Europe-Wide Atmospheric Radionuclide Dispersion by Unprecedented Wildfires in the Chernobyl Exclusion Zone, April 2020.

From early April 2020, wildfires raged in the highly contaminated areas around the Chernobyl nuclear power plant (CNPP), Ukraine. For about 4 weeks, the fires spread around and into the Chernobyl exclusion zone (CEZ) and came within a few kilometers of both the CNPP and radioactive waste storage facilities. Wildfires occurred on several occasions throughout the month of April. They were extinguished, but weather conditions and the spread of fires by airborne embers and smoldering fires led to new fires starting at different locations of the CEZ. The forest fires were only completely under control at the beginning of May, thanks to the tireless and incessant work of the firefighters and a period of sustained precipitation. In total, 0.7-1.2 TBq 137Cs were released into the atmosphere. Smoke plumes partly spread south and west and contributed to the detection of airborne 137Cs over the Ukrainian territory and as far away as Western Europe. The increase in airborne 137Cs ranged from several hundred µBq·m-3 in northern Ukraine to trace levels of a few µBq·m-3 or even within the usual background level in other European countries. Dispersion modeling determined the plume arrival time and was helpful in the assessment of the possible increase in airborne 137Cs concentrations in Europe. Detections of airborne 90Sr (emission estimate 345-612 GBq) and Pu (up to 75 GBq, mostly 241Pu) were reported from the CEZ. Americium-241 represented only 1.4% of the total source term corresponding to the studied anthropogenic radionuclides but would have contributed up to 80% of the inhalation dose.

Origin and percolation times of Milandre Cave drip water determined by tritium time series and beryllium-7 data from Switzerland.

Early observations of the tritium (3H) activity in precipitation from Switzerland started in 1967 in Bern and a monitoring program with improved analytical techniques was carried out between 1971 and 2009. Between 2012 and 2015, we performed tritium analyses on daily precipitation samples from north-western Switzerland to better constrain the tritium variability. We also collected waters dripping inside Milandre Cave (Jura Mountains) with a 4-6 months' resolution in order to estimate the age of the drip water, which is mandatory to interpret the high-resolution speleothem (cave carbonate deposits) records. Over the monitoring period, the mean tritium concentration in the daily precipitation samples was approximately 8.7 ± 4.7 TU with distinct seasonality showing higher values in spring and summer (from April-May to August-September). The similarity in trends with the other cosmogenic radionuclide beryllium-7 (7Be) suggests that, for the study site, tritium in precipitation essentially originates from stratospheric input in spring. In winter, precipitation dynamics with increasing moisture originating from the Atlantic and diluted during transport contribute to low values close to the Atlantic background. In cave drip water, the depleted activity level of tritium indicates a relatively long percolation time from the surface to the cave of several years. A residual superimposed short tritium variability provides evidence for the occurrence of fast flowing water paths. The contribution from both components were quantified based on a two-component lumped parameter-mixing model. Finally, we show that tritium concentrations in cave drip water are linked to the moisture origin and atmospheric transport pathways.

Radioisotopes demonstrate changes in global atmospheric circulation possibly caused by global warming.

In this paper, we present a new method to study global atmospheric processes and their changes during the last decade. A cosmogenic radionuclide measured at ground-level, beryllium-7, is utilized as a proxy to study atmospheric dynamics. Beryllium-7 has two advantages: First, this radionuclide, primarily created in the lower stratosphere, attaches to aerosols that are transported downwards to the troposphere and travel around the globe with the general atmospheric circulation. By monitoring these particles, we can provide a global, simple, and sustainable way to track processes such as multi-annual variation of the troposphere, tropopause heightening, position and speed of atmospheric interface zones, as well as the poleward movement and stalling patterns of jet streams. Second, beryllium-7 is a product of cosmic rays which are themselves directly linked to solar activity and the earth magnetic field. This study shows whether beryllium-7 observed concentration changes are correlated with such natural processes or independent of them. Our work confirms that major changes in the atmospheric circulation are currently ongoing, even though timeseries are too short to make climatological assessments. We provide solid evidence of significant and progressive changes of the global atmospheric circulation as well as modifications of tropopause heights over the past decade. As the last decade happened to be the warmest on record, this analysis also indicates that the observed changes are, at least to some extent, attributable to global warming.

A vibrating beam MEMS accelerometer for gravity and seismic measurements.

This paper introduces a differential vibrating beam MEMS accelerometer demonstrating excellent long-term stability for applications in gravimetry and seismology. The MEMS gravimeter module demonstrates an output Allan deviation of 9 µGal for a 1000 s integration time, a noise floor of 100 µGal/√Hz, and measurement over the full ±1 g dynamic range (1 g = 9.81 ms-2). The sensitivity of the device is demonstrated through the tracking of Earth tides and recording of ground motion corresponding to a number of teleseismic events over several months. These results demonstrate that vibrating beam MEMS accelerometers can be employed for measurements requiring high levels of stability and resolution with wider implications for precision measurement employing other resonant-output MEMS devices such as gyroscopes and magnetometers.

