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 CaCO₃ and the other half left as an unlimed control. The transfer of ⁹⁰Sr from water into fish decreased on average by 50% during the first year after liming. However, at the same time the ⁹⁰Sr concentration in water increased, reaching a maximum within 6 months after liming. Approximately 50% more ⁹⁰Sr was detected in water in the limed part of the lake than on control side during the first year. ⁹⁰Sr 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 ⁹⁰Sr into water from sediment and decreased transfer of ⁹⁰Sr from water into fish), the ⁹⁰Sr 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 ¹³⁷Cs, the effect of liming was less pronounced. ¹³⁷Cs activity concentration in water increased in the first year by 20% and uptake by fish decreased by 20%.     

Cesium accumulation by fish following acute input to lakes: a comparison of experimental and Chernobyl-impacted systems

An uptake parameter u (L kg⁻¹ d⁻¹) and a loss rate parameter k (d⁻¹) were estimated for the patterns of accumulation and loss of ¹³³Cs by three fish species following an experimental ¹³³Cs addition into a pond in South Carolina, USA. These u and k parameters were compared to similar estimates for fish from other experimental ponds and from lakes that received ¹³⁷Cs deposition from Chernobyl. Estimates of u from ponds and lakes declined with increasing potassium concentrations in the water column. Although loss rates were greater in the experimental ponds, the times required to reach maximum Cs concentrations in fish were similar between ponds and lakes, because ponds and lakes had similar retentions of Cs in the water column. The maximum Cs concentrations in fish were largely determined by initial Cs concentrations in the water column. These maximum concentrations in fish and the times required to reach these maxima are potentially useful indicators for assessments of risks to humans from fish consumption.     

Will the application of Ammonium-Ferric-Hexacyano-Ferrate enhance the vertical migration of radiocaesium?

The consideration of a possible enhanced vertical migration of radiocaesium with the application of ammonium-ferric-hexacyano-ferrate (AFCF) as countermeasure, due to the colloidal nature of AFCF, made us set up a series of migration experiments. For the study two soil types were considered, which were either left unplanted or cultivated with ryegrass. Two AFCF concentrations, 1 and 10 g m^-2, and an untreated control were applied. A simple diffusion–convection model was fitted to the data.The application of AFCF did not enhance the downward migration of radiocaesium in the profile. Moreover, for an unplanted sandy soil the application of AFCF significantly retarded the migration: 10 g AFCF m^-2 decreased the convection term, V, from 0·78 to 0·42 cm a^-1 and the diffusion component, D, from 0·21 to 0·09 cm² a^-1. For all other experimental conditions (unplanted loamy soil, ryegrass cultivated sandy and loamy soil), the application of AFCF did not have any effect on radiocaesium migration. Since AFCF does not promote the vertical migration of radiocaesium, enhanced groundwater contamination is improbable.     

Does soil adhesion matter when predicting radiocaesium transfer to animals?

A sward will often have significant amounts of soil adhered to the vegetation surfaces which will be ingested by grazing animals. If the soil is contaminated by radioactive fallout then it can serve as a dietary source of radionuclides, in addition to any root uptake by the plants. This study is an attempt to quantitatively assess the importance of soil adhesion as a source of radiocaesium to sheep using the RUINS model which simulates radiocaesium transfer in grazing systems.The method of simulating the contamination of vegetation surfaces used by the RUINS model is described, and the importance of the availability of radiocaesium associated with adhered soil relative to plant incorporated radiocaesium discussed. Two sets of simulations are presented: one in which the soil is treated as a medium providing a uniform availability of radiocaesium, and the second in which account is taken of the partitioning of radiocaesium in the soil between ‘fixed’ and ‘labile’ phases.The results demonstrate that, because of the reduced absorption in the gut of radiocaesium associated with soil, animals grazing pastures with significant amounts of radiocaesium associated with adhered soil will not be as contaminated as radiocaesium activity concentrations measured in bulk vegetation samples would suggest. Therefore, the extent of soil adhesion needs to be considered if predictions of radiocaesium contamination of animal products are to be made on the basis of measured activities of sampled vegetation. However, soil adhesion is unlikely to be a significant dietary source of available radiocaesium, unless the soil concerned exhibits an unusually high bioavailability of radiocaesium. Moreover the simulation results indicate that differences in availability between soil types observed experimentally are consistent with the partitioning between fixed and labile soil compartments made by the RUINS model.     

