Large-scale and Long-term Environmental Behaviour of Transuranic Elements as Modelled through European Surface Water Systems.
Introduction. Previous research shows that interactive processes in surface water can result in locally concentrated Pu, Am and Np activities at sites remote from source. Pu, Am and Np were introduced to global environmental systems from a series of nuclear weapons tests that started 50 years ago. European terrestrial and aquatic sites show a wide range of environmental fluxes and concentrations of the fallout Pu and Am. As contributions from existing nuclear industry are still very limited, the variations in Pu and Am levels are clearly related to large-scale and long-term forcing processes. Such processes are poorly understood especially when considering the complexity of environmental changes and the diversity of European freshwater systems. The journey of surface water begins with tropospheric condensation of water vapour, followed by hydrological interactions in the landscape "between-system interactions" that lead to physical, geochemical and biogeochemical transportation and mobilisation of Pu and Am to surface water via rivers and catchments. The retention of water in lakes serves as a vehicle for a chain of "within-system interactions" via bioproduction, internal redistributions, accumulation and final preservation in historical archives. There are a set of critical factors that influence the dynamics and rates of mobilisation and accumulation of Pu and Am through these external and internal interactions. The most important of these are: hydrology and water circulation pattern, soil characteristics, land use activities, erosion, resuspension, reduction/oxidation, speciation and calcite precipitation, acidification, eutrophication, diagenetic processes and microbial activities.
Objectives. Three lake systems characterising different European types of freshwater and existing in dynamic regions with a wide-range of natural and environmental conditions are used here to uncover the major forcing processes that influence the behaviour of the transuranic elements in surface water. Härsvatten is a severely acidified softwater lake; Stechlin is an oligotrophic hardwater lake with some signs of eutrophication; and Blelham is a freshwater lake with land use effects. Through reconstruction of highly resolved temporal and spatial records of Pu and Am in the sediments of the lakes, the influence of large-scale redistribution processes and delayed mobilisation in surface water will be quantitatively determined.
Lake Härsvatten, Sweden.
Since the environmental and limnological evolution of these lakes are very well documented it would be possible to evaluate the individual lakes and to compare them in terms of: (1) acid deposition and the internal functioning of acidified water which is still unclear since previous studies were not designed to reflect the overall forcing processes regulating the internal distribution; (2) the sedimentation in hardwater lakes is fairly unknown on quantitative terms and a great need exists to explore the self-cleaning capacities in these lakes under changing environmental conditions, especially the increasing anthropogenic loads (including nutrients) that can modify calcite precipitation and thereby the delayed effects; (3) landuse activities and the increasing pressures on the utilisation of land cause delayed translocations with the influence of eutrophication due to farm runoff and sewage from local plants.

Acidification and eutrophication due to industrial and land use activities are among major Global Changes modifying the behaviour of hazardous radionuclides. This work gives important data concerning objectively based evaluations on radiation risks for better radioecological strategies, restoration and rehabilitation, with consideration to Global Change aspects.
Results. Lake Härsvatten. The spatial distribution of reactive radionuclides, such as the transuranics, are generally uniform in the individual basins (Fig. 1) with no focusing effects. Shallow areas can accumulate and trap high amounts of radioactive and non-radioactive species. The inventories in the northern basin in relation to the southern one are 1.7 for 234+240Pu and 241Am and 1.3 for 210Pb. The northern basin is more effective in accumulating Pu and Am which are relatively more associated with coarser particulates. Acidification enhances water clarity and promotes the expansion of algal and Sphagnum mats especially in shallow areas. This effect changed the sedimentation and accumulation dynamics, which are forced by processes other than the normal settling of particulates, i.e. the trapping action of the algal/Sphagnum mats.

