclear emergency

In case of a nuclear accident, adequate protection of the public and the environment requires timely assessment of the short- and long-term radiological exposure. Measurements of the radiation dose and the radioactive contamination in the environment are essential for the optimization of radiation protection and the decision making process. In the early phase, however, such measurements are rarely available or sufficient.To compensate for the lack of monitoring data during nuclear emergencies, especially in the early phase of the emergency, mathematical models are frequently used to assess the temporal and spatial distribution of radioactive contamination. During the transition and recovery phase, models are typically used to optimise remediation strategies by assessing the cost-effectiveness of different countermeasures. A prerequisite of course is that these models are fit for purpose. Different models may be needed during different phases of the accident. In this paper, we discuss the role of radioecological models during a nuclear emergency, and give an outlook on the scientific challenges which need to be addressed to further improve our predictions of human and wildlife exposure.

Towards dynamic and process-based modelling of radionuclides cycling in terrestrial radioecology

Mathematical models are frequently used in terrestrial radioecology to interpret observations and to assess the detrimental impacts of radioactive releases to the environment. Conventional radioecological models are largely based on equilibrium and empirical relationships with reasonable data requirements, making them practical tools for long-term assessments. But conventional models may be inadequate to simulate radionuclide dynamics in terrestrial environments realistically. Specifically, the structure of such models seldom conforms to the physics of water flow and solute transport in soils. The equilibrium relationships may fail to predict seasonality in radionuclide transfer between environmental compartments; model transferability between sites is often hampered by its empirical nature. Numerous studies have highlighted the need to circumvent these limitations. In this paper, we introduce dynamic and process-based modelling to a conventional radioecological model by coupling an empirical plant module to a process-based soil module that simulates water flow, solute transport and root uptake in the soil column. Illustrative simulations are presented using the coupled model and stable chlorine cycling in a temperate Scots pine (Pinus sylvestris L.) stand as an example. The model satisfactorily reproduced soil moisture dynamics and the inventory of inorganic chlorine in the tree and forest floor compartments. The inventory of organic chlorine in the stand, however, was overestimated, indicating that processes pertinent to organochlorine cycling at the stand were missing from the model. The approach proposed in this paper is a step towards dynamic and process-based modelling in terrestrial radioecology and impact assessment. It can be particularly useful for modelling transfer of elements, such as redox-sensitive radionuclides, whose behaviour in soil-plant systems is moisture-dependent.

Advanced approach for screening soil with a low radiocesium transfer to brown rice in Fukushima based on exchangeable and nonexchangeable potassium

Phytoavailable K in soil is a key to control the transfer factor of radiocesium from soil to brown rice. The transfer factors were determined for paddy fields cultivated in 2017 and 2018 under different K fertilization regimes in Fukushima Prefecture, Japan. Two phytoavailable forms of K, the exchangeable and nonexchangeable K contents were investigated for the surface soil sampled after the transplanting and fertilization as well as after harvest of rice in the same paddy fields. The exchangeable K content largely decreased from after transplanting and fertilization to after harvest, and the exchangeable K of the soil after harvest was negatively correlated with the transfer factor (r_s = -0.70, p < .001). Most soil samples after harvest showed that the transfer factors exponentially increased as the exchangeable K decreased; however, some of the samples indicated considerably low transfer factors (<0.005) despite being exchangeable K deficient, i.e., exchangeable K < 25 mg K₂O 100 g^-1. Even though this value before usual fertilization has been effectively used as a threshold to determine whether supplemental K fertilization is required to reduce the radiocesium content in brown rice, additional screening was needed to estimate this radiocesium transfer more precisely. Thus, we found that not only the exchangeable K but also nonexchangeable K contents had a negative correlation with the transfer factor (r_s = -0.60, p < .001) of the soil samples after harvest but were not correlated with each other (r_p = -0.10). Furthermore, the results revealed that soil with nonexchangeable K > 50 mg K₂O 100 g^-1 indicated a considerably low transfer factor, even if exchangeable K deficient. Thus, via our field-scale experiments, we concluded that the criterion nonexchangeable K > 50 mg K₂O 100 g^-1 can be used as another threshold for use along with that of exchangeable K to differentiate soil with a low radiocesium transfer rate from exchangeable K deficient soil.

Deriving probabilistic soil distribution coefficients (Kd). Part 2: Reducing caesium Kd uncertainty by accounting for experimental approach and soil properties

