Chlorine Distribution in Soil and Vegetation in Boreal Habitats along a Moisture Gradient from Upland Forest to Lake Margin Wetlands

The assumed dominance of chloride (Cl^-) in terrestrial ecosystems is challenged by observations of extensive formation of organically bound Cl (Cl_org), resulting in large soil Cl storage and internal cycling. Yet, little is known about the spatial distribution of Cl in ecosystems. We quantified patterns of Cl distribution in different habitats along a boreal hillslope moisture gradient ranging from relatively dry upland coniferous forests to wet discharge areas dominated by alder. We confirmed that dry habitats are important for Cl storage but found that Cl pools tended to be larger in moist and wet habitats. The storage of Cl_org was less important in wet habitats, suggesting a shift in the balance between soil chlorination and dechlorination rates. Cl concentrations in the herb layer vegetation were high in wet and moist sites attributed to a shift in plant species composition, indicating plant community-dependent ecosystem Cl cycling. Mass-balance calculations showed that internal Cl cycling increased overall ecosystem Cl residence times at all sites and that plant uptake rates of Cl^- were particularly high at wet sites. Our results indicate that habitat characteristics including plant communities and hydrology are key for understanding Cl cycling in the environment.

Approaches to the definition of potentially exposed groups and potentially exposed populations of biota in the context of solid radioactive waste

A methodology for addressing the biosphere in safety assessments for solid radioactive waste disposal was developed through theme 1 of the IAEA coordinated research project on BIOsphere Modelling and ASSessment (BIOMASS) that ran from 1996 to 2001. This methodology provided guidance on how the biosphere can be addressed in safety assessments for disposal of solid radioactive waste. Since the methodology was developed, it has proven useful and has been widely referenced in assessments in a diversity of contexts encompassing both near-surface and deep geological disposal of solid radioactive waste. The principles that could be adopted for defining potentially exposed groups (PEGs) were an important aspect in the original BIOMASS methodology as the endpoint of an assessment usually includes the evaluation of individual dose or risk to human health. Identification of PEGs and definition of their characteristics are usually made to be consistent with the biosphere system description being developed, acknowledging that due to inherent uncertainties in projecting future human behaviour, the biosphere models adopted for assessing safety of a disposal system can only be illustrative. Since the publication of the original BIOMASS methodology, consideration has been extended to include potentially exposed populations of biota (PEPs), in the context of dose assessment and protection of the environment. Considering the need for the development of transfer pathways from a source term to an end point (for either PEGs or PEPs), the exposure modes that may occur and those to be assessed quantitatively should be identified. Within an expert working group (WG6) of the second phase of the IAEA coordinated project Modelling and Data for Radiological Impact Assessments (MODARIA II), the experience of participating organisations has been collected on topics associated with the definition of PEGs and PEPs using a questionnaire. The objective of the questionnaire was to review the current status and on-going discussions on the handling of issues related to definitions of PEGs and PEPs as an input to the development of biosphere models for assessing radiological impacts on human health and the environment. The answers received to the questionnaire provided a clear overview of the progress that has been made since the original BIOMASS methodology was published, together with the lessons learned from the application of that methodology in the development of safety cases. This paper summarises the questionnaire responses in five subject areas: (1) environment of the PEGs and its evolution; (2) linking the choice of PEGs to these environments; (3) food habits and consumption rates; (4) populations of non-human biota (PEPs) and (5) national and international regulations and guidance. We illustrate how the results of the questionnaire have been used to enhance the original BIOMASS methodology (IAEA Enhanced BIOMASS Methodology Report in press).

Safety assessments undertaken using the BIOMASS methodology: lessons learnt and methodological enhancements

