Review and assessment of models used to predict the fate of radionuclides in lakes

A variety of models for predicting the behaviour of radionuclides in fresh water ecosystems have been developed and tested during recent decades within the framework of many international research projects. These models have been implemented in Computerised Decision Support Systems (CDSS) for assisting the appropriate management of fresh water bodies contaminated by radionuclides. The assessment of the state-of-the-art and the consolidation of these CDSSs has been envisaged, by the scientific community, as a primary necessity for the rationalisation of the sector. The classification of the approaches of the various models, the determination of their essential features, the identification of similarities and differences among them and the definition of their application domains are all essential for the harmonisation of the existing CDSSs and for the possible development and improvement of reference models that can be widely applied in different environmental conditions. The present paper summarises the results of the assessment and evaluation of models for predicting the behaviour of radionuclides in lacustrine ecosystems. Such models were developed and tested within major projects financed by the European Commission during its 4th Framework Programme (1994-1998). The work done during the recent decades by many modellers at an international level has produced some consolidated results that are widely accepted by most experts. Nevertheless, some new results have arisen from recent studies and certain model improvements are still necessary.     

Review and assessment of models for predicting the migration of radionuclides through rivers

The present paper summarises the results of the review and assessment of state-of-the-art models developed for predicting the migration of radionuclides through rivers. The different approaches of the models to predict the behaviour of radionuclides in lotic ecosystems are presented and compared. The models were classified and evaluated according to their main methodological approaches. The results of an exercise of model application to specific contamination scenarios aimed at assessing and comparing the model performances were described. A critical evaluation and analysis of the uncertainty of the models was carried out. The main factors influencing the inherent uncertainty of the models, such as the incompleteness of the actual knowledge and the intrinsic environmental and biological variability of the processes controlling the behaviour of radionuclides in rivers, are analysed.     

Fractionation of radionuclide species in the environment

Naturally occurring and artificially produced radionuclides in the environment may be present in different physico-chemical forms (i.e., radionuclide species) varying in size (nominal molecular mass), charge properties and valence, oxidation state, structure and morphology, density, degree of complexation, etc. Low molecular mass (LMM) species are believed to be mobile and potentially bioavailable, while high molecular mass (HMM) species such as colloids, polymers, pseudocolloids and particles are considered inert. Due to time-dependent transformation processes such as mobilisation of radionuclide species from solid phases or interactions of mobile and reactive radionuclide species with components in soils and sediments, the original distribution of radionuclides deposited in ecosystems will change over time. To assess the environmental impact from radionuclide contamination, information on radionuclide species deposited, interactions within affected ecosystems and the time-dependent distribution of radionuclide species influencing mobility and biological uptake is essential. The development of speciation techniques to characterize radionuclide species in waters, soils and sediments should therefore be essential for improving the prediction power of impact and risk assessment models. The present paper reviews available fractionation techniques which can be utilised for radionuclide speciation purposes.     

Integration of environmental and human health risk assessment for industries using hazardous materials: a quantitative multi criteria approach for environmental decision makers

Environmental management, for which environmental and human health risk assessment is the first stage, is a requirement for industries both before construction and during operation in order to sustain improved quality of life in the ecosystem. Therefore, the aim of this study is to propose an approach that integrates environmental and human health risk assessment for industries using hazardous materials in order to support environmental decision makers with quantitative and directive results. Analytic hierarchy process and fuzzy logic are used as tools to handle problems caused by complexity of environment and uncertain data. When the proposed approach is implemented to a scenario, it was concluded that it is possible to define risk sources with their risk classes and related membership degrees in that classes which enable the decision maker to decide which risk source has priority. In addition, they can easily point out and rank the factors contributing those risk sources owing to priority weights of them. As a result, environmental decision makers can use this approach while they are developing management alternatives for unfounded and on-going industrial plants using hazardous materials.     

Assessment, validation and intercomparison of operational models for predicting tritium migration from routine discharges of nuclear power plants: the case of Loire River

During last decades, a number of projects have been launched to validate models for predicting the behaviour of radioactive substances in the environment. The project of the "Aquatic" working group of the project EMRAS (Environmental Modelling for Radiation Safety) organised by the International Atomic Energy Agency (IAEA) was based on the validation and assessment of models for predicting the behaviour of radionuclides in the aquatic ecosystems. The present paper describes a blind test of models aimed at assessing the dispersion of tritium releases in the Loire River (France), on a large domain ( approximately 350km) and on a period of six months, by comparing the results obtained by operational-to-experimental values of tritium concentration at Angers, a city along the Loire River. The common conclusion is that the models used by the different participants namely 1D models and models based on a schematic hydraulic (box models) are reliable tools for tritium transport modelling. Nevertheless, the importance of proper and detailed hydrological data for the appropriate prediction of pollutant migration in water is demonstrated by the example provided during this study.     