A review and test of predictive models for the bioaccumulation of radiostrontium in fish

Empirical relations between the ⁹⁰Sr concentration factor (CF) and the calcium concentration in freshwater aquatic systems have previously been determined in studies based on data obtained prior to the Chernobyl accident. The purpose of the present research is to review and compare these models, and to test them against a database of post-Chernobyl measurements from rivers and lakes in Ukraine, Russia, Belarus and Finland. It was found that two independently developed models, based on pre-Chernobyl empirical data, are in close agreement with each other, and with empirical data. Testing of both models against new data obtained after the Chernobyl accident confirms the models’ predictive ability. An investigation of the influence of fish size on ⁹⁰Sr accumulation showed no significant relationship, though the data set was somewhat limited.     

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.     

Predicting the transfer of radiocaesium from organic soils to plants using soil characteristics

A model predicting plant uptake of radiocaesium based on soil characteristics is described. Three soil parameters required to determine radiocaesium bioavailability in soils are estimated in the model: the labile caesium distribution coefficient (kd1), K+ concentration in the soil solution [mK] and the soil solution-->plant radiocaesium concentration factor (CF, Bq kg-1 plant/Bq dm-3). These were determined as functions of soil clay content, exchangeable K+ status, pH, NH4+ concentration and organic matter content. The effect of time on radiocaesium fixation was described using a previously published double exponential equation, modified for the effect of soil organic matter as a non-fixing adsorbent. The model was parameterised using radiocaesium uptake data from two pot trials conducted separately using ryegrass (Lolium perenne) on mineral soils and bent grass (Agrostis capillaris) on organic soils. This resulted in a significant fit to the observed transfer factor (TF, Bq kg-1 plant/Bq kg-1 whole soil) (P < 0.001, n = 58) and soil solution K+ concentration (mK, mol dm-3) (P < 0.001, n = 58). Without further parameterisation the model was tested against independent radiocaesium uptake data for barley (n = 71) using a database of published and unpublished information covering contamination time periods of 1.2-10 years (transfer factors ranged from 0.001 to 0.1). The model accounted for 52% (n = 71, P < 0.001) of the observed variation in log transfer factor.     

Transfer of Sr into fish in Finnish lakes

The long-term behavior of ⁹⁰Sr 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. ⁹⁰Sr 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 ⁹⁰Sr, with elevated levels in fish and water and very short observed half-life in fish, less than 2 years. Concentration factors of ⁹⁰Sr 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 ⁹⁰Sr 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.     

Assessment of state-of-the-art models for predicting the remobilisation of radionuclides following the flooding of heavily contaminated areas: the case of Pripyat River floodplain

The performances of models are assessed to predict the wash-off of radionuclides from contaminated flooded areas. This process should be accounted for in the proper management of the aftermath of a nuclear accident. The contamination of the Pripyat River water following the inundation of a floodplain heavily contaminated by (90)Sr and (137)Cs of Chernobyl origin is used as the basis for modelling. The available experimental evidence demonstrated that remobilisation of radiostrontium is an important process implying a significant secondary radioactive load of water flowing over the contaminated floodplain. On the contrary, there is no empirical evidence of a similar behaviour for radiocaesium. In general, state-of-the-art models properly predicted the remobilisation of strontium, whereas they significantly overestimated radiocaesium concentrations in water. The necessary model improvements for a more accurate prediction of radiocaesium contamination levels include a reassessment of the values of the model parameters controlling the remobilisation process.     