The International Atomic Energy Agency has coordinated an international project addressing enhancements of methods for modelling in post-closure safety assessments of solid radioactive waste disposal. The project used earlier published work from the IAEA biosphere modelling and assessment (BIOMASS) project to further develop methods and techniques. The task was supported by a parallel on-going project within the BIOPROTA forum. The output from the project is described in detail in a forthcoming IAEA report. Here an overview of the work is given to provide researchers in the broader fields of radioecology and radioactive waste disposal with a summarised review of the enhanced BIOMASS methodology and the work that has been undertaken during the project. It is hoped that such dissemination will support and promote integrated understanding and coherent treatment of the biosphere component within the overall assessment process. The key activities undertaken in the project were: review and identification of those parts of the original BIOMASS methodology that needed enhancement, discussions on lessons learned from applying the BIOMASS method, using real examples to assess the methodology and its usefulness, and writing of those parts of the methodology that were considered could benefit from refinement or for which new guidance was required to take account of scientific developments. The work has shown that the overall approach in the original BIOMASS methodology has proven sound. However, the enhanced version clarifies the need for an iterative and holistic approach with system understanding central to the approach. Specifically, experience, especially in site-specific contexts, has emphasised that adequate system understanding is essential in underpinning safety assessments for radioactive waste disposal. The integral role of the biosphere within safety assessment is also emphasised in the enhanced methodology.

System understanding as a scientific foundation in radioactive waste disposal, legacy site and decommissioning programmes

Ongoing national programmes and International forums have in recent decades developed and enhanced methods and strategies in how to address the characterisation of potentially suitable sites for radioactive waste repositories. Siting processes, site selection and site investigation programmes have been conducted for near surface and geological repositories and plans for construction are in progress or have already been implemented. Lessons learned from these national and international programmes are available and results are published. In this paper we synthesise the methods and our lessons learned in how to plan, conduct, and achieve site understanding. Effective site understanding should incorporate a multi-disciplinary and integrated view of geosphere and biosphere information for a site, together with the designed parts of a repository or installation that constitute the total system. We argue that this integrated approach, following a staged program of repository development and adopting a graded approach to assessment at each stage, is to be recommended. The recommendation is supported by the results of international cooperation and progress with national programmes (e.g. the Swedish SKB). Further, we argue that this strategy is valid as a foundation for planning and execution of other types of radioactive waste management programmes such as decommissioning, legacy site management and remediation projects.

Integration of ecosystem science into radioecology: A consensus perspective

In the Fall of 2016 a workshop was held which brought together over 50 scientists from the ecological and radiological fields to discuss feasibility and challenges of reintegrating ecosystem science into radioecology. There is a growing desire to incorporate attributes of ecosystem science into radiological risk assessment and radioecological research more generally, fueled by recent advances in quantification of emergent ecosystem attributes and the desire to accurately reflect impacts of radiological stressors upon ecosystem function. This paper is a synthesis of the discussions and consensus of the workshop participant's responses to three primary questions, which were: 1) How can ecosystem science support radiological risk assessment? 2) What ecosystem level endpoints potentially could be used for radiological risk assessment? and 3) What inference strategies and associated methods would be most appropriate to assess the effects of radionuclides on ecosystem structure and function? The consensus of the participants was that ecosystem science can and should support radiological risk assessment through the incorporation of quantitative metrics that reflect ecosystem functions which are sensitive to radiological contaminants. The participants also agreed that many such endpoints exit or are thought to exit and while many are used in ecological risk assessment currently, additional data need to be collected that link the causal mechanisms of radiological exposure to these endpoints. Finally, the participants agreed that radiological risk assessments must be designed and informed by rigorous statistical frameworks capable of revealing the causal inference tying radiological exposure to the endpoints selected for measurement.

A carbon mass-balance budget for a periglacial catchment in West Greenland - Linking the terrestrial and aquatic systems

Climate change is predicted to have far reaching consequences for the mobility of carbon in arctic landscapes. On a regional scale, carbon cycling is highly dependent on interactions between terrestrial and aquatic parts of a catchment. Despite this, studies that integrate the terrestrial and aquatic systems and study entire catchments using site-specific data are rare. In this work, we use data partly published by Lindborg et al. (2016a) to calculate a whole-catchment carbon mass-balance budget for a periglacial catchment in West Greenland. Our budget shows that terrestrial net primary production is the main input of carbon (99% of input), and that most carbon leaves the system through soil respiration (90% of total export/storage). The largest carbon pools are active layer soils (53% of total carbon stock or 13 kg C m^-2), permafrost soils (30% of total carbon stock or 7.6 kg C m^-2) and lake sediments (13% of total carbon stock or 10 kg C m^-2). Hydrological transport of carbon from the terrestrial to aquatic system is lower than in wetter climates, but the annual input of 4100 kg C yr^-1 (or 3.5 g C m^-2 yr^-1) that enters the lake via runoff is still three times larger than the eolian input of terrestrial carbon. Due to the dry conditions, the hydrological export of carbon from the catchment is limited (5% of aquatic export/storage or 0.1% of total export/storage). Instead, CO₂ evasion from the lake surface and sediment burial accounts for 57% and 38% of aquatic export/storage, respectively (or 0.8% and 0.5% of total export/storage), and Two-Boat Lake acts as a net source of carbon to the atmosphere. The limited export of carbon to downstream water bodies make our study system different from wetter arctic environments, where hydrological transport is an important export pathway for carbon.