The significance of agricultural vs. natural ecosystem pathways in temperate climates in assessments of long-term radiological impact

Recent developments in performance assessment biosphere models have begun to emphasise the importance of natural accumulation pathways. In contrast to the agricultural pathways, the database for natural ecosystem pathways is less well developed, leading to a mismatch in quality of representations of the two types of system. At issue is the lack of reliable soil-plant and animal ingestion transfer factors for key radionuclides in natural ecosystems. The relative importance of the agricultural vs. natural ecosystem pathways is investigated here, in the context of a temperate site in present day, Eastern France. The BIOMASS Candidate Critical Group (CCG) methodology has been applied to map a set of eight candidate critical groups derived from the present-day societal context onto physical locations within a simple model of a river catchment system. The overall assessment model has been implemented using the Aquabios code. Annual individual dose to each of the CCGs has been calculated for each of the key radionuclides (79Se, 94Nb, 99Tc, 129I, 135Cs and 237Np) released to the valley aquifer and river. In addition to the traditional agricultural pathways, lifestyle groups exploiting natural habitats are explicitly addressed. Results show the susceptibility of different candidate critical groups to different radionuclides. A reference database typical of those employed in long-term performance assessment models is employed. Doses from external exposure (94Nb) and dust inhalation (237Np) are shown to dominate agricultural food consumption by factors of more than six, but, with the reference data set, foodstuffs obtained from natural ecosystems do not contribute significantly to critical group dose and, at most, show similar exposures to the agricultural pathways. This may lead to the conclusion that natural food can be ruled out of consideration in performance assessment models. However, systematic parametric sensitivity studies carried out on soil-plant and animal ingestion transfer factors restrict the validity of this observation and demonstrate the limitations of existing databases. Remaining uncertainties can be reduced by improving structural models for performance assessment and by better characterisation of sources of locally obtained foods. Improved characterisation of radionuclide accumulation in natural ecosystems in temperate as well as alternative future climate states should complement the modelling approach.     

Dynamic model for the assessment of radiological exposure to marine biota

A generic approach has been developed to simulate dynamically the uptake and turnover of radionuclides by marine biota. The approach incorporates a three-compartment biokinetic model based on first order linear kinetics, with interchange rates between the organism and its surrounding environment. Model rate constants are deduced as a function of known parameters: biological half-lives of elimination, concentration factors and a sample point of the retention curve, allowing for the representation of multi-component release. The new methodology has been tested and validated in respect of non-dynamic assessment models developed for regulatory purposes. The approach has also been successfully tested against research dynamic models developed to represent the uptake of technetium and radioiodine by lobsters and winkles. Assessments conducted on two realistic test scenarios demonstrated the importance of simulating time-dependency for ecosystems in which environmental levels of radionuclides are not in equilibrium.     

Modelling multiple dispersion of radionuclides through the environment

The migration of a contaminant through the environment is the result of the transport by a variety of biotic and abiotic carriers which move according to different dispersion mechanisms. Consequently, the patterns of the distribution of a pollutant in the environment cannot be ever explained on the basis of a single migration process or assuming that the concentrations of contaminant in the different kinds of carriers quickly reach the equilibrium condition. The present work discusses two examples (wash-off from catchments and transport through soils of radionuclides) that clearly demonstrate the inadequacy of “single dispersion” models to predict these patterns. On the contrary, models based on multiple dispersion can successfully simulate the particular features of the above mentioned processes. It was demonstrated that the time behaviour of radionuclide migration rates from catchment of different rivers vary within small ranges as a consequence of multiple dispersion. This result can be useful for the development of generic predictive models.     

Radionuclide speciation and its relevance in environmental impact assessments

To assess the long-term environmental impact of radioactive contamination of ecosystems, information on source terms including radionuclide speciation, mobility and biological uptake is needed. A major fraction of refractory radionuclides released from nuclear sources such as nuclear weapons tests and reactor accidents is present as radioactive particles, whilst radionuclides are also present as colloids or low molecular mass species in effluents from nuclear installations. Low molecular mass species are more mobile (lower apparent K(d)) and bioavailable (higher apparent BCF) than colloids and particles. Soils and sediments act as sinks for particles and colloids. Due to particle weathering, associated radionuclides can be remobilised over time. Thus, information on particle characteristics such as composition, particle size, crystalline structures and oxidation states influencing weathering rates and subsequent mobilisation is essential. The present article summarises current knowledge on radioactive particles released from different sources, and the relevance of radionuclide speciation for mobility and biological uptake.     