Depth profile of ²³⁶U/²³⁸U in soil samples in La Palma, Canary Islands

The vertical distribution of the ²³⁶U/²³⁸U isotopic ratio was investigated in soil samples from three different locations on La Palma (one of the seven Canary Islands, Spain). Additionally the ²⁴⁰Pu/²³⁹Pu atomic ratio, as it is a well establish tool for the source identification, was determined. The radiochemical procedure consisted of a U separation step by extraction chromatography using UTEVA^® Resin (Eichrom Technologies, Inc.). Afterwards Pu was separated from Th and Np by anion exchange using Dowex 1x2 (Dow Chemical Co.). Furthermore a new chemical procedure with tandem columns to separate Pu and U from the matrix was tested. For the determination of the uranium and plutonium isotopes by alpha spectrometry thin sources were prepared by microprecipitation techniques. Additionally these fractions separated from the soil samples were measured by Accelerator Mass Spectrometry (AMS) to get information on the isotopic ratios ²³⁶U/²³⁸U, ²⁴⁰Pu/²³⁹Pu and ²³⁶U/²³⁹Pu, respectively. The ²³⁶U concentrations [atoms/g] in each surface layer (∼2 cm) were surprisingly high compared to deeper layers where values around two orders of magnitude smaller were found. Since the isotopic ratio ²⁴⁰Pu/²³⁹Pu indicated a global fallout signature we assume the same origin as the probable source for ²³⁶U. Our measured ²³⁶U/²³⁹Pu value of around 0.2 is within the expected range for this contamination source.     

137Cs in marine samples from the Brazilian southeastern coastal region

The present work presents the results of ¹³⁷Cs concentration in seawater, fish and sediments samples collect in 11 sampling points, crossing the Brazilian Southeastern coastal region, from Vitória (ES) to Santos (SP), on a routine basis from 1997 to 2002. This monitoring program was carried out by the Instituto de Radioproteção e Dosimetria (IRD/CNEN/MCT), in cooperation with the Instituto de Estudos do Mar Almirante Paulo Moreira (IEAPM/Brazilian Navy), aiming at determining artificial radionuclides in marine samples. Additionally to the ¹³⁷Cs results, ⁹⁰Sr concentrations in fish samples from 1998 to 2002 are also reported.The ¹³⁷Cs in seawater follows a lognormal distribution, with a geometric mean of 1.8 Bq m^-3 and a geometric standard deviation of 1.4 (n=54), decay corrected to June/2002. For ¹³⁷Cs levels in fish samples a geometric mean of 0.19 Bq kg⁻¹ and a geometric standard deviation of 2.9 (n=39), decay corrected to June/2002, with a range of 0.03 to 1.7 Bq kg⁻¹, were obtained. Based on the ¹³⁷Cs mean concentration in fish as well as in seawater, a transfer factor of 1 × 10² was calculated, which is quite in agreement with the recommended value found in the Safety Report Serie 19.     

Uptake and elimination of radiocaesium in fish and the "size effect."

A number of hypotheses have previously been developed concerning the rates of uptake and elimination of radiocaesium (137Cs) in fish. These include the influence of potassium and other water chemical parameters on both uptake and elimination, and the effect of fish size on accumulation. In order to test these hypotheses, we have assembled a data set comprising more than 1,000 measurements of radiocaesium (137Cs) in predatory fish (perch, pike and brown trout) in nine European lakes during the years after Chernobyl. These data have been analysed using simple models for uptake and excretion of 137Cs in fish, showing that: 1. Fish-water concentration factors (CF) were inversely proportional to potassium [K+] concentration of the different lakes, in agreement with previous studies. 2. The uptake rate of 137Cs in fish was negatively correlated with lake [K+], but excretion rate was independent of [K+]. 3. Lower than expected CF values were found in one lake, Iso Valkj?rvi, Finland. This is attributed to inhibition of the K+ (and therefore 137Cs) high affinity transport system in aquatic plants and fish by low pH and/or low Ca2+. 4. The inclusion of fish weight as a parameter in our dynamic model significantly improves the ability of the model to fit the observed measurements of 137Cs. 5. The model developed from the above hypotheses was able to fit the data from nine different lakes to within approximately a factor of 3 of the observed values.     

