Long-term behavior of (90)Sr and (137)Cs in the environment: Case studies in Switzerland

Steep declines in radioactive caesium after 30 years of monitoring alpine plants in mountain areas of central Norway

The Chernobyl accident exposed large areas of northern Europe to radiocaesium (137Cs). We investigated temporal and spatial variation in concentrations of radiocaesium among five functional groups of alpine plants at two mountain areas in central Norway over a 31-year period from 1991 to 2022. Average concentrations of radiocaesium were initially high in lichens and bryophytes at around 4600-6400 Bq/kg dry weight during 1991-1994 but then decreased dramatically over three decades to current concentrations of <200 Bq/kg for all plant groups in 2019-2022. The effective half-life of radiocaesium was estimated to be 4-6 years in lichens and mosses, 7-13 years in herbaceous plants, and 22-30 years in woody plants, which were less than the physical half-life of 30.2 years. Concentrations of radiocaesium were greater at the nutrient-poor site than at the nutrient-rich site, probably due to greater deposition levels at higher elevations and the geographical pattern of the deposition. Functional groups of plants differed with higher concentrations among non-vascular than vascular plants. Common heather Calluna vulgaris was unusual among woody plants with high concentration of radiocaesium, especially in the new shoots. Our new estimates of concentrations and dynamics of radiocaesium for alpine plants in natural environments will be useful for modelling herbivore exposure and evaluating potential impacts on wildlife and human health.

Do transient hydrological processes explain the variability of strontium-90 activity in groundwater downstream of a radioactive trench near Chernobyl?

The Chernobyl Pilot Site (CPS) was created in 2000 in order to study radionuclide migration processes to the geosphere from radioactive material of the Red forest buried in a trench. In this article, the data collected in the CPS up to 2015 are analyzed to identify the links between hydrological conditions and release of strontium-90 (⁹⁰Sr) from the trench. Then, a flow-and-transport model is used for simulating distribution of ⁹⁰Sr both in the unsaturated and saturated zones downstream of the trench. The results show that the ⁹⁰Sr activity in groundwater is strongly transient in time, due to the high inter-annual variability of both the recharge rate and the groundwater level (some particularly wet winters resulted in saturation of the bottom part of the trench). In addition, the parameters that govern the sorption of ⁹⁰Sr in trench material appear to vary significantly in space (the retardation factor ranges from 10 to 50 depending on the location). This spatiotemporal variability could hide some critical processes, e.g., related to a long-term trend, and needs to be characterized through an appropriate sampling frequency.

Effective half-lives for 137Cs in dairy milk from alpine ecosystems and the controlling factors

Alpine regions in the federal state of Salzburg (Austria) have been intensively contaminated by Chernobyl fallout, necessitating long-term monitoring programs. The sites predominately affected are those in areas with soil developed on silicate bedrock, as these soils tend to be acidic, favouring high transfer factors for ¹³⁷Cs. In addition, nutrient deficiency, low mineral and high organic matter content, and tough climatic conditions are causing the slow migration of ¹³⁷Cs in the soil, which are associated with long effective half-lives in the biosphere. As a quantitative measure for effective half-lives, milk has been collected at nine alpine seasonal stock farming sites since 1988; at four sites, the monitoring is still ongoing (2020). For the period between 1999 and 2020, the decrease of ¹³⁷Cs can be reasonably fitted with one effective half-life describing the time-trend. The effective half-lives obtained by this procedure vary between 9.3 ± 0.9 years and 18.8 ± 3.4 years. The effective half-lives show a weak negative correlation with the half-value depth of ¹³⁷Cs, defined as the depth of the upper soil layer containing half of the deposited fallout inventory. The majority of the inventory is bound in the rooting zone of 0-10 cm, which is reflected by the small half value depths in the range between 3.2 and 4.4 cm. The soils investigated are acidic with pH values between 3.78 and 4.88, showing a pronounced negative correlation with the effective half-lives of ¹³⁷Cs in milk. The data indicate that in these soils rich in organic matter, which are also almost totally devoid of clay minerals and have a very low clay size fraction, pH may be the dominating factor influencing the effective half-lives of ¹³⁷Cs plant uptake and the subsequent contamination of milk.