Climate change and landscape development in post-closure safety assessment of solid radioactive waste disposal: Results of an initiative of the IAEA

The International Atomic Energy Agency has coordinated an international project addressing climate change and landscape development in post-closure safety assessments of solid radioactive waste disposal. The work has been supported by results of parallel on-going research that has been published in a variety of reports and peer reviewed journal articles. The project is due to be described in detail in a forthcoming IAEA report. Noting the multi-disciplinary nature of post-closure safety assessments, here, an overview of the work is given to provide researchers in the broader fields of radioecology and radiological safety assessment with a review of the work that has been undertaken. It is hoped that such dissemination will support and promote integrated understanding and coherent treatment of climate change and landscape development within an overall assessment process. The key activities undertaken in the project were: identification of the key processes that drive environmental change (mainly those associated with climate and climate change), and description of how a relevant future may develop on a global scale; development of a methodology for characterising environmental change that is valid on a global scale, showing how modelled global changes in climate can be downscaled to provide information that may be needed for characterising environmental change in site-specific assessments, and illustrating different aspects of the methodology in a number of case studies that show the evolution of site characteristics and the implications for the dose assessment models. Overall, the study has shown that quantitative climate and landscape modelling has now developed to the stage that it can be used to define an envelope of climate and landscape change scenarios at specific sites and under specific greenhouse-gas emissions assumptions that is suitable for use in quantitative post-closure performance assessments. These scenarios are not predictions of the future, but are projections based on a well-established understanding of the important processes involved and their impacts on different types of landscape. Such projections support the understanding of, and selection of, plausible ranges of scenarios for use in post-closure safety assessments.

Application of an ecosystem model to evaluate the importance of different processes and food web structure for transfer of 13 elements in a shallow lake

In environmental risk assessments of nuclear waste, there is need to estimate the potential risks of a large number of radionuclides over a long time period during which the environment is likely to change. Usually concentration ratios (CRs) are used to calculate the activity concentrations in organisms. However, CRs are not available for all radionuclides and they are not easily scalable to the varying environment. Here, an ecosystem transport model of elements, which estimates concentrations in organisms using carbon flows and food transfer instead of CR is presented. It is a stochastic compartment model developed for Lake Eckarfjärden at Forsmark in Sweden. The model was based on available data on carbon circulation, physical and biological processes from the site and identifies 11 functional groups of organisms. The ecosystem model was used to estimate the environmental transfer of 13 elements (Al, Ca, Cd, Cl, Cs, I, Ni, Nb, Pb, Se, Sr, Th, U) to various aquatic organisms, using element-specific distribution coefficients for suspended particles (K_{d PM}) and upper sediment (K_{d sed}), and subsequent transfer in the foodweb. The modelled CRs for different organism groups were compared with measured CRs from the lake and literature data, and showed good agreement for many elements and organisms, particularly for lower trophic levels. The model is, therefore, proposed as an alternative to measured CR, though it is suggested to further explore active uptake, assimilation and elimination processes to get better correspondence for some of the elements. The benthic organisms (i.e. bacteria, microphytobenthos and macroalgae) were identified as more important than pelagic organisms for transfer of elements to top predators. The element transfer model revealed that most of the radionuclides were channelled through the microbial loop, despite the fact that macroalgae dominated the carbon fluxes in this lake. Thus, element-specific adsorption of elements to the surface of aquatic species, that may be food sources for organisms at higher trophic levels, needs to be considered in combination with generic processes described by carbon fluxes.