Development of a dynamic transfer model of (14)C from the atmosphere to rice plants

A dynamic compartment model was investigated to describe ¹⁴C accumulation in rice plants exposed to atmospheric ¹⁴C with temporally changing concentrations. In the model, rice plants were regarded to consist of three compartments: the ear and the mobile and immobile carbon pools of the shoot. Photosynthetically fixed carbon moves into the ear and the mobile carbon pool, and these two compartments release a part of this carbon into the atmosphere by respiration. Carbon accumulated in the mobile carbon pool is redistributed to the ear, while carbon transferred into the immobile carbon pool from the mobile one is accumulated there until harvest. The model was examined by cultivation experiments using the stable isotope, ¹³C, in which the ratios of carbon photosynthetically fixed at nine times during plant growth to the total carbon at the time of harvest were determined. The model estimates of the ratios were in relatively good agreement with the experimental observations, which implies that the newly developed compartment model is applicable to estimate properly the radiation dose to the neighboring population due to an accidental release of ¹⁴C from nuclear facilities.     

A critical review of measures to reduce radioactive doses from drinking water and consumption of freshwater foodstuffs.

Following a radioactive fallout event, there are a number of possible intervention measures to reduce radioactive doses to the public via the surface water pathway. We have critically reviewed the options available to decision-makers in the event of radioactive contamination of surface waters. We believe that the most effective and viable measures to reduce radioactivity in drinking water are those which operate at the water treatment and distribution stage. Intervention measures to reduce concentrations of radioactivity in rivers and reservoirs are expected to be much less viable and efficient at reducing doses via the drinking water pathway. Bans on consumption of freshwater fish can be effective, but there are few viable measures to reduce radioactivity in fish prior to the preparation stage. Lake liming and biomanipulation have been found to be ineffective for radiocaesium, although the addition of potassium to lakewaters appears promising in some situations. Lake liming may be effective in reducing radiostrontium in fish, though this has not, to our knowledge, been tested. De-boning fish contaminated by strontium is probably the most effective food preparation measure, but salting and freezing can also reduce radiocaesium concentrations in fish. The provision of accurate information to the public is highlighted as a key element of countermeasure implementation.     

Vertical distributions of (99)Tc and the (99)Tc/(137)Cs activity ratio in the coastal water off Aomori, Japan

Using a sector-field ICP-MS the vertical distributions of the ⁹⁹Tc concentration and ⁹⁹Tc/¹³⁷Cs activity ratio were measured in the coastal waters off Aomori Prefecture, Japan, where a spent-nuclear-fuel reprocessing plant has begun test operation. The ⁹⁹Tc concentrations in surface water ranged from 1.8 to 2.4 mBq/m³, no greater than the estimated background level. Relatively high ⁹⁹Tc/¹³⁷Cs activity ratios (10-12 × 10⁻⁴) would be caused by the inflow of the high-⁹⁹Tc/¹³⁷Cs water mass from the Japan Sea. There is no observable contamination from the reprocessing plant in the investigated area. The ⁹⁹Tc concentration and the ⁹⁹Tc/¹³⁷Cs activity ratio in water column showed gradual decreases with depth. Our results implied that ⁹⁹Tc behaves in a more conservative manner than ¹³⁷Cs in marine environments.     

Distribution of radionuclides in the guano sediments of Xisha Islands, South China Sea and its implication

Several natural and anthropogenic radionuclides (²¹⁰Pb, ²²⁶Ra and ¹³⁷Cs) in guano-phosphatic coral sediments and pure guano particles collected from Ganquan, Guangjin, Jinqing and Jinyin Islands of the Xisha archipelago, South China Sea, were analyzed. The Constant Initial Concentration (CIC) model and the Constant Rate of Supply (CRS) model were applied for age calculation. The average supply rate of ²¹⁰Pb was 126 Bq m⁻² a⁻¹, very close to the flux of northern hemisphere average (125 Bq m⁻² a⁻¹). The activities of anthropogenic radionuclides in the sediments were very low, indicating that human nuclear tests did not notably impact this region. The main source of radionuclides in the sediments was from atmospheric precipitation, and the organic matter derived from plant and produced by nutrient-rich guano could further enhance them.     

