Effective Ecological Half-life Research Articles

Comparison of 137Cs uptake, depuration and continuous uptake, originating from feed, in five salmonid fish species

The origin of 137Cs contamination, contamination levels, and its effective ecological half-life can differ among fish species, but until recently it was unknown whether interspecific differences in radiocaesium metabolism existed. We compare the trophic transfer of 137Cs in five salmonid species under controlled conditions. Fish were fed a diet containing 200 Bq kg−1 wet wt concentration of 137Cs for a fixed period of time. While there were almost no differences in contamination levels among individual fish within a species during fixed periods, the rate of 137Cs uptake was statistically different among the species examined, for which the effective half-life of 137Cs ranged 49–84 days. No significant difference in rate of decline in the quantity of 137Cs was apparent among species in two experiments after correcting for differences in growth. The effective half-life was shortest in Oncorhynchus mykiss and O. masou—two species exhibiting the best growth rate in both experiments—indicating an influence of the dilution effect associated with growth. When continuously fed 200 Bq kg−1 wet wt concentration of 137Cs for a period of 209–294 days, contamination levels in the five species ranged 216–240 Bq kg−1 wet wt, and reached a steady state value. Our results indicate metabolic rate, although accompanying a change of body size, did not affect 137Cs contamination levels in fish, which implies that contamination levels in the diet strongly determined contamination levels in fish.

Reservoir sediments as a long-term source of dissolved radiocaesium in water system; a mass balance case study of an artificial reservoir in Fukushima, Japan

Because of their large mobility and high bioavailability, it is necessary to elucidate the origins and dynamics of dissolved radionuclides in river and reservoir systems to assess the transfer of those radionuclides from water to crops and aquatic organisms. Elution from contaminated reservoir sediments, a potential source of dissolved radionuclides, presents a long-term concern, particularly for long-lived radionuclides. In this study, we systematically investigated caesium-137 (137Cs) concentrations using a time-series suite of input and output water samples collected from 2014 to 2019 from the Ogaki Dam Reservoir, which has a catchment with a high 137Cs inventory due to the Fukushima Dai-ichi Nuclear Power Plant accident. The results of our study showed that dissolved 137Cs concentration was significantly higher in the output water than that in the main input water, and that the effective ecological half-life of dissolved 137Cs in the output water was longer than in the main input water. We quantitatively evaluated the mass balance of dissolved 137Cs in the reservoir to elucidate how much dissolved 137Cs from the rivers and production from reservoir sediments contribute to 137Cs in the reservoir output. The annual output of dissolved 137Cs was significantly higher than the total input of dissolved 137Cs, with approximately 32%–40% of the dissolved 137Cs in the output water presumably being produced from reservoir sediments. Consequently, the estimated dissolved 137Cs fluxes from reservoir sediments to overlying water were 0.57–1.3 × 104 Bq m−2 y−1. This implies that approximately 0.04%–0.09% of 137Cs accumulated in the sediments was released through elution to the overlying water each year. Reservoir sediments containing high 137Cs levels may thus become even more important as sources of bioavailable dissolved 137Cs in the future.

Radiocesium dynamics in the aquatic ecosystem of Lake Onuma on Mt. Akagi following the Fukushima Dai-ichi Nuclear Power Plant accident

Understanding ecosystem dynamics of radionuclides is necessary to ensure effective management for food safety. The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011 released large amounts of radiocesium (134Cs and 137Cs) and contaminated the environment across eastern Japan. In this study, we aimed to elucidate the temporal dynamics of 137Cs in the aquatic ecosystem of Lake Onuma on Mt. Akagi. The effective ecological half-life (Teff) of 137Cs in fishes, western waterweed (Elodea nuttallii), seston (phytoplankton and zooplankton), and lake water was estimated using survey data of 137Cs concentration collected from 2011 to 2016, and single- and two-component decay function models (SDM and TDM, respectively). The decay processes of 137Cs concentrations in wakasagi (Hypomesus nipponensis), pale chub (Zacco platypus), phytoplankton, and total 137Cs concentrations of the water column (WC) in the lake were well suited by the TDMs. The Teff in the fast component of the TDMs in these samples ranged from 0.49 to 0.74years. The Teff in the slow component of the TDMs could converge towards the physical half-life of 137Cs. Nearly five and a half years after the FDNPP accident, we concluded that 137Cs concentrations approached a state of dynamic equilibrium between some aquatic organisms (wakasagi, pale chub, and phytoplankton) and the environment (lake water). However, the decay processes of 137Cs concentrations in Japanese dace (Tribolodon hakonensis), western waterweed, zooplankton, and particulate- and dissolved-forms in the WC were better predicted for the SDM. The total 137Cs concentrations in inflowing river and spring waters were one to two orders of magnitude lower than lake water under normal flow conditions. However, particulate 137Cs contamination level in the river water was high after heavy rains. Overall, 137Cs contamination levels have significantly decreased in Lake Onuma, but monitoring surveys should be continued for further understanding of the reduction processes.