Evaluation of factors influencing accumulation of stable Sr and Cs in lake and coastal fish

As a result of nuclear accidents and weapons tests, the radionuclides Cs-137 and Sr-90 are common contaminants in aquatic ecosystems. Concentration ratios (CR) based on concentrations of stable Cs and Sr in biota and media are used for the estimation of transfer of their radioisotopes for radiation dose calculations in environmental and human safety assessments. Available element-specific CRs vary by over an order of magnitude for similar organisms, thus affecting the dose estimates proportionally. The variation could be reduced if they were based on a better understanding of the influence of the underlying data and how that affects accumulation and potential biomagnification of stable Cs and Sr in aquatic organisms. For fish, relationships have been identified between water concentrations of K and CR of Cs-137, and between water concentrations of Ca and CR of Sr-90. This has not been confirmed for stable Cs and Sr in European waters. In this study, we analysed an existing dataset for stable Cs and Sr, as well as K and Ca, in four Swedish lakes and three Baltic Sea coastal areas, in order to understand the behaviour of these elements and their radioisotopes in these ecosystems. We found significant seasonal variations in the water concentrations of Cs, Sr, K and Ca, and in electrical conductivity (EC), especially in the lakes. CR values based on measurements taken at single or few time points may, therefore, be inaccurate or introduce unnecessarily large variation into risk assessments. Instead, we recommend incorporating information about the underlying variation in water concentrations into the CR calculations, for example by using the variation of the mean. The inverse relationships between fish CR(Cs)-[K]water and fish CR(Sr)-[Ca]water, confirmed that stable Cs and Sr follow the same trends as their radioisotopes. Thus, they can be used as proxies when radioisotope data are lacking. EC was also strongly correlated with K and Ca concentrations in the water and could potentially be used as a quick and cost-effective method to estimate water chemistry to obtain less variable CR. We also recommend some simple improvements to data collection that would greatly enhance our ability to understand Cs and Sr uptake by fish.

Addressing ecological effects of radiation on populations and ecosystems to improve protection of the environment against radiation: Agreed statements from a Consensus Symposium

This paper reports the output of a consensus symposium organized by the International Union of Radioecology in November 2015. The symposium gathered an academically diverse group of 30 scientists to consider the still debated ecological impact of radiation on populations and ecosystems. Stimulated by the Chernobyl and Fukushima disasters' accidental contamination of the environment, there is increasing interest in developing environmental radiation protection frameworks. Scientific research conducted in a variety of laboratory and field settings has improved our knowledge of the effects of ionizing radiation on the environment. However, the results from such studies sometimes appear contradictory and there is disagreement about the implications for risk assessment. The Symposium discussions therefore focused on issues that might lead to different interpretations of the results, such as laboratory versus field approaches, organism versus population and ecosystemic inference strategies, dose estimation approaches and their significance under chronic exposure conditions. The participating scientists, from across the spectrum of disciplines and research areas, extending also beyond the traditional radioecology community, successfully developed a constructive spirit directed at understanding discrepancies. From the discussions, the group has derived seven consensus statements related to environmental protection against radiation, which are supplemented with some recommendations. Each of these statements is contextualized and discussed in view of contributing to the orientation and integration of future research, the results of which should yield better consensus on the ecological impact of radiation and consolidate suitable approaches for efficient radiological protection of the environment.

The impact of low and intermediate-level radioactive waste on humans and the environment over the next one hundred thousand years

In order to assess the potential radiological risk to humans and the environment from a geological repository for radioactive waste, a safety assessment must be performed. This implies that the release and transfer of radionuclides from the repository into the surface environment are calculated and that the effects in the biosphere are evaluated for an assessment period up to one hundred thousand years according to Swedish regulations. This paper discusses the challenges associated with the modelling of surface ecosystems over such long time scales, using the recently completed assessment for the extension of the existing repository for the low- and intermediate-level nuclear waste (called SFR) in Forsmark, Sweden as an applied example. In the assessment, natural variation and uncertainties in climate during the assessment period were captured by using a set of climate cases, primarily reflecting different expectations on the effects of global warming. Development of the landscape at the site, due to post-glacial isostatic rebound, was modelled, and areas where modelling indicated that radionuclides could discharge into the biosphere were identified. Transfers of surface water and groundwater were described with spatially distributed hydrological models. The projected release of radionuclides from the bedrock was then fed into a biosphere radionuclide transport model, simulating the transport and fate of radionuclides within and between ecosystems in the landscape. Annual doses for human inhabitants were calculated by combining activity concentrations in environmental media (soil, water, air and plants) with assumptions on habits and land-use of future human inhabitants. Similarly, dose rates to representative organisms of non-human biota were calculated from activity concentrations in relevant habitats, following the ERICA methodology. In the main scenario, the calculated risk for humans did not exceed the risk criteria or the screening dose rate for non-human biota, indicating that the repository design is sufficient to protect future populations and the environment. Although the combination of radionuclides, land-uses/habitats, type of most exposed population and area of exposure that contribute most to the total dose shifts over time, the total calculated dose shows limited variability. Significant reductions in the dose only occur during submerged periods and under periglacial climate conditions. As several different water and food pathways were equally important for endpoint results, it is concluded that it would be difficult to represent the biosphere with one or a set of simplified models. Instead, we found that it is important to maintain a diversity of food and water pathways, as key pathways for radionuclide accumulation and exposure partly worked in parallel.