A REVIEW OF 137CS TRANSFER FACTOR STUDIES FOR INDIAN ENVIRONS

In recent years there have been conscious efforts put by researchers in India to generate the soil-to-plant 137Cs transfer factor values. This review study is aimed at providing a comprehensive background of 137Cs transfer rates around the environs of a few nuclear power plant sites of India. As a part of this, the study looked into 137Cs transfer rates from soil to various plant species. The effort was made to quantify the transfer rates of 137Cs and various parameters affecting the movement of 137Cs in the terrestrial environment. The past and current knowledge on 137Cs transfer factors available in the literature were reviewed thoroughly and highlighted the need for incorporating lesser known and overlooked aspects of radionuclide behaviour for future studies. The paper also tried to identify and highlight the gaps and inconsistencies in the reported values of 137Cs transfer factors.

Novel 90Sr analysis of environmental samples by Ion-Laser InterAction Mass Spectrometry

The sensitive analysis of ⁹⁰Sr with accelerator mass spectrometry (AMS) was developed to advance environmental radiology. One advantage of AMS is the ability to analyze environmental samples with ⁹⁰Sr/⁸⁸Sr atomic ratios of 10^-14 in following a simple chemical separation. Three different IAEA samples with known ⁹⁰Sr concentrations (moss-soil, animal bone, Syrian soil: 1 g each) were analyzed to assess the validity of the chemical separation and the AMS measurement. The ⁹⁰Sr measurements were conducted on the AMS system VERA combined with the Ion Laser InterAction Mass Spectrometry (ILIAMS) setup at the University of Vienna, which has excellent isobaric separation performance. The isobaric interference of ⁹⁰Zr in the ⁹⁰Sr AMS was first largely removed by chemical separation. The separation factor of Zr in two-step column chromatography with Sr resin and anion exchange resin was 10⁶. The ⁹⁰Zr remaining in the sample was effectively suppressed by ILIAMS. This procedure achieved a limit of detection <0.1 mBq in the ⁹⁰Sr AMS, which is lower than typical β-ray detection. The agreement between AMS measurements and nominal values for the ⁹⁰Sr concentrations of IAEA samples indicated that the new highly-sensitive ⁹⁰Sr analysis in the environmental samples with AMS is reliable.

Establishing historical 90Sr activity in seawater of the China seas from 1963 to 2018

Historical ⁹⁰Sr activity in seawater was established in the China seas from 1963 to 2018. Based on the exponential decrease in ⁹⁰Sr activity in seawater, the effective half-life (EHL) of ⁹⁰Sr was quantified to be 11.5 ± 1.6 a, 16.5 ± 2.4 a, 27.2 ± 6.2 a, and 26.7 ± 4.3 a in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea, respectively. We found contrasting patterns in the EHL of ⁹⁰Sr and ¹³⁷Cs in the marginal seas and open oceans that were closely related to the subtly different pathways of ⁹⁰Sr and ¹³⁷Cs in marine environment. Additionally, we demonstrated that Fukushima-derived ⁹⁰Sr (<0.01 Bq/m³) would be difficult to identify in the China seas. Our study not only provided the key parameter of the EHL in marine models for predicting the ⁹⁰Sr activity in the China seas in the post-Fukushima era but also enhanced our understanding of ⁹⁰Sr behavior and its fate in marine environments.

Assessment of residual radionuclide levels at the Bokak and Bikar Atolls in the northern Marshall Islands

The Bikar and Bokak Atolls, located in the northern Marshall Islands, are extremely isolated and consist of pristine marine and terrestrial ecosystems. Both atolls may have experienced significant radioactive deposition following the nuclear weapon testing conducted at Bikini and Enewetak proving grounds. Here we report activity concentrations of artificial radionuclides (²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴¹Am, ¹³⁷Cs and ⁹⁰Sr) in marine and terrestrial samples collected from Bikar and Bokak Atolls. Artificial radionuclides in soil from the Majuro Atoll are also reported and form a radiological baseline against which the levels at the other atolls can be compared. We observed low levels of artificial radionuclides in soil from Majuro and Bokak, but significantly higher levels in soil from Bikar. The residual radioactivity in the Bikar environment is comparable to the levels previously reported for other nearby atolls, including Taka and Utrik, but lower than for Rongerik, Rongelap, Bikini and Enewetak. An analysis of ²⁴⁰Pu/²³⁹Pu isotope ratios and estimations of the dates of contamination from ²⁴¹Am/²⁴¹Pu activity ratios both indicated that the Bikar Atoll was contaminated mainly by radioactive fallout from the Castle Bravo test in 1954. We compare the results of our measurements at Bikar and Bokak to data from other atolls in the Marshall Islands and to regions of the world affected by both global and regional fallout from atmospheric nuclear weapons testing and nuclear accidents.