Effects of model complexity on uncertainty estimates

In the Model Complexity working group of BIOMOVS II, models of varying complexity have been applied to a theoretical problem concerning downward transport of radionuclides in soils. The purpose was to study how uncertainty in model predictions varies with model complexity and how model simplifications can suitably be made. A scenario describing a case of surface contamination of a pasture soil was defined. Three different radionuclides with different environmental behavior and radioactive half-lives were considered: ¹³⁷Cs, ⁹⁰Sr and ¹²⁹I. A detailed specification of the parameters required by different kinds of models was given, together with reasonable values for the parameter uncertainty. A total of seven modelling teams participated in the study using 13 different models. Four of the modelling groups performed uncertainty calculations using nine different modelling approaches. The models ranged in complexity from analytical solutions of a 2-box model using annual average data to numerical models coupling hydrology and transport using data varying on a daily basis.     

Assessment of radiological effects on the regional environment due to the operation of the Tokai Reprocessing Plant

The regional environmental radiological effects were assessed for the past 25-year operation of the Tokai Reprocessing Plant (TRP). The assessment was basically performed with the environmental radiological monitoring data around the TRP. For the environmental monitoring, various kinds of terrestrial and marine samples including air dust, surface soil, polished rice grain, leafy vegetable, milk, seawater, seabed sediments, fish, shellfish and seaweed were collected in the surrounding environment of the TRP. Radionuclides such as (3)H, (14)C, (90)Sr, (137)Cs and (239,240)Pu in the environmental samples were determined by radiochemical methods. However, they showed no significant short-term increase or long-term accumulation of radionuclides discharged from the TRP. Therefore, the public dose was evaluated using the mathematical models and the discharge data of radionuclides. The estimated annual effective dose for the public was about 0.1% of the annual effective dose limit recommended by the ICRP. The assessment showed that there were no significant radiological effects on the environment and the public due to the 25-year operation of the TRP.     

Environmental monitoring and radioecology: a necessary synergy

Environmental monitoring primarily aims through sampling or by the use of direct detection equipment to quantify the levels of radioactive substances and ionising radiation resulting from human activities and natural sources in the different compartments of the environment. Its objectives are very practical and include the quantification of the environmental sources of ionising radiation and the verification of compliance with regulatory requirements and permit limits for industrial, research and medical activities, as stated by their specific licence. Radioecology is a multidisciplinary science, which attempts to understand and to quantify the behaviour of radionuclides in the environment and the processes ruling their transport through natural and agricultural ecosystems to various receptors such as plants, animals and humans. A second facet of this science covers the assessment of the radiological dose to and effects on man and its environment from present, past or future, even hypothetical, nuclear activities. Despite their different immediate objectives, environmental monitoring and radioecology are complementary. Many examples illustrate the connections between these two approaches. For instance, transfer parameters generated by radioecological studies are necessary to estimate through models the radiological exposure of population, derive from the contamination level measured in a bio-indicator the quantity of radioactivity released from a nuclear installation, or identify potentially important pathways to be monitored. On the other hand, monitoring data will confirm important pathways suggested by radioecological modelling and provide site-specific data for the estimation of model parameters or actual data sets for the validation of transfer models.     

Economic water management decisions: trade-offs between conflicting objectives in the sub-middle region of the São Francisco watershed

Hydro-economic models can measure the economic effects of different reservoir operating rules, environmental restrictions, maintenance of ecosystems, technical constraints, institutional constraints, land use change, and climate change. To determine the optimal economic water allocation, for its main uses in the sub-middle of the São Francisco River Basin, a hydro-economic optimization model was developed and applied. Demand curves were used rather than fixed requirements for water resources. The results show that operation rules of reservoirs and institutional constraints, such as priorities for human consumption, have high impacts on costs and benefits of the principal economic uses in the study area. Especially, costs of environmental demands, like minimum ecological river flow, have high impacts on the water resource management. Scarcity costs of irrigation users associated with maintaining ecosystems and environmental constraints are particularly significant. The results from this study provide a better understanding of the water trade-offs for future policymaking and efficient water management. Policymaking for the water resources should consider the food-water-energy-environment nexus at a regional scale to minimize environmental and economic cost under water scarcity and land use change.     

Sector field inductively coupled plasma mass spectrometry, another tool for plutonium isotopes and plutonium isotope ratios determination in environmental matrices

The transfer of radio nuclides into the different compartments of the environment are widely studied and leads to the elaboration of transfer models in order to evaluate potential impact onto the environment and humans. Accurate experimental data are needed to validate these models for all types of matrices (air, water, sediments, soils, biota and food...). Among these radionuclides, 238Pu, 239Pu, 240Pu and 241Pu, are often mentioned. They have been released into the environment by nuclear weapon tests, nuclear facilities, reactors or satellite accidents. These different sources have different 240Pu/239Pu ratios and therefore this ratio is used to provide information on the source of contamination into the environment. The most conventional analytical tools used for plutonium isotope determination are liquid scintillation and alpha spectrometry, and thermal ionisation mass spectrometry (TIMS) is still considered as the primary method for determination of plutonium isotope ratios. During the last decade, mass spectrometers equipped with plasma ion sources and sector field analysers were developed and can offer now another alternative method for the accurate determination of isotope content and ratios of long-lived radionuclides in environmental samples. This paper presents and discusses the results obtained for 239Pu, 240Pu and 241Pu content and isotope ratios by sector field ICP-MS in different environmental matrices.     