Cs and K in fruiting bodies of different fungal species collected in a single forest in southern Poland

Fruiting bodies of fungi belonging to more than 70 species were collected within a few thousand square meter area of one forest during 2006 and 2007. The soil profile was collected to check the cumulative deposition of ¹³⁷Cs, which was relatively high, equal to 64 ± 2 kBq/m² (calculated for October 2006). The majority of this activity was in the first 6 cm. Fruitbodies were analyzed for radiocesium and ⁴⁰K by means of gamma-spectrometry. The highest ¹³⁷Cs activity was 54.1 ± 0.7 kBq/kg (dry weight) for a sample of Lactarius helvus collected in 2006. The results for 2006 were higher than those for 2007. In a few cases the traces of short-lived (T_1/2 = 2.06 a) ¹³⁴Cs were still found in samples. The importance of mycorrhizal fungi for radiocesium accumulation is confirmed. The differences in activity among the species are discussed in relation to observations and predictions from previous studies, where the change in relative accumulation between fruiting bodies of different species was at least partially explained by the differences in the depth of the mycelium localization in a litter/soil system. It is concluded that in some cases, such as Boletus edulis and Xerocomus badius, this prediction is fulfilled and therefore this explanation confirmed.     

Long-term dynamics of radioactive <Superscript>90</Superscript>Sr and <Superscript>137</Superscript>Cs contamination of small mammals in the Chernobyl zone

Between 1986 and 1994, a decrease in nonalimentary ⁹⁰Sr and ¹³⁷Cs intake and changes in the accessibility of radionuclides in the soil-plant link of their cycle resulted in a 10-to 100-fold decrease in their specific activity (SA) in the bodies of small mammals inhabiting the Chernobyl zone, and a similar decrease was observed in the radionuclide transition factor (TF) in the soil-animal chain. Between 1995 and 2005, no consistent increase or decrease in SA or TF could be revealed against the background of a combined effect of different physicochemical and ecological factors. It is suggested that subsequent changes in the level of radioactive contamination of small mammals will generally reflect only the dynamics of physical ⁹⁰Sr and ¹³⁷Cs decay, but, nevertheless, seasonal and local variations in this level will be significant.     

Radium and radium-daughter nuclides in carbonates: a brief overview of strategies for determining chronologies

Radium isotopes have been used extensively to trace the movement of groundwater as well as oceanic water masses, but these radionuclides (and their daughters) are also useful chronometers for the determination of the time scales of other Earth and environmental processes. The purpose of this overview is to present the application of Ra and Ra daughters in the dating of carbonates. We show that the choice of dating method (decay of excess radionuclide or ingrowth of daughter) depends strongly on the parent/daughter activity ratios in the water in which the carbonate was precipitated. Thus freshly precipitated carbonates uniformly show excesses of ²²⁶Ra relative to its parent ²³⁰Th, and ²²⁶Ra decay can provide ages of carbonates over Holocene time scales. In contrast, carbonates are precipitated in waters of greatly varying ²¹⁰Pb/²²⁶Ra. Corals, deep-sea hydrothermal vent clams and the shelled cephalopod Nautilus live in waters with significant dissolved ²¹⁰Pb and all show excesses of ²¹⁰Pb in their carbonate. Bivalve molluscs from nearshore and coastal waters, and carbonates deposited from groundwater environments (e.g. travertines) in which ²¹⁰Pb is efficiently scavenged from solution, show deficiencies of ²¹⁰Pb relative to ²²⁶Ra. In contrast, fish otoliths strongly discriminate against ²¹⁰Pb regardless of the environment in which the fish lives. Deficiencies of ²²⁸Th relative to ²²⁸Ra are common in all carbonates. Useful time ranges for the ²¹⁰Pb/²²⁶Ra and ²²⁸Th/²²⁸Ra chronometers are ∼100 y and ∼10 y, respectively.