Caesium-137 in mountain flora with emphasis on reindeer's diet - Spatial and temporal trends

The present study summarizes three decades of studies on ¹³⁷Cs transfer to various species of lichens, graminoids, herbs and woody plants across a ~3000 km² area used as mountain pasture for reindeer and other ruminants. The investigation comprised of field studies covering the period 2011-2016, and a compilation of studies and data for the preceding period (1986-2010). Altogether, more than 700 individual vegetation samples were considered. For lichens, relatively fast decrease in contamination levels was observed during the first decade after the Chernobyl fallout (ecological half-time of about 3 years). For later years there seems to be a continuous re-contamination which results in a "steady state" where time-trends are mainly governed by physical decay of ¹³⁷Cs. For green plants, decline in transfer factors (TF) (i.e. the ratio between activity concentration in vegetation and activity density in soil) during the period 1986-2012 was not as pronounced as for lichens: Some species showed significant decrease with time, while others did not. 25-30 years after the Chernobyl accident, ¹³⁷Cs levels in lichens and green plants were significantly dependent on the levels in soil (R² between 0.53 and 0.57), but there were also some significant differences in transfer between sampling sites. Moreover, marked variability in TFs was found between different plant species growing at the same site, whereas such differences were not found for reindeer lichens.

Long term decrease of 137Cs bioavailability in French pastures: Results from 25 years of monitoring

Long term radioactivity monitoring programs contribute to the understanding of the behavior of radionuclides in the environment. This work aims to investigate the long term behavior of Cesium-137 in pasture ecosystem (root soil, grass and cow's milk) by using of more than twenty five years monitoring data collected at ten of French pasture sites contaminated by atmospheric fallouts from Chernobyl and nuclear atmospheric tests. We estimated with a simple exponential model the long term effective half-lives of radiocesium in root soil, grass vegetation and cow's milk along with their associated uncertainties. The average values of the effective half-lives over all the investigated sites were determined as 17, 11 and 9 years for soil, grass and milk respectively. Those values compare favorably to those estimated in previous studies in literature. These findings further enable us to quantify the decrease of ¹³⁷Cs bioavailability which ranges from 0.008 to 0.044 year^-1 with an average value of 0.026 year^-1 (i.e. effective half lives ranging from 16 to 87 years with an average value of 26 years in soil).

Modelling the mitigation speeds of 137Cs, 90Sr and 131I in the topsoils and assessment of the radiological hazards

Fate modelling of artificial radionuclides (ARs) in top soils are necessary to assess the radiological effects to population. Among ARs, ¹³⁷Cs, ⁹⁰Sr and ¹³¹I are very important since the large abundances in the environment. In this study, the fates of ¹³⁷Cs, ⁹⁰Sr and ¹³¹I in the surface soil layers were simulated by the soil model which was developed by the Canadian Centre for Environmental Modelling and Chemistry (CEMC). The scenario that ¹³⁷Cs, ⁹⁰Sr and ¹³¹I contaminated in topsoil in the exclusion of Fukushima Daiichi nuclear power plant (NPP) accident was evaluated. The results show the expected time for the minimum hazardous level of exposure. It is 115.5 days after the exposure, when the total effective dose is 1 mSv y^-1 in which 0.46 mSv y^-1 from ingestion and 0.54 mSv y^-1 from gamma exposure. Hazard levels due to exposure progresses are varied in order gamma exposure (82.14%) > ingestion (17.47%) > inhalation (0.39%). The hazard levels from radionuclides are varied in order ¹³⁷Cs (63.34%) > ¹³¹I (33.48%) > ⁹⁰Sr (3.18%).

