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.

Effect of liming on the behaviour of (90)Sr and (137)Cs 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%.

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.

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.

A liquid-scintillation-based primary standardization of 210Pb.

A new radioactivity solution standard of 210Pb has been developed and will be disseminated by the National Institute of Standards and Technology (NIST) as standard reference material (SRM) 4337. This new 210Pb solution standard is contained in a 5 mL flame-sealed borosilicate glass ampoule, consists of (5.133+/-0.002)g of a nominal 1mol L(-1) nitric acid solution, has a density of (1.028+/-0.002)g mL(-1) at 20 degrees C, has carrier ion concentrations of about 11 microg Pb2+ and 21 microg Bi3+ per gram of solution, and is certified to contain a massic activity (9.037+/-0.22)kBq g(-1) as of the reference time 1200 EST, 15 June 2006. All of the uncertainties cited above correspond to standard uncertainties multiplied by a coverage factor k=2. The standardization for the (210)Pb content of the solution was based on 4pialphabeta liquid scintillation (LS) measurements using CIEMAT/NIST (3)H-standard efficiency tracing (CNET). Confirmatory determinations were also performed by high-resolution HPGe gamma-ray spectrometry, by 2pialpha spectrometry with a Si surface barrier detector of separated 210Po, and by 4pibeta(LS)-gamma(NaI) anticoincidence counting.

A note on the half-life of 209Po.

The widely adopted value of (102+/-5)a for the (209)Po half-life, which is based on a single determination reported in 1956, appears to be in error by a large factor. Decay data from two separate primary standardizations of a (209)Po solution standard, conducted approximately 12 years apart, are inconsistent with the adopted value and its assigned uncertainty. An estimated half-life, larger than the adopted value by about 25%, is more consistent with the standardization data. A longer half-life is also supported by measurements on a recently standardized (210)Pb solution standard.

Effect of industrial pollution on the distribution of 137Cs in soil and the soil-to-plant transfer in a pine forest in SW Finland.

The objective of this study was to evaluate the effect of industrial pollution on the distribution of radiocaesium in soil and on its transfer from soil to plants. The study was started in September 2000 in four Scots pine stands located at distances of 0.5, 2, 4 and 8 km along a transect running SE from the Cu-Ni smelter at Harjavalta in SW Finland. Annual emissions from the smelter in 1990 were 80 t of Cu, 31 t of Ni and 9000 t of SO(2), and in 1999 these were 5.9, 0.8 and 3400 t, respectively. At each site, soil profiles were sampled with a corer, and samples were separated into litter (L), organic soil layer (O) and mineral soil layers (B, E). Mushrooms, lichens (Cladina spp. and Cetraria islandica), lingonberry (Vaccinium vitis-idaea) and crowberry (Empetrum nigrum) plants were collected at each site, except at a distance of 0.5 km, where only mushrooms were available. In the organic soil layer, 137Cs activity decreased from 8000 Bq/m(2) at a distance of 8 km from the smelter to 1500 Bq/m(2) at a distance of 0.5 km; in litter, 137Cs concentration increased from 6300 Bq/m(2) at 8 km to 14000 Bq/m(2) at 0.5 km. 137Cs activity concentration decreased significantly in plants, mushrooms and lichens as the pollution load increased. In lichens, 137Cs activity decreased from 910 Bq/kg at 8 km to 170 Bq/kg at 2 km, while in lingonberry it decreased from 1470 to 20 Bq/kg and in crowberry from 310 to 20 Bq/kg. Aggregated transfer factors for 137Cs decreased in a similar way in lingonberry from 7.6x10(-2) m(2)/kg at 8 km to 7.7x10(-4) m(2)/kg at 2 km and in crowberry from 1.6x10(-2) to 7.9x10(-4) m(2)/kg.