Iisa Outola

Establishing a database of radionuclide transfer parameters for freshwater wildlife

Environmental assessments to evaluate potentials risks to humans and wildlife often involve modelling to predict contaminant exposure through key pathways. Such models require input of parameter values, including concentration ratios, to estimate contaminant concentrations in biota based on measurements or estimates of concentrations in environmental media, such as water. Due to the diversity of species and the range in physicochemical conditions in natural ecosystems, concentration ratios can vary by orders of magnitude, even within similar species. Therefore, to improve model input parameter values for application in aquatic systems, freshwater concentration ratios were collated or calculated from national grey literature, Russian language publications, and refereed papers. Collated data were then input into an international database that is being established by the International Atomic Energy Agency. The freshwater database enables entry of information for all radionuclides listed in ICRP (1983), in addition to the corresponding stable elements, and comprises a total of more than 16,500 concentration ratio (CRwo-water) values. Although data were available for all broad wildlife groups (with the exception of birds), data were sparse for many organism types. For example, zooplankton, crustaceans, insects and insect larvae, amphibians, and mammals, for which there were CRwo-water values for less than eight elements. Coverage was most comprehensive for fish, vascular plants, and molluscs. To our knowledge, the freshwater database that has now been established represents the most comprehensive set of CRwo-water values for freshwater species currently available for use in radiological environmental assessments.

Transfer of 90Sr into fish in Finnish lakes

The long-term behavior of (90)Sr was investigated from 1987 to 1997 in fifteen lakes in southern and central Finland following the 1986 Chernobyl accident. Both water and fish samples (perch, pike, vendace) were analyzed. (90)Sr stays long in the freshwater ecosystem; the observed half-lives during the study period were generally around 10 years in water and even longer in fish. One lake exhibited very different behavior for (90)Sr, with elevated levels in fish and water and very short observed half-life in fish, less than 2 years. Concentration factors of (90)Sr in fish (Bqkg(-1) f.w. in fish/Bqkg(-1) in water) among the studied lakes significantly correlated with both the Ca concentration and electrical conductivity of the water. More (90)Sr was transferred into fish in lakes with a low electrical conductivity and a low Ca concentration. Among other water parameters evaluated were pH, color, total nitrogen, and phosphorus.

Overview of strontium-89,90 deposition measurements in Finland 1963―2005

In Finland the deposition of strontium-89 (90Sr) and strontium-90 (90Sr) has been monitored since the early 1960s. The measured cumulative 90Sr deposition in 1963-2005 is on average 1200 Bq m(-2), of which 150 Bq m(-2) originates from the Chernobyl accident. Adding to this the deposition in 1945-1962 produces a value of 2040 Bq m(-2) for the cumulative deposition in Finland. The nuclear explosion-derived deposition up to 1985 obtained in this study, 1850 Bq m(-2), is in good agreement with the zonal 90Sr deposition of 1740 Bq m(-2) in the 60 degrees N-70 degrees N latitude band estimated by UNSCEAR. The regional deposition patterns of 89Sr and 90Sr following the Chernobyl accident resemble those of the refractory nuclides such as 239,240Pu and 95Zr. The total deposition of Chernobyl-derived 90Sr in Finland was about 5.3 x 10(13) Bq. This activity corresponds to 0.027% of the reactor core inventory and 0.66% of the atmospheric emissions from the accident. The corresponding figures for 89Sr are 4.5 x 10(14) Bq, 0.023% and 0.56%, respectively.