The Pu and Am inventories in the lake are generally much lower (54%) than the inventories in the catchment. The inventories of 210Pb are similar to those of the catchment. The rainfall data and the global inventories in soils and sediments demonstrate a deficiency in the lake via the outflow. There is evidence that the dry deposition due to high sea salt and S-, N- gas and particulate matter is generally higher or much higher than wet deposition in Härsvatten. However, since the precipitation in the area is generally lower during the spring and outflow from the lake is higher in this period less sediment inventories can be expected.
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The temporal evolution is very different in Härsvatten. There is a collective behaviour for all radionuclides and particulate matter in the northern basin regardless their origin. This is forced by a particulate-sorting mechanism that continuously mobilises finer particulates upwards. Since most of course particulates exist in shallow areas, the deep sediments of the southern basin mainly reflect the accumulation of fine particulates. The temporal records of the southern basin (Fig. 2) indicate that 15-30% of the inventories came from the catchment and during a relatively short period after 1963. The current delay effects are lower than 0.03% per year.
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Lake Stechlin. The spatial inventories of the Pu and Am indicate some increase with water depth down to 30m (Fig. 3) where no further increase exists. However, the 238U inventories show a strong and clear increase in the upper 30m. The accumulation of all the radionuclides is strongly associated with the net sedimentation (primary/secondary production minus destruction due to bacterial/chemical actions) to the bottom. Indeed, the accumulation of radionuclides by organic matter is highest at lower carbonate contents. This indicates that the uptake and/or accumulation of radionuclides, via primary and secondary production, are enhanced at low carbonate contents. The inventories of Pu, Am and 210Pb in the lake are similar to or slightly lower than those in the catchment. There are no clear indications of inputs from the catchment as is seen with 238U where it is probably introduced to the lake via carbonate-soluble uranyl in groundwater. The situation of 137Cs is somewhat similar to the U.
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Figure (4) shows considerable delay effects in the accumulation of Pu and Am to the sediments. Since there is no clear evidence of Pu/Am inputs from the catchment, these effects must be attributed to internal cycling due the bacterial activity in the water. In surface water, the bioproduction is on cumulative manner increasing with depth. The passage of high organic material via oxygenated water allows an efficient aerobic mineralization and the destruction of organic cells in deep water. Neither the cumulative organic matter nor the inventories of radionuclides are, therefore, expected to increase with water depths of more than 30m. The delay effects due to successive inputs from the catchment via groundwater, which seem to be limited for Pu and Am, are not known at this stage.
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Lake Blelham Tarn. Atmospheric fluxes of natural (210Pb) and artificial (137Cs, 239+240Pu) have been determined at Blelham Tarn, UK, from direct measurements in rainfall and records in soil cores. Sediment cores from Blelham Tarn have been used to determine the spatial distribution of fallout radionuclides over the bed of the lake (Fig. 5). Mass balance calculations indicate that between 25-30% of the 210Pb and 137Cs in the sediments derives from erosive inputs from the catchment. For 239+240Pu the value rises to 44%. Calculations of transport rates suggest that mean residence times of radionuclides in the catchment are c. 1760 years for 210Pb and c. 1150 years for 239+240Pu. The inputs from the catchment are concentrated near one of the major input streams. Away from this part of the lake, inputs to the sediment record are dominated by direct atmospheric fallout.
The chronology of inputs has been studied in detail using cores from Blelham Tarn and Esthwaite Water. These were dated precisely and the results show that 210Pb fluxes at individual sites within the lake have increased in response to accelerating sedimentation rates.

Calculations still in progress are intended to determine a more precise history of the transfer of 239+240Pu from the catchment to the lakes (Fig. 6). Initial results suggest that present day transport rates have declined to c. 0.0002 y-1 (corresponding to a residence time of c. 5000 y) as the radionuclides become less available due to migration into the deeper soil layers.
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Implications. The results of this project indicate that the suggested field, laboratory and modelling approaches are very useful in quantifying the transportation and mobilisation of the particulate-reactive fallout radionuclides in various freshwater systems. The large-scale and long-term environmental behaviour of fallout radionuclides, including the transuranic elements, is forced by the functioning and metabolism of their freshwater systems. Indeed, various delay effects can be studied and modelled by following the major processes and interactions that regulate freshwater systems. This is only possible through integrated studies on the atmosphere, soil, and surface water compartments. It is, therefore, essential to have detailed records on the atmospheric inputs not only concerning hazardous radionuclides but also on reference radionuclides, such as 210Pb and 222Rn. The behaviour of 238U in hardwater can be used as a reference for radioecological studies. These radionuclides are important tracers for studying and modelling the large-scale and long-term environmental behaviour of hazardous radionuclides in aquatic systems.
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Environmental changes in surface water systems due to acidification, eutrophication, industrial/land use activities, and related anthropogenic pollution in general influence the delay effected and modify the radioecological behaviour in surface water systems. It is, therefore, important to understand and evaluate the radioecological behaviour of hazardous radionuclides in relation to the role of Global Change aspects on the natural functioning and metabolism of surface water systems. Without proper and detailed understanding of "within-system" and "between-system" interactions, it would not be possible to develope either operational models or sustainable rehabilitation policies. Future radioecological rehabilitation and remedial actions as well as radiation protection policies must consider the increasing pressures on existing land-water resources. Mechanisms and technologies that are able to remedy, strengthen and stabilise natural cleaning processes, such as sedimentation in surface water systems, the filtering and holding capacity of soils, are essential for establishing sustainable resources.

Studies of the aqueous behaviour of soluble and conservative radionuclides must consider the role of recent environmental changes on the functioning and metabolism of surface water systems on an integrated scale. Small-scale and large-scale water cleansing technologies are important aspects in future radiation protection policies.



Coordinator: Dr. F. El-Daoushy (Uppsala University, Sweden).

Partners:
Dr. F. El-Daoushy (Uppsala University, Sweden)
Prof. P. G. Appleby (University of Liverpool, the United Kingdom);
Prof. M. García-León (Universidad de Sevilla, Spain);
Dr. P. Casper (Forschungsverbund Berlin e.V.-Gemeinsame Verwaltung, Germany);
Prof. G. Ardisson (Université de Nice, France).