Temporal changes in the radiocesium distribution in forests over the five years after the Fukushima Daiichi Nuclear Power Plant accident

To elucidate the temporal changes in the radiocesium distribution in forests contaminated by the Fukushima Daiichi Nuclear Power Plant accident, we monitored the 137Cs concentration and inventory within forests from 2011 to 2015 across nine plots containing variable tree species and different contamination levels. The 137Cs concentrations in needles and branches decreased exponentially at all coniferous plots, with effective ecological half-lives of 0.45–1.55 yr for needles and 0.83–1.69 yr for branches. By contrast, the 137Cs concentration in deciduous konara oak leaves did not change over the five years. The concentration of 137Cs in oak wood increased by 37–75%, whereas that in Japanese red pine decreased by 63% over the five years. In Japanese cedar and hinoki cypress, the 137Cs concentration in wood showed an increasing trend in half of the plots. The changes in 137Cs in the organic and mineral soil layers were not strongly related to the tree species or contamination level. Our multi-site, multi-species monitoring results revealed that the pattern of temporal changes in radiocesium in the 9 forest plots was similar overall; however, changes in 137Cs in needles/leaves and wood differed among tree species.

ワカサギ中の放射性セシウム濃度の体重依存性と経年変化

The accident of the Fukushima Dai-ichi Nuclear Power Plant in March, 2011, released large amounts of radiocesium into the atmosphere, and contaminated the environment of Gunma Prefecture in eastern Japan. In particular, 640 Bq kg–1-wet of radiocesium concentration was found in wakasagi (Hypomesus nipponensis) in Lake Onuma on Mt. Akagi in August, 2011. Thus, to elucidate the body-size effect in weight-dependent and dynamics of radiocesium in the ecosystem of Lake Onuma, we determined the age of wakasagi, the body-weight class of the radiocesium concentration in wakasagi and the effective ecological half-life (Teff) of radiocesium in wakasagi 0+ from 2012 to 2016. The body-size effect was found for the 137Cs concentration of wakasagi fished from 2012 to 2015, i.e., the concentration of 137Cs increased with the increase in its body weight. On the other hand, no body size effect was found in 2016. This result may be caused by the following two factors: the 137Cs concentration of the lake water reached a steady state after May 2014; wakasagi is a small plankton-feeding fish, while it is known that larger piscivorous fishes show the strong body size effect. Teff of the 137Cs concentration in wakasagi 0+ consists of two components, fast- and slow-term ones, and the decay rate of the 137Cs concentration in wakasagi 0+ was greatly reduced. As stated above, the radiocesium contamination in Lake Onuma has still been lasting; we are thus continuing our monitoring studies.

Temporal Development of Radiocaesium and Radiostrontium Concentrations in the Hydrosphere—Methods of Evaluation

A significant amount of artificial radionuclides has been introduced into the environment in the last century during atmospheric nuclear weapons tests and the Chernobyl accident. In this study, we investigated the temporal changes of concentrations and amounts of these radionuclides (90Sr and 137Cs) in surface water and river bed sediments. In order to evaluate the artificial radionuclide contamination diminution, we used and compared two different approaches: using a kinetic equation of the first order and, if needed, dividing the monitored period into two intervals, and in addition expressing the whole process in one equation with a series of exponential functions. Effective ecological half-lives were estimated as rates of decrease. In most cases, the ecological processes were proven to affect the radionuclide removal from the hydrosphere besides their radioactive decay. Furthermore, based on the assessment made, the 90Sr and 137Cs data were extrapolated and the radionuclide concentrations, which occurred in the hydrosphere after the fallout deposition in 1986, were estimated.