Short- and long-term patterns of 137Cs in fish and other aquatic organisms of small forest lakes in southern Finland since the Chernobyl accident

We summarize the patterns of ¹³⁷Cs activity concentrations and transfer into fish and other biota in four small forest lakes in southern Finland during a twenty-year period following the Chernobyl accident in April 1986. The results from summer 1986 showed fastest accumulation of ¹³⁷Cs into planktivorous fishes, i.e. along the shortest food chains. Since 1987, the highest annual mean values of ¹³⁷Cs have been recorded in fish occupying the highest trophic levels, for perch (Perca fluviatilis) 13,600 Bq/kg (ww) and for pike (Esox lucius) 20,700 Bq/kg (ww). At the same time, activity concentrations of ¹³⁷Cs in crustacean zooplankton and Asellus aquaticus have ranged between 1000 and 19,500 Bq/kg (dw). In 2006, 5-28% of the 1987 ¹³⁷Cs activity concentration levels were still present in perch and pike. Since 1989 their ¹³⁷Cs activity concentrations in oligohumic seepage lakes have remained significantly higher than in polyhumic drainage lakes due to the increased transfer of ¹³⁷Cs into fish in the seepage lakes with lower electrolyte concentrations, longer water retention times and lower sedimentation rate.

A comparison of marine radionuclide dispersion models for the Baltic Sea in the frame of IAEA MODARIA program.

Four radionuclide dispersion models have been applied to simulate the transport and distribution of (137)Cs fallout from Chernobyl accident in the Baltic Sea. Models correspond to two categories: box models and hydrodynamic models which solve water circulation and then an advection/diffusion equation. In all cases, interactions of dissolved radionuclides with suspended matter and bed sediments are included. Model results have been compared with extensive field data obtained from HELCOM database. Inventories in the water column and seabed, as well as (137)Cs concentrations along 5 years in water and sediments of several sub-basins of the Baltic, have been used for model comparisons. Values predicted by the models for the target magnitudes are very similar and close to experimental values. Results suggest that some processes are not very relevant for radionuclide transport within the Baltic Sea, for instance the roles of the ice cover and, surprisingly, water stratification. Also, results confirm previous findings concerning multi-model applications.

Modelling of marine radionuclide dispersion in IAEA MODARIA program: Lessons learnt from the Baltic Sea and Fukushima scenarios☆

State-of-the art dispersion models were applied to simulate (137)Cs dispersion from Chernobyl nuclear power plant disaster fallout in the Baltic Sea and from Fukushima Daiichi nuclear plant releases in the Pacific Ocean after the 2011 tsunami. Models were of different nature, from box to full three-dimensional models, and included water/sediment interactions. Agreement between models was very good in the Baltic. In the case of Fukushima, results from models could be considered to be in acceptable agreement only after a model harmonization process consisting of using exactly the same forcing (water circulation and parameters) in all models. It was found that the dynamics of the considered system (magnitude and variability of currents) was essential in obtaining a good agreement between models. The difficulties in developing operative models for decision-making support in these dynamic environments were highlighted. Three stages which should be considered after an emergency, each of them requiring specific modelling approaches, have been defined. They are the emergency, the post-emergency and the long-term phases.

Effect of liming on the behaviour of 90Sr and 137Cs in a lake ecosystem

Liming of lakes is considered one possible remedial action to reduce the accumulation of radionuclides into fish in the case of a radiological accident. These responses were tested in field conditions in a small acidified lake that was divided into two parts: one limed with CaCO(3) and the other half left as an unlimed control. The transfer of (90)Sr from water into fish decreased on average by 50% during the first year after liming. However, at the same time the (90)Sr concentration in water increased, reaching a maximum within 6 months after liming. Approximately 50% more (90)Sr was detected in water in the limed part of the lake than on control side during the first year. (90)Sr was most probably released from the sediment as the Ca concentration and pH of the water increased. As a result of these two processes, which counterbalanced each other (increased release of (90)Sr into water from sediment and decreased transfer of (90)Sr from water into fish), the (90)Sr concentration in fish did not notably differ between the limed and control sides of the lake. Liming may only be suitable as a remedial action if carried out immediately after a radiological accident, before significant amounts of radionuclides have been deposited in lake sediments. In the case of (137)Cs, the effect of liming was less pronounced. (137)Cs activity concentration in water increased in the first year by 20% and uptake by fish decreased by 20%.