Advancing the Modelling Environment for the Safety Assessment of the Swedish LILW Repository at Forsmark

An extension of the Swedish final repository for short-lived radioactive waste (SFR) is planned and a safety assessment has been performed as part of the licensing process. Within this work, steps have been taken to advance the modelling environment to better integrate its individual parts. It is desirable that an integrating modelling environment provides the framework to set up and solve a consistent hierarchy of models on different scales. As a consequence, the consistent connection between software tools and models needs to be considered, related to the full assessment domain. It should also be possible to include the associated geometry and material descriptions, minimizing simplifications of conceptual understanding.

The usefulness of the analysis software Comsol Multiphysics as component of an integrating modelling environment has been tested. Here, we present two examples of hierarchical models. Consistent properties and boundary conditions have been extracted form regional hydrogeology and surface hydrology models when setting up repository scale models. CAD models of the repository have been imported into the analysis software, representing tunnel systems and storage vaults with engineered structures and barriers. Data from geographic information systems such as digital elevation models and geological formations have been also directly implemented into model geometries.

The repository scale hydrology models have provided a basis for further developments focussed on the modelling of coupled processes. An interface between Comsol Multiphysics and the geochemical simulator Phreeqc has been developed to support reactive solute transport studies. An important test case concerns radionuclide transport in a 3D, near-surface model of a catchment area. The dynamic surface hydrology has been simulated with MIKE SHE and connected to Comsol Multiphysics and Phreeqc for detailed hydro-geo-chemical modelling of radionuclide migration through soils and sediments.

A biosphere assessment of high-level radioactive waste disposal in Sweden

Licence applications to build a repository for the disposal of Swedish spent nuclear fuel have been lodged, underpinned by myriad reports and several broader reviews. This paper sketches out the technical and administrative aspects and highlights a recent review of the biosphere effects of a potential release from the repository. A comprehensive database and an understanding of major fluxes and pools of water and organic matter in the landscape let one envisage the future by looking at older parts of the site. Thus, today's biosphere is used as a natural analogue of possible future landscapes. It is concluded that the planned repository can meet the safety criteria and will have no detectable radiological impact on plants and animals. This paper also briefly describes biosphere work undertaken after the review. The multidisciplinary approach used is relevant in a much wider context and may prove beneficial across many environmental contexts.

Using an Ecosystem Approach to complement protection schemes based on organism-level endpoints

Radiation protection goals for ecological resources are focussed on ecological structures and functions at population-, community-, and ecosystem-levels. The current approach to radiation safety for non-human biota relies on organism-level endpoints, and as such is not aligned with the stated overarching protection goals of international agencies. Exposure to stressors can trigger non-linear changes in ecosystem structure and function that cannot be predicted from effects on individual organisms. From the ecological sciences, we know that important interactive dynamics related to such emergent properties determine the flows of goods and services in ecological systems that human societies rely upon. A previous Task Group of the IUR (International Union of Radioecology) has presented the rationale for adding an Ecosystem Approach to the suite of tools available to manage radiation safety. In this paper, we summarize the arguments for an Ecosystem Approach and identify next steps and challenges ahead pertaining to developing and implementing a practical Ecosystem Approach to complement organism-level endpoints currently used in radiation safety.