Origin and stability of uranium accumulation-layers in an Alpine histosol.

Uranium (U) accumulation in organic soils is a common phenomenon that can lead to high U concentration in montane wetlands. The stability of the immobilized U in natural wetlands following redox fluctuations and re-oxidation events, however, is not currently known. In this study, we investigated a saturated histosol that had accumulated up to 6000 ppm of U at 30 cm below ground level (bgl). Uranium in the waters feeding the wetland originates from the weathering of surrounding gneiss rocks, a process releasing trace amounts (<3 ppb) of soluble U into nearby streams. Redox oscillations in the first 20 cm bgl led to the accumulation of U, Ca, S in low permeability layers at 30 and 45 cm bgl. XRF measurements along the core showed that U strongly correlates with sulfur (S) and calcium (Ca), but not iron (Fe). We tested the stability of uranium in the histosol over a nine-month laboratory amendment of a large core of the histosol (∅ 30 cm; length 55 cm) with up to 500 ppm nitrate. Nitrate addition was followed by complete nitrate reduction and re-generation of oxidizing Eh conditions in the top 25 cm of the soil without U release to the soil pore waters above background levels (1-2 ppb). Our results demonstrate that, fast reduction of nitrate, sulfate, and Fe(III) occur in the soil without U release. The remarkable stability of sorbed U in the histosol may result from buffering by sulfide and Sn° and/or strong U(IV)-OM or U(VI)-OM enhanced by organic S moieties or bridging complexation by Ca. That U in the soil was immobile under nitrate addition for up to 9 months can inform remediation strategies based on the use of artificial wetlands to limit U mobility in contaminated sites.

Variation in airborne 134Cs, 137Cs, particulate 131I and 7Be maximum activities at high-altitude European locations after the arrival of Fukushima-labeled air masses.

The Fukushima-labeled air mass arrival, and later the cesium-134 (134Cs), cesium-137 (137Cs) and particulate iodine-131 (hereafter noted 131Ip) maximum levels were registered in Europe at different dates depending on the location. Most of those data were obtained at low-altitude sampling areas. Here, we compare the airborne levels registered at different high-altitude European locations (from 850 m to about 3500 m). The integrated 137Cs activity concentration was not uniform with regard to the altitude even after a long travel time/distance from Japan. Moreover, the relation of integrated 137Cs vs. altitude showed a linear decrease up to an altitude of about 3000 m. A similar trend was noticed for 131Ip (particulate fraction) while it increased above 3000 m. Comparison with 7Be activity concentration showed that, as far as the high altitude location is concerned, the 137Cs and 134Cs maximum concentrations corresponded to the 7Be maximum, suggesting downdraft movements from high tropospheric or stratospheric layers to be responsible for 137,134Cs increase and peak values. This was also confirmed by high potential vorticity and low relative humidity registered during the peak values.

Probing the Kinetic Parameters of Plutonium-Naturally Occurring Organic Matter Interactions in Freshwaters Using the Diffusive Gradients in Thin Films Technique.

The interaction of trace metals with naturally occurring organic matter (NOM) is a key process of the speciation of trace elements in aquatic environments. The rate of dissociation of metal-NOM complexes will impact the amount of free metal available for biouptake. Assessing the bioavailability of plutonium (Pu) helps to predict its toxic effects on aquatic biota. However, the rate of dissociation of Pu-NOM complexes in natural freshwaters is currently unknown. Here, we used the technique of diffusive gradients in thin films (DGT) with several diffusive layer thicknesses to provide new insights into the dissociation kinetics of Pu-NOM complexes. Results show that Pu complexes with NOM (mainly fulvic acid) are somewhat labile (0.2 ≤ ξ ≤ 0.4), with kd = 7.5 × 10(-3) s(-1). DGT measurements of environmental Pu in organic-rich natural water confirm these findings. In addition, we determined the effective diffusion coefficients of Pu(V) in polyacrylamide (PAM) gel in the presence of humic acid using a diffusion cell (D = 1.70 ± 0.25 × 10(-6) cm(2) s(-1)). These results show that Pu(V) is a more mobile species than Pu(IV).

Speciation and Bioavailability Measurements of Environmental Plutonium Using Diffusion in Thin Films.