Temporal and spatial changes in the concentrations of radiocaesium and radiostrontium in the Vltava river basin affected by the operation of the nuclear power plant at Temelín

Concentrations of radiocaesium and radiostrontium were assessed in surface water, sediment, fish and aquatic flora affected and notaffected by waste water discharges from the Temelín Nuclear Power Plant. The assessment included residual contamination originatingfrom nuclear weapon tests in the atmosphere over the last century and the accident at Chernobyl in 1986. Results of long-term monitoring(since 1990) were used to derive effective ecological half-lives of these radionuclides in the hydrosphere. Possible effect of waste waterdischarged from Temelín power plant on the concentrations of radiocaesium and radiostrontium in the Vltava River was assessed by usingdata recorded in the period 2001–2010.

Artificial radioactivity in environmental media (air, rainwater, soil, vegetation) in Austria after the Fukushima nuclear accident

Several environmental media in Austria were monitored for artificial radionuclides released during the Fukushima nuclear accident. Air (up to 1.2 mBq/m(3) particulate (131)I) and rainwater (up to 5.2 Bq/L (131)I) proved to be the media best suited for the environmental monitoring, allowing also a temporal resolution of the activity levels. Significant regional differences in the wet deposition of (131)I with rain could be observed within the city of Vienna during the arrival of the contaminated air masses. Forward-trajectory analysis supported the hypothesis that the contaminated air masses coming from the northwest changed direction to northeast over Northern Austria, leading to a strong activity concentration gradient over Vienna. In the course of the environmental monitoring of the Fukushima releases, this phenomenon-significant differences of (131)I activity concentrations in rainwater on a narrow local scale (8.1 km)-appears to be unique. Vegetation (grass) was contaminated with (131)I and/or (137)Cs at a low level. Soil (up to 22 Bq/kg (137)Cs) was only affected by previous releases (nuclear weapon tests, Chernobyl). Here, also significant local differences can be observed due to different deposition rates during the Chernobyl accident. The effective ecological half-lives of (137)Cs in soil were calculated for four locations in Austria. They range from 7 to 30 years. No Austrian sample investigated herein exceeded the detection limit for (134)Cs; hence, the Fukushima nuclear accident did not contribute significantly to the total radiocesium inventory in Austrian environmental media. The levels of detected radioactivity were of no concern for public health.

Radiocaesium contamination of beef in Croatia after the Chernobyl accident

Long-term investigations of radiocaesium activity concentrations in beef in the Republic of Croatia are presented. The radiocaesium levels in beef decreased exponentially and the effective ecological half-life of 137Cs was estimated to be 0.57±0.06 years for 1986–1990 period and 5.21±0.27 years for the 1991–2005 period. For 1986–1990 period, the effective ecological half-life of 134Cs in was estimated to be about 0.69±0.05 while 134Cs activity concentrations after year 1990 were below the detection limit of the instruments.The 134Cs:137Cs activity ratio in beef has been found to be similar to the ratio that has been observed in other foodstuffs and environmental samples.Radioecological sensitivity for beef meat, i.e., the transfer coefficient from fallout to sample was estimated to be 1.15× 10−2Bqykg−1/(Bqm−2).For an adult member of Croatian population annual effective doses received by 134Cs and 137Cs intake due to consumption of beef are small, as per caput effective dose for the overall 1986–2005 period was estimated to be 24.6μSv. Consequently, after the Chernobyl accident beef consumption was not a critical pathway for the transfer of radiocaesium from fallout to humans in Croatia.