Radionuclide transfer in marine coastal ecosystems, a modelling study using metabolic processes and site data

This study implements new site-specific data and improved process-based transport model for 26 elements (Ac, Ag, Am, Ca, Cl, Cm, Cs, Ho, I, Nb, Ni, Np, Pa, Pb, Pd, Po, Pu, Ra, Se, Sm, Sn, Sr, Tc, Th, U, Zr), and validates model predictions with site measurements and literature data. The model was applied in the safety assessment of a planned nuclear waste repository in Forsmark, Öregrundsgrepen (Baltic Sea). Radionuclide transport models are central in radiological risk assessments to predict radionuclide concentrations in biota and doses to humans. Usually concentration ratios (CRs), the ratio of the measured radionuclide concentration in an organism to the concentration in water, drive such models. However, CRs vary with space and time and CR estimates for many organisms are lacking. In the model used in this study, radionuclides were assumed to follow the circulation of organic matter in the ecosystem and regulated by radionuclide-specific mechanisms and metabolic rates of the organisms. Most input parameters were represented by log-normally distributed probability density functions (PDFs) to account for parameter uncertainty. Generally, modelled CRs for grazers, benthos, zooplankton and fish for the 26 elements were in good agreement with site-specific measurements. The uncertainty was reduced when the model was parameterized with site data, and modelled CRs were most similar to measured values for particle reactive elements and for primary consumers. This study clearly demonstrated that it is necessary to validate models with more than just a few elements (e.g. Cs, Sr) in order to make them robust. The use of PDFs as input parameters, rather than averages or best estimates, enabled the estimation of the probable range of modelled CR values for the organism groups, an improvement over models that only estimate means. Using a mechanistic model that is constrained by ecological processes enables (i) the evaluation of the relative importance of food and water uptake pathways and processes such as assimilation and excretion, (ii) the possibility to extrapolate within element groups (a common requirement in many risk assessments when initial model parameters are scarce) and (iii) predictions of radionuclide uptake in the ecosystem after changes in ecosystem structure or environmental conditions. These features are important for the longterm (>1000 year) risk assessments that need to be considered for a deep nuclear waste repository.

Radionuclide transport and uptake in coastal aquatic ecosystems: a comparison of a 3D dynamic model and a compartment model

In safety assessments of underground radioactive waste repositories, understanding radionuclide fate in ecosystems is necessary to determine the impacts of potential releases. Here, the reliability of two mechanistic models (the compartmental K-model and the 3D dynamic D-model) in describing the fate of radionuclides released into a Baltic Sea bay is tested. Both are based on ecosystem models that simulate the cycling of organic matter (carbon). Radionuclide transfer is linked to adsorption and flows of carbon in food chains. Accumulation of Th-230, Cs-135, and Ni-59 in biological compartments was comparable between the models and site measurements despite differences in temporal resolution, biological state variables, and partition coefficients. Both models provided confidence limits for their modeled concentration ratios, an improvement over models that only estimate means. The D-model enables estimates at high spatio-temporal resolution. The K-model, being coarser but faster, allows estimates centuries ahead. Future developments could integrate the two models to take advantage of their respective strengths.

Land use and food intake of future inhabitants: outlining a representative individual of the most exposed group for dose assessment

The radiation doses to humans resulting from a potential release of radionuclides from a geological repository for long-lived waste are assessed over tens or even hundreds of thousands of years. Ingestion is expected to be the major exposure pathway, and the group with the highest exposures will be those that consume the most contaminated food. In this paper, we characterize the group of individuals with the highest exposures by considering the physical and biological characteristics of the contaminated area and human requirements for energy and nutrients. We then calculate intake rates based on land-use scenarios drawn from self-sustained communities spanning prehistoric times to an industrial-age agrarian culture. The approach is illustrated by simulating groundwater release of four radionuclides to an expected discharge area. We argue that the derived intake rates may serve as credible bounding cases when projected doses are evaluated for compliance with regulatory criteria.

Humans and ecosystems over the coming millennia: overview of a biosphere assessment of radioactive waste disposal in Sweden

This is an overview of the strategy used to describe the effects of a potential release from a radioactive waste repository on human exposure and future environments. It introduces a special issue of AMBIO, in which 13 articles show ways of understanding and characterizing the future. The study relies mainly on research performed in the context of a recent safety report concerning a repository for spent nuclear fuel in Sweden (the so-called SR-Site project). The development of a good understanding of on-site processes and acquisition of site-specific data facilitated the development of new approaches for assessment of surface ecosystems. A systematic and scientifically coherent methodology utilizes the understanding of the current spatial and temporal dynamics as an analog for future conditions. We conclude that future ecosystem can be inferred from a few variables and that this multidisciplinary approach is relevant in a much wider context than radioactive waste.