The solid-liquid distribution coefficient (K_d) is a key input parameter in radioecological models. However, its large variability hampers its usefulness in modelling transport processes as well as its accuracy in representing soil-radionuclide interactions. For the specific case of radiocaesium, the analyses of a Cs K_d soil dataset (769 entries) showed that values varied over a five order of magnitude range, and the resulting Cs K_d best estimate (calculated as a geometric mean = 2.5 × 10³ L kg^-1) lacked reliability and representativity. Grouping data and creation of partial datasets based on the experimental approach (short-term (< ~1 yr) vs. long-term experiments (> ~1 yr)) and soil factors affecting Cs interaction (i.e., the ratio of the radiocaesium interception potential (RIP) to the potassium content in soil solution (K_ss); organic matter content (OM) and soil texture) succeeded in reducing variability a few orders of magnitude, with Cs K_d best estimates also differing by one-two orders of magnitude depending on the type of soil and experimental approach. The statistical comparison of the Cs K_d best estimates and related cumulative distribution functions of the partial datasets revealed a relevant effect of the sorption dynamics on Cs K_d values (with long-term values systematically higher than short-term ones), and that the RIP/K_ss ratio was an excellent predictor of Cs K_d for short-term scenarios, whereas the RIP parameter could be predicted on the basis of texture information. The OM threshold to distinguish between OM threshold to distinguish between Mineral and Organic soils subclasses, regarding Cs interaction was determined to be 50% and 90% OM for short- and long-term scenarios, respectively. It was then recommended to select the Cs K_d input data depending on the soils and scenarios to be assessed (e.g., short- vs. long-term; OM %) to improve the reliability and decrease the uncertainty of the radioecological models.

Can models based on phylogeny be used to predict radionuclide activity concentrations in crops?

The modelling of transfer of radionuclides from soils to plants generally relies upon empirical soil-plant concentration ratios. Concentration ratios are often highly uncertain and are not available for many plant-radionuclide combinations. A number of papers published over the last 20 years have suggested that phylogenetic models could be used to make predictions of the radionuclide transfer to plants. Such a modelling approach would have the advantage that site factors (typically related to soils) are taken into account. For the first time we have compared predictions of Cs and Sr transfer to a range of crops grown on different soils. Predictions for both elements were generally acceptable (within an order of magnitude of observed data) but Sr concentrations were over predicted in fruits and tubers. This over prediction of Sr concentrations is likely to be because the phylogenetic models were fitted to data for green shoots. We conclude that phylogenetic models offer a number of advantages, but that they must be validated and, in future, parametrisation datasets need to include data on concentrations in edible plant parts and not just green shoots.

Baseline radioecological data for the soil and selected bioindicator organisms in the temperate forest of Plitvice Lakes National Park, Croatia

The aim of this study was to provide baseline radioecological data for the temperate forest ecosystem in Plitvice Lakes National Park. Emphasis was placed on the determination of naturally occurring radionuclides since there is an acknowledged lack of data for these radionuclides in non-accident conditions in wildlife, even for bioindicator organisms. Activity concentrations of ²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²³²Th, ⁴⁰K, ¹³⁴Cs, and ¹³⁷Cs were measured by gamma spectrometry in soil and bioindicators: earthworms, conifer needles, mosses, and lichens. From the measured activity concentrations, concentration ratios were calculated to quantify the transfer of these radionuclides from soil to bioindicators. Our results show that soil activity concentrations are biased toward results from other studies conducted within the Dinaric mountain region. However, in moss and lichen samples, we measured higher activity concentrations of ²²⁶Ra and lower activity concentrations of ⁴⁰K and ¹³⁷Cs in comparison to similar studies. Also, we estimated lower concentration ratios for all radionuclides from soil to these organisms, except for ²¹⁰Pb, in comparison to generic values. The transfer of ²³⁸U was generally low for all of the bioindicator organisms. For conifer needles, a correlation was found between activity concentrations of ²²⁶Ra and ¹³⁷Cs in soil and related concentration ratios. Correlation was also found between the activity concentration of ⁴⁰K in soil and transfer of ⁴⁰K and ¹³⁷Cs to mosses and lichens. A comparison with literature data highlighted the lack of ²²⁶Ra related concentration ratios for conifer trees and especially for earthworms. Therefore, the results of this study could supplement the sparse data currently available on radionuclide background data in similar ecosystems and related soil-to-wildlife transfer of radionuclides. Dose rate assessments, performed by the ERICA Tool, estimated that 96% of the overall exposure of wildlife in the Park area is due to the background dose rates, while 0.06 μGy h^-1 on average can be attributed as an incremental dose rate from ¹³⁴Cs and ¹³⁷Cs.

Long term decrease of 137Cs bioavailability in French pastures: Results from 25 years of monitoring

Long term radioactivity monitoring programs contribute to the understanding of the behavior of radionuclides in the environment. This work aims to investigate the long term behavior of Cesium-137 in pasture ecosystem (root soil, grass and cow's milk) by using of more than twenty five years monitoring data collected at ten of French pasture sites contaminated by atmospheric fallouts from Chernobyl and nuclear atmospheric tests. We estimated with a simple exponential model the long term effective half-lives of radiocesium in root soil, grass vegetation and cow's milk along with their associated uncertainties. The average values of the effective half-lives over all the investigated sites were determined as 17, 11 and 9 years for soil, grass and milk respectively. Those values compare favorably to those estimated in previous studies in literature. These findings further enable us to quantify the decrease of ¹³⁷Cs bioavailability which ranges from 0.008 to 0.044 year^-1 with an average value of 0.026 year^-1 (i.e. effective half lives ranging from 16 to 87 years with an average value of 26 years in soil).