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

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.

Marine ecosystem modeling beyond the box: using GIS to study carbon fluxes in a coastal ecosystem

Studies of carbon fluxes in marine ecosystems are often done by using box model approaches with basin size boxes, or highly resolved 3D models, and an emphasis on the pelagic component of the ecosystem. Those approaches work well in the ocean proper, but can give rise to considerable problems when applied to coastal systems, because of the scale of certain ecological niches and the fact that benthic organisms are the dominant functional group of the ecosystem. In addition, 3D models require an extensive modeling effort. In this project, an intermediate approach based on a high resolution (20x20 m) GIS data-grid has been developed for the coastal ecosystem in the Laxemar area (Baltic Sea, Sweden) based on a number of different site investigations. The model has been developed in the context of a safety assessment project for a proposed nuclear waste repository, in which the fate of hypothetically released radionuclides from the planned repository is estimated. The assessment project requires not only a good understanding of the ecosystem dynamics at the site, but also quantification of stocks and flows of matter in the system. The data-grid was then used to set up a carbon budget describing the spatial distribution of biomass, primary production, net ecosystem production and thus where carbon sinks and sources are located in the area. From these results, it was clear that there was a large variation in ecosystem characteristics within the basins and, on a larger scale, that the inner areas are net producing and the outer areas net respiring, even in shallow phytobenthic communities. Benthic processes had a similar or larger influence on carbon fluxes as advective processes in inner areas, whereas the opposite appears to be true in the outer basins. As many radionuclides are expected to follow the pathways of organic matter in the environment, these findings enhance our abilities to realistically describe and predict their fate in the ecosystem.

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.

A carbon budget of a small humic lake: an example of the importance of lakes for organic matter cycling in boreal catchments

Lakes play an important role in the cycling of organic matter in the boreal landscape, due to the frequently high extent of bacterial respiration and the efficient burial of organic carbon in sediments. Based on a mass balance approach, we calculated a carbon budget for a small humic Swedish lake in the vicinity of a potential final repository for radioactive waste in Sweden, in order to assess its potential impact on the environmental fate of radionuclides associated with organic matter. We found that the lake is a net heterotrophic ecosystem, subsidized by organic carbon inputs from the catchment and from emergent macrophyte production. The largest sink of organic carbon is respiration by aquatic bacteria and subsequent emission of carbon.dioxide to the atmosphere. Although the annual burial of organic carbon in the sediment is a comparatively small sink, it results in the build-up of the largest carbon pool in the lake. Hence, lakes may simultaneously disperse and accumulate organic-associated radionuclides leaking from a final repository.

Modeling water exchange and contaminant transport through a Baltic coastal region

The water exchange of the Baltic coastal zone is characterized by its seasonally varying regimes. In the safety assessment of a potential repository for spent nuclear fuel, it is important to assess the consequences of a hypothetical leak of radionuclides through the seabed into a waterborne transport phase. In particular, estimates of the associated residence times in the near-shore coastal zone are of interest. There are several methods to quantify such measures, of which three are presented here. Using the coastal location of Forsmark (Sweden) as an example, methods based on passive tracers, particle trajectories, and the average age distribution of exogeneous water parcels are compared for a representative one-year cycle. Tracer-based methods can simulate diffusivity more realistically than the other methods. Trajectory-based methods can handle Lagrangian dispersion processes due to advection but neglect diffusion on the sub-grid scale. The method based on the concept of average age (AvA) of exogeneous water can include all such sources simultaneously not only boundary water bodies but also various (fresh)-water discharges. Due to the inclusion of sub-grid diffusion this method gives a smoother measure of the water renewal. It is shown that backward in time trajectories and AvA-times are basically equipollent methods, yielding correlated results within the limits set by the diffusivity.

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.