Modelling radiocaesium transfer and long-term changes in reindeer

A dynamic model on 137Cs in reindeer is presented, taking into consideration short- and long-term mechanisms, including the effect of ground deposition, transfer to vegetation, reindeer diet, feed intake, absorption and depletion of radiocaesium in the reindeer body. The model was optimised to fit measured activity concentrations in Swedish reindeer after the Chernobyl fallout. For comparison, regression analyses were made and aggregated transfer factors and effective ecological half-lives were estimated. The fit of the simulated model to observed activity concentrations was slightly better than the fit obtained by linear regressions. Improved knowledge about radiocaesium in vegetation would make the model more accurate for predictive purposes. Presently, the use of Tag and T eff is probably better for predictions, provided that their temporal and geographical limitations are taken into consideration. The dynamic model describes mechanisms better and may explain how changes in the system influence on activity concentrations of radiocaesium in the animal.

Long-term investigations of radiocaesium activity concentrations in carp in North Croatia after the Chernobyl accident

Long-term investigations of radiocaesium activity concentrations in carp in the Republic of Croatia are presented. The radiocaesium levels in carp decreased exponentially and the effective ecological half-life of 137Cs was estimated to be about 1 year during 1987–2002 and 5 years during 1993–2005. The observed 134Cs: 137Cs activity ratio in carp was found to be similar to the ratio observed in other environmental samples. The concentration factor for carp (wet weight) was estimated to be 128 ± 74 L kg −1, which is in reasonable agreement with model prediction based on K + concentrations in water. Estimated annual effective dose received by adult members of the Croatian population due to consumption of carp contaminated with 134Cs and 137Cs are small: per capita dose from this source during 1987–2005 was estimated to be 0.5 ± 0.2 μSv. Due to minor freshwater fish consumption in Croatia and low radiocaesium activity concentrations in carp, it can be concluded that carp consumption was not a critical pathway for the transfer of radiocaesium from fallout to humans after the Chernobyl accident.

Effective ecological half-lives of Cs-137 for fishes controlled by their surrounding sea-waters

National Research Institute of Fisheries Science (NRIFS) has carried out the long term monitoring program for radioactive pollution in marine organisms caught around Japan in order to confirm the safety of marine organisms as food source. Recently, the effective ecological half-lives have been introduced to estimate the recovery time from radioactive pollution. In this study, the effective ecological half-lives of Cs-137 for some fishes were calculated from our monitoring data. It is known that fish species have each effective ecological half-lives. However, it has been unclear what change these half-lives of Cs-137 for fishes. Fishes intake Cs-137 through food chain and directly from their surrounding seawater. Therefore, the effective ecological half-lives of Cs-137 for some fishes would be controlled by the effective environment half-lives of Cs-137 for their surrounding seawater. There is difference in the effective environment half-lives between the open ocean and the coastal seawaters. Some fishes move between the open ocean and the coastal areas, and therefore their effective ecological half-lives are influenced by the effective environment half-lives for seawaters of both areas. Consequently, the rate of the influence by seawater in each area would produce the differences in the effective ecological half-lives among fish species.

Modelling of long-term behaviour of caesium and strontium radionuclides in the Arctic environment and human exposure

In this paper a compartment model of the highly vulnerable Arctic terrestrial food chain “lichen–reindeer–man” is outlined. Based upon an analysis of measured 137Cs and 90Sr contents in lichen and reindeer meat from 1961 up to 2001, site specific model parameters for two regions in north-western Arctic Russia and for Kautokeino municipality in Arctic Norway have been determined. The dynamics of radionuclide activity concentrations in the “lichen–reindeer–man” food chain for all areas was satisfactorily described by a double exponential function with short-term and long-term effective ecological half-lives between 1–2 and 10–12 years, respectively, for both 137Cs and 90Sr. Using parameter values derived from the model, life-time internal effective doses due to consumption of reindeer meat by reindeer-breeders after an assumed single pulse deposit of 1 kBq m −2 of 137Cs were estimated to be 11.4 mSv (Kola Peninsula), 5 mSv (Nenets Autonomous Area), and 2 mSv (Kautokeino, Norway). Differences in vulnerability to radiocaesium deposition were due to differences in transfer between lichen and reindeer and in diet between the three regions.