Regional-scale application of the decision support system MOIRA-PLUS: an example of assessment of the radiological impact of the Chernobyl accident on the fresh water ecosystem in Italy

The present work illustrates the customisation and application of the decision support system MOIRA-PLUS (a MOdel-based computerised system for management support to Identify optimal remedial strategies for Restoring radionuclide contaminated Aquatic ecosystems and drainage areas) to the fresh water environment in Italy. MOIRA-PLUS is aimed at evaluating the behaviour of radiocaesium and radiostrontium in fresh water ecosystems and at assessing the appropriateness of suitable strategies for the management of contaminated water bodies by the application of multi-attribute analysis techniques. MOIRA-PLUS can be applied to complex networks of lakes, rivers and tributaries and can be straightforwardly customised utilising data and information from readily accessible sources such as official websites provided by scientific or government organisations. The present work shows an application of the decision system to 10 lakes and 18 rivers in Italy contaminated with ¹³⁷Cs of Chernobyl origin. Site-specific values of some aggregated transfer parameters were estimated for the most important Italian lakes. Although high values of fish and water consumptions were hypothesised, very low doses to public from the fresh water pathway following the accident were calculated.     

Customisation of the decision support system MOIRA-PLUS for applications to the marine environment

The present short communication describes a technique to customise the decision system MOIRA-PLUS for applications to the marine environment. MOIRA-PLUS was originally designed to predict the behaviour of ¹³⁷Cs and ⁹⁰Sr in fresh water ecosystems and to evaluate the environmental, social and economic impacts of selected countermeasures aimed at restoring the polluted environment and at reducing the doses to man. An example of application for predicting the concentration of radiocaesium of Chernobyl origin in the Mediterranean Sea is described and discussed. The technique allows the user to easily integrate existing state-of-the-art box models of sea water circulation into the MOIRA-PLUS decision system.     

Review and assessment of models for predicting the migration of radionuclides from catchments

The present paper summarises the results of the review and assessment of models developed for predicting the migration of radionuclides from catchments to water bodies. The models were classified and evaluated according to their main methodological approaches. A retrospective analysis of the principles underpinning the model development in relation to experimental finding and results was carried out. It was demonstrated that most of the various conceptual approaches of different modellers can be integrated in a general, harmonised perspective supported by a variety of experimental evidences. Shortcomings and advantages of the models were discussed.     

Radioactive discharges from an alkaline pulp mill located in the south of Spain

Artificial and natural radionuclides present in environmental samples (Eucalyptus wood and bark) can be transferred in an alkaline pulp mill and concentrated in the recycling process. Some part of the radionuclides can be released into the environment associated with the discharges from such a factory. An alkaline pulp mill located in the South of Spain was extensively sampled during 1993 and different radionuclides analysed in the collected samples. Thus the concentrations and fluxes of natural (40K, 226Ra, 228Ra and 228Th) and artificial (137Cs and 90Sr) radionuclides associated with the samples collected along the pulp production process are presented. It is shown that radium and thorium radionuclides behave similarly to strontium and calcium and, consequently, concentrate within the Ca-cycle. Furthermore, 40K and 137Cs are concentrated during the Na-cycle. Finally, the results obtained by the authors are compared with the levels found in similar factories, though situated in the North of Europe, by other researchers.     

From spatial models to spatial evaluation in the analysis of wetland restoration in the Vecht river basin

Water tables are strictly controlled throughout much of the Netherlands. Higher water tables could permit the restoration of wetland ecosystems. Deriving benefits from this requires the spatial matching of physical planning, eco-hydrological processes, and economic activities. This paper reports on a study combining spatial analysis and disciplinary integration to assess management strategies for the Vecht river basin in the Netherlands. The paper addresses two research questions. How can the information generated by spatial models be condensed into performance indicators for the evaluation of strategies? How can spatial information be retained in the evaluation, and does this lead to different rankings and/or insights? Procedures for the construction of performance indicators for three criteria – net present value, environmental quality and spatial equity – are presented. The evaluation concludes that recreation and higher water tables provide a means for restoration, recouping of costs, and stimulating regional incomes. Maintenance of spatial detail led to different insights, and in particular indicated uncertainty about this conclusion.