Model of the long-term transport and accumulation of radionuclides in future landscapes

Assessments of radiological impacts on humans and other biota from potential releases to the biosphere from a deep geologic repository for spent nuclear fuel are associated with several challenges. Releases, if any, will likely occur in a far future and to an environment that will have experienced substantial transformations. Such releases would occur over very long periods during which environmental conditions will vary continuously due to climate change and ecosystem succession. Assessments of radiological impacts must therefore be based on simulations using models that can describe the transport and accumulation of radionuclides for a large variety of environmental conditions. In this paper we describe such a model and show examples of its application in a safety assessment, taking into account results from sensitivity and uncertainty analyses of the model predictions.

Can ECOPATH with ECOSIM enhance models of radionuclide flows in food webs? An example for 14C in a coastal food web in the Baltic Sea

In this study it was evaluated whether the ECOPATH with ECOSIM software could be used as a platform to facilitate the construction of models and study of transport and accumulation of radionuclides in aquatic food webs. The evaluation was based upon a food web model of carbon (C) and carbon-14 ((14)C) flow for a coastal area in the Baltic Sea, the ECOPATH, the ECOSIM and the ECOTRACE models. The original carbon flows and assumptions were easily incorporated into the ECOPATH and ECOSIM modelling environment. The new model was also well suited to drive a (14)C flow model (ECOTRACE) for each of the organisms included. ECOTRACE estimated steady-state concentrations of (14)C that were between 73 and 142% of the original flows. The results clearly show that there is great potential for a successful development of this approach for integrating scientific knowledge about food webs and radioecological models for aquatic systems.

Modeling the long-term transport and accumulation of radionuclides in the landscape for derivation of dose conversion factors

To evaluate the radiological impact of potential releases to the biosphere from a geological repository for spent nuclear fuel, it is necessary to assess the long-term dynamics of the distribution of radionuclides in the environment. In this paper, we propose an approach for making prognoses of the distribution and fluxes of radionuclides released from the geosphere, in discharges of contaminated groundwater, to an evolving landscape. The biosphere changes during the temperate part (spanning approximately 20,000 years) of an interglacial period are handled by building biosphere models for the projected succession of situations. Radionuclide transport in the landscape is modeled dynamically with a series of interconnected radioecological models of those ecosystem types (sea, lake, running water, mire, agricultural land and forest) that occur at present, and are projected to occur in the future, in a candidate area for a geological repository in Sweden. The transformation between ecosystems is modeled as discrete events occurring every thousand years by substituting one model by another. Examples of predictions of the radionuclide distribution in the landscape are presented for several scenarios with discharge locations varying in time and space. The article also outlines an approach for estimating the exposure of man resulting from all possible reasonable uses of a potentially contaminated landscape, which was used for derivation of Landscape Dose Factors.

Pools and fluxes of organic matter in a boreal landscape: implications for a safety assessment of a repository for nuclear waste

To provide information necessary for a license application for a deep repository for spent nuclear fuel, the Swedish Nuclear Fuel and Waste Management Co is carrying out site investigations, including extensive studies of different parts of the surface ecosystems, at two sites in Sweden. Here we use the output from detailed modeling of the carbon dynamics in the terrestrial, limnic and marine ecosystems to describe and compare major pools and fluxes of organic matter in the Simpevarp area, situated on the southeast coast of Sweden. In this study, organic carbon is used as a proxy for radionuclides incorporated into organic matter. The results show that the largest incorporation of carbon into living tissue occurs in terrestrial catchments. Carbon is accumulated in soil or sediments in all ecosystems, but the carbon pool reaches the highest values in shallow near-land marine basins. The marine basins, especially the outer basins, are dominated by large horizontal water fluxes that transport carbon and any associated contaminants into the Baltic Sea. The results suggest that the near-land shallow marine basins have to be regarded as focal points for accumulation of radionuclides in the Simpevarp area, as they receive a comparatively large amount of carbon as discharge from terrestrial catchments, having a high NPP and a high detrital accumulation in sediments. These focal points may constitute a potential risk for exposure to humans in a future landscape as, due to post-glacial land uplift, previous accumulation bottoms are likely to be used for future agricultural purposes.