Long-term studies on transfer of 137Cs from soil to vegetation and to grazing lambs in a mountain area in Northern Sweden

Studies were made during 1990–1997 on the transfer of 137Cs from soil to vegetation (herbage) and to grazing lambs on a mountain farm with an uncultivated grazing area of about 10 km 2 . The farm is situated in an area in Northern Sweden which was contaminated by the Chernobyl fallout in 1986. The mean concentration of 137Cs in the soil to a depth of 10 cm for eight sampling sites observed in the 8-year period was 14.51 kBq/m 2, while in the cut herbage the average concentration was 859 Bq/kg d.w. and in lamb meat 682 Bq/kg w.w. A slow vertical migration of 137Cs in the 0–10 cm soil layer was indicated. Although the 137Cs concentration in herbage gradually decreased, the concentration in lamb meat varied from year to year. Soil ingestion by the lambs as a pathway for activity transfer was shown to be negligible, while ingestion of fungi with high concentrations of 137Cs was demonstrated to occur, as large numbers of fungi spores were counted in samples of the lambs’ faeces. Fungi ingestion might therefore partly explain the varying mean yearly 137Cs concentrations in lamb muscle. The mean transfer parameters were as follows: for “soil to herbage” 61.3 Bq/kg d.w. herbage per kBq/m 2 soil, for “herbage to lamb meat” 0.81 Bq/kg w.w. meat per Bq/kg d.w. herbage, and for “soil to lamb meat” 47.1 Bq/kg w.w. meat per kBq/m 2 soil. A trend of decreasing values of the transfer parameter for “soil to herbage” indicated that 137Cs was becoming less available for root-uptake with time. The effective ecological half-life of 137Cs in soil, herbage and lamb meat was calculated to be 19, 7 and 16 years, respectively. It can be concluded that natural areas are vulnerable to 137Cs contamination, resulting in high concentrations in plants, fungi and lamb meat for a long time.

Time-dependent behaviour of radiocaesium: A new method to compare the mobility of weapons test and Chernobyl derived fallout

Environmental radiocaesium ( 137 Cs ) originates primarily from two sources, atmospheric nuclear weapons testing, and the Chernobyl accident. It has not, to our knowledge, previously been possible statistically to compare changes in the environmental mobility of 137 Cs from these two sources since the weapons test fallout varied in a complex manner over a number of years. A novel technique is presented for curve-fitting measurements with a time-dependent input function such as that for weapons test fallout. Different models were fitted to measurements of both pre- and post-Chernobyl 137 Cs activity concentrations in five major Finnish rivers. It was shown that there was no significant difference in the temporal changes in 137 Cs mobility from these two sources during the years after fallout. Transport parameters derived from weapons test measurements gave good predictions of the long-term contamination of these rivers by Chernobyl fallout. Changes in 137 Cs activity concentrations in rivers after Chernobyl have previously been shown to decline as a result of slow sorption to clay minerals in catchment soils. It is shown that weapons test fallout also exhibited this slow decline over time. Rates of decline in 137 Cs activity concentrations 10 years after fallout correspond to effective ecological half-lives ( T eff ) in the range 10–30 years. Removal of activity from the catchment was found to have no significant effect on the long-term decline in 137 Cs activity concentrations in these rivers.

Long-term variation of radiocaesium in the food chain of lamb in the Faroe Islands

Radiocaesium has been measured in the food chain of lamb for eight years in nine uncultivated pastures in the Faroe Islands. Soil, grass and lamb meat were studied in 1990–1997, and lamb faeces in 1995–1997. The deposition is attributable to weapon tests and the Chernobyl accident with the largest proportion deriving from the weapon tests. The deposition in the top 10 cm of the soil was 3–8 kBq/m 2, and a significant decrease with time was noted for only one pasture. Effective ecological half-lives of 137Cs concentration in grass and lamb meat could be estimated for some pastures (2.0–11.4 years for grass and 4.0–12.6 years for meat). The observed activities and transfer estimates of 137Cs vary significantly between as well as within the pastures. Chemical characteristics of the soil and precipitation rates in the pastures were considered in connection with the observations.