The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can potentially contribute to the exposure of biota and humans. The diffusive gradients in thin films technique is introduced here for in-situ measurements of Pu bioavailability and speciation. A diffusion cell constructed for laboratory experiments with Pu and the newly developed protocol make it possible to simulate the environmental behavior of Pu in model solutions of various chemical compositions. Adjustment of the oxidation states to Pu(IV) and Pu(V) described in this protocol is essential in order to investigate the complex redox chemistry of plutonium in the environment. The calibration of this technique and the results obtained in the laboratory experiments enable to develop a specific DGT device for in-situ Pu measurements in freshwaters. Accelerator-based mass-spectrometry measurements of Pu accumulated by DGTs in a karst spring allowed determining the bioavailability of Pu in a mineral freshwater environment. Application of this protocol for Pu measurements using DGT devices has a large potential to improve our understanding of the speciation and the biological transfer of Pu in aquatic ecosystems.

A DGT technique for plutonium bioavailability measurements.

The toxicity of heavy metals in natural waters is strongly dependent on the local chemical environment. Assessing the bioavailability of radionuclides predicts the toxic effects to aquatic biota. The technique of diffusive gradients in thin films (DGT) is largely exploited for bioavailability measurements of trace metals in waters. However, it has not been applied for plutonium speciation measurements yet. This study investigates the use of DGT technique for plutonium bioavailability measurements in chemically different environments. We used a diffusion cell to determine the diffusion coefficients (D) of plutonium in polyacrylamide (PAM) gel and found D in the range of 2.06-2.29 × 10(-6) cm(2) s(-1). It ranged between 1.10 and 2.03 × 10(-6) cm(2) s(-1) in the presence of fulvic acid and in natural waters with low DOM. In the presence of 20 ppm of humic acid of an organic-rich soil, plutonium diffusion was hindered by a factor of 5, with a diffusion coefficient of 0.50 × 10(-6) cm(2) s(-1). We also tested commercially available DGT devices with Chelex resin for plutonium bioavailability measurements in laboratory conditions and the diffusion coefficients agreed with those from the diffusion cell experiments. These findings show that the DGT methodology can be used to investigate the bioaccumulation of the labile plutonium fraction in aquatic biota.

Cosmogenic (7)Be and (22)Na in ground level air in Switzerland (1994-2011).

We report monthly averages of weekly (7)Be and (22)Na concentrations in aerosol samples collected with high volume aerosol filters at 5 sampling sites in Switzerland from 1994 to 2011 ((7)Be) and from 2000 to 2011 ((22)Na). Monthly average concentrations of the two cosmogenic isotopes varied between 2600 and 4600 µBq/m(3) for (7)Be and between 0.2 µBq/m(3) and 0.5 µBq/m(3) for (22)Na. The (22)Na concentration in ground level air strongly increased from March to May, while a corresponding (7)Be increase was seen from March until July. The observed variations of the (7)Be and (22)Na activities together with the changes in the (7)Be/(22)Na ratio indicate input of stratospheric air between March and May, increased mixing of upper tropospheric air from June to August, and less exchange between the upper and lower troposphere in autumn and winter. Additionally, the 11-year solar cycle is clearly seen in the annual averages of the (7)Be concentrations.

Long-term and long-range migration of radioactive fallout in a Karst system.

Mountainous areas are often covered by little evolved soils from which deposited radionuclides can potentially leak into the vadose zone. In the Swiss Jura mountains, we observed unusual isotopic ratios of nuclear weapon test (NWTs) fallout with an apparent loss of NWTs plutonium relative to ¹³7Cs of Chernobyl origin in thinner soils. Here, we studied the karstic watershed of a vauclusian spring to determine the residence times of plutonium, ²4¹Am, and 9°Sr deposited by global fallout and their respective mobility in carbonaceous soils. The results show that 9°Sr is washed most efficiently from the watershed with a residence time of several hundred years. The estimated plutonium residence time is more than 10 times higher (in the range of 5000-10,000 years), and the ²4¹Am residence time is double that of plutonium. The spring water ²4¹Am/²³9+²4°Pu isotopic ratio is lower (0.12 - 0.28) than found in watershed soils (0.382 ± 0.077). Similar differences are found in aquatic mosses (²4¹Am/²³9+²4°Pu isotopic ratio 0.05-0.12), which are permanently submerged in spring waters. In contrast to plutonium, 9°Sr is leached from these mosses with 0.5M HCl, demonstrating that strontium is probably associated with calcium carbonate precipitations on the mosses. The higher plutonium to americium isotopic ratio found in the samples of spring water and mosses at the outlet of the karst shows that plutonium mobility is enhanced.

Binding of ²³9Pu and 9°Sr to organic colloids in soil solutions: evidence from a field experiment.

Colloidal transport has been shown to enhance the migration of plutonium in groundwater downstream from contaminated sites, but little is known about the adsorption of 9°Sr and plutonium onto colloids in the soil solution of natural soils. We sampled soil solutions using suction cups, and separated colloids using ultrafiltration to determine the distribution of ²³9Pu and 9°Sr between the truly dissolved fraction and the colloidal fraction of the solutions of three Alpine soils contaminated only by global fallout from the nuclear weapon tests. Plutonium was essentially found in