Radiostrontium transport in plants and phytoremediation

Radiostrontium is a common product of nuclear fission and was emitted into the environment in the course of nuclear weapon tests as well as from nuclear reactor accidents. The release of ⁹⁰Sr and ⁸⁹Sr into the environment can pose health threats due to their characteristics such as high specific activities and easy access in human body due to its chemical analogy to calcium. Radiostrontium enters the human food chain by the consumption of plants grown on sites comprising fission-derived radionuclides. For humans, Sr is not an essential element, but, due to solubility in water and homology with calcium, once interred in the body, it gets deposited in bones and in teeth. This concern has drawn the attention of researchers throughout the globe to develop sustainable treatment processes to remediate soil and water resources. Nowadays, phytoremediation has become a promising approach for the remediation of large extents of toxic heavy metals. Some of the plants have been reported to accumulate Sr inside their biomass but detailed mechanisms at genetic level are still to be uncovered. However, there is inadequate information offered to assess the possibility of this remediation approach. This review highlights phytoremediation approach for Sr and explains in detail the uptake mechanism inside plants.

Strontium adsorption and desorption in wetlands: Role of organic matter functional groups and environmental implications

Strontium (Sr) is a chemical element that is often used as a tracer in hydrogeochemical studies, and is ubiquitously distributed as a radioactive contaminant in nuclear sites in the form of strontium-90 (Sr-90). At the interface between groundwater and surface water, wetlands possess unique hydrogeochemical properties whose impact on Sr transport has not been investigated thoroughly. In this study, the adsorption and desorption of Sr was investigated on six natural wetland substrates and two mixes of exogenous media and wetland sediment: winter and summer wetland sediments, decayed cattails, wood, leaf litter, moss, bone charcoal, and clinoptilolite. The composition of the organic matter was characterized using carbon-13, solid phase Nuclear Magnetic Resonance analysis. The range of the substrates' adsorption coefficients obtained could be explained by factors indicative of proteins in the organic matter, which were shown to support strong and poorly reversible Sr adsorption. In contrast, the proportion of carbohydrates and lignin were found to be indicative of lower adsorption coefficients and higher desorption. The implications of these results for Sr pollution remediation in wetlands are discussed.

210Pb and 137Cs as tracers of recent sedimentary processes in two water reservoirs in Cuba

Hanabanilla and Paso Bonito Reservoirs are the main fresh water sources for about half a million inhabitants in central Cuba. Prior to this investigation precise information about the losses of storage capacity was not available. Sedimentation is the dominant process leading to reduction in water storage capacity. We investigated the sedimentation process in both reservoirs by analyzing environmental radionuclides (e.g. ²¹⁰Pb, ²²⁶Ra and ¹³⁷Cs) in sediment cores. In the shallow Paso Bonito Reservoir (mean depth of 6.5 m; water volume of 8 × 10⁶ m³), we estimated a mean mass accumulation rate (MAR) of 0.4 ± 0.1 g cm^-2y^-1 based on ²¹⁰Pb chronologies. ¹³⁷Cs was detected in the sediments, but due to the recent construction of this reservoir (1975), it was not possible to use it to validate the ²¹⁰Pb chronologies. The estimated MAR in this reservoir is higher than the typical values reported in similar shallow fresh water reservoirs worldwide. Our results highlight a significant loss of water storage capacity during the past 30 years. In the deeper and larger Hanabanilla Reservoir (mean depth of 15.5 m; water volume of 292 × 10⁶ m³), the MAR was investigated in three different sites of the reservoir. The mean MARs based on the ²¹⁰Pb chronologies varied between 0.15 and 0.24 g cm^-2y^-1. The MARs calculated based on the ¹³⁷Cs profiles further validated these values. We show that the sediment accumulation did not change significantly over the last 50 years. A simple empirical mixing and sedimentation model that assumes ¹³⁷Cs in the water originated from both, direct atmospheric fallout and the catchment area, was applied to interpret the ¹³⁷Cs depth profiles. The model consistently reproduced the measured ¹³⁷Cs profiles in the three cores (R² > 0.9). Mean residence times for ¹³⁷Cs in the water and in the catchment area of 1 y and 35-50 y, respectively were estimated. The model identified areas where the catchment component was higher, zones with higher erosion in the catchment, and sites where the fallout component was quantitatively recorded in the sediments.