Fluxes of radiocaesium in selected rural study sites in Russia and Ukraine

Food production and food harvesting systems common in the areas contaminated by the Chernobyl accident in Russia and Ukraine can be grouped into three major categories: collective farm produce, private farming produce and foods collected from natural ecosystems. The contribution of each of these sources to radiocaesium intake by people living in rural settlements in the mid 1990s has been estimated at two major study sites, one in each country. The collective farm system provided the smallest contribution (7–14%) to the intake of radiocaesium at both sites. Natural food was the major contributor to intake at the Russian site (83%). Whereas private farm produce was the major contributor (68%) at the Ukrainian study site. The difference between the two sites was mainly because private milk production was stopped at the Russian site due to the contamination in 1986. A retrospective assessment of the situation 1 year after the accident shows that collective farming could have been a minor contributor to radiocaesium intake (8%), whilst private farming would have been the major contributor wherever private milk production and consumption continued. The extent to which inhabitants consume natural foods from forests has a considerable effect on their radiocaesium intake. The comparative importance of food products from natural ecosystems increases with time due to the long effective ecological half-lives of radiocaesium in unimproved pastures and forests. Estimation of the fluxes of radiocaesium from the different production and harvesting systems showed that the contribution from private farming and food harvesting from natural ecosystems may be significant, contributing 14–30% to the total fluxes of radiocaesium from an area even if the quantity of food produced in these systems is small. However, the major contributor to the flux exported from an area was the collective farming system, accounting for about 70–86% of the total.

The influence of fungi on the long-term behaviour of radiocaesium in Norwegian sheep

Results from 11 years of live monitoring of sheep from a grazing area in Norway formed the basis for a study of the importance of ingested fungal fruit bodies in determining radiocaesium activity concentrations in sheep grazing freely on mountain pasture. The 137Cs activity concentration due to ingested vegetation decreased with an estimated effective ecological half-life of 7.0±2.6 years. The contribution from ingested fungal fruit bodies to radiocaesium activity concentrations in the sheep was up to 75–85% in the years when fruit bodies were most abundant. The study demonstrates that using a simple exponential function in assessments of long-term consequences of radiocaesium fallout for grazing sheep may be inappropriate.

Accumulation and long term behaviour of radiocaesium in Norwegian fungi

The accumulation and long term behaviour of radiocaesium in mushrooms were studied at selected sites in Norway. A large variation in transfer factors from soil to mushroom were found with the lowest value of 0.002 m2/kg for Leccinum versipelle up to 0.5 m2/kg for species of Amanita fulva and Amanita vaginata. Rozites caperata and Cortinarius armillatus were also found to accumulate radiocaesium very efficiently. For the study of long term behaviour of radiocaesium in mushrooms the same species were collected from the same location from 1989 to 1995. Two of four data sets showed a significant decrease in activity levels in the study period with an estimated effective ecological half-life in the range of 1.5–8 years, while a non-significant decrease and no decrease were indicated by the other two.

Transfer of 137Cs to cow's milk: Investigations on dairy farms in Sweden

Since 1986, the year of the nuclear accident at Chernobyl, 137Cs activity concentrations in cow's milk on dairy farms were studied in Sweden. Transfer coefficients, F m , of 137Cs from pasture and fodder to cow's milk were determined on farms in the counties of Uppsala, Gävleborg and Västmanland in central Sweden for one month on winter-fodder, and for the first month in 1987 and in 1988 on pasture. The average F m for all investigations (of 10 farms on winter-fodder and 11 farms on pasture in 1987 and 4 farms on pasture in 1988) south of Gävle was estimated to be 0.0055 with a range of 0.0039 to 0.0080. The 137Cs activity concentration in milk decreased with time. In summer 1992 and 1993, 137Cs in milk, on the farms still producing milk, was determined. On these farms, 137Cs activity concentration in milk was found to be < 2–21 Bq kg −1. The effective ecological half-life from 1987 was estimated to be 1.4 ± 0.5 (sd) years for milk from 10 farms with a range of 0.8–2.0 years. One farm where 137Cs in milk decreased at a slower rate, or not at all, used semi-natural and uncultivated pasture, forest meadows. On intensely managed farms, where potassium fertilizer was distributed, ploughing was performed and, in 1986, forage was cut at a higher stubble-height, the decrease of 137Cs in milk was observed to be faster.