A strategy for describing the biosphere at candidate sites for repositories of nuclear waste: linking ecosystem and landscape modeling

To provide information necessary for a license application for a deep repository for spent nuclear fuel, the Swedish Nuclear Fuel and Waste Management Co. has started site investigations at two sites in Sweden. In this paper, we present a strategy to integrate site-specific ecosystem data into spatially explicit models needed for safety assessment studies and the environmental impact assessment. The site-specific description of ecosystems is developed by building discipline-specific models from primary data and by identifying interactions and stocks and flows of matter among functional units at the sites. The conceptual model is a helpful initial tool for defining properties needed to quantify system processes, which may reveal new interfaces between disciplines, providing a variety of new opportunities to enhance the understanding of the linkages between ecosystem characteristics and the functional properties of landscapes. This type of integrated ecosystem-landscape characterization model has an important role in forming the implementation of a safety assessment for a deep repository.

Rural landscape, production and human consumption: past, present and future

Production and consumption of food and in a rural area over the last 400 years were reconstructed for a parish in south east Sweden. This was based on a number of different data sources, including historical maps and official demographic and agricultural statistics. Changes in population (and thus consumption) and the production from arable land and livestock were calculated and used to provide an estimate of the area's supply and demand over time, and of the historical sustainability of the area. Overall food productivity was remarkably constant over time, at approximately 0.04 kgC m(-2) y(-1), despite recent changes in population size and the area of cultivated land. The empirical results from the past and the present, together with the future land changes due to shoreline displacement were used to predict the situation in the future. These final estimates can be used in the assessment of risk for exposure to contaminated food for the future population in the area.

Transport and fate of radionuclides in aquatic environments--the use of ecosystem modelling for exposure assessments of nuclear facilities

In safety assessments of nuclear facilities, a wide range of radioactive isotopes and their potential hazard to a large assortment of organisms and ecosystem types over long time scales need to be considered. Models used for these purposes have typically employed approaches based on generic reference organisms, stylised environments and transfer functions for biological uptake exclusively based on bioconcentration factors (BCFs). These models are of non-mechanistic nature and involve no understanding of uptake and transport processes in the environment, which is a severe limitation when assessing real ecosystems. In this paper, ecosystem models are suggested as a method to include site-specific data and to facilitate the modelling of dynamic systems. An aquatic ecosystem model for the environmental transport of radionuclides is presented and discussed. With this model, driven and constrained by site-specific carbon dynamics and three radionuclide specific mechanisms: (i) radionuclide uptake by plants, (ii) excretion by animals, and (iii) adsorption to organic surfaces, it was possible to estimate the radionuclide concentrations in all components of the modelled ecosystem with only two radionuclide specific input parameters (BCF for plants and Kd). The importance of radionuclide specific mechanisms for the exposure to organisms was examined, and probabilistic and sensitivity analyses to assess the uncertainties related to ecosystem input parameters were performed. Verification of the model suggests that this model produces analogous results to empirically derived data for more than 20 different radionuclides.

Consideration of environmental change in performance assessments

Depending on the particular circumstances in which a post-closure performance assessment of a radioactive waste repository is made, it may be appropriate to follow simple or more complex approaches in characterising the biosphere. Several different Example Reference Biospheres were explored in BIOMASS Theme 1 to address a range of issues that arise. Here, consideration is given to Example Reference Biospheres relevant to representing the implications of changes that may occur within the biosphere system during the period over which releases of radionuclides from a disposal facility might take place. Mechanisms of change considered include those extrinsic and intrinsic to the system of interest. An overall methodology for incorporating environmental change into assessments is proposed. This includes screening of primary mechanisms of change; identification of possible time sequences of change; development of a coherent description of the regional landscape response for each time sequence; integration of source term and geosphere-biosphere interface information; identification and description of one or more time series of assessment biospheres; and evaluation of the advantages and disadvantages of simulating the effects of sequences of biosphere systems and the transitions between them, or of defining a set of biosphere systems to be represented individually in a non-sequential analysis. The usefulness of the methodology is explored in two site-specific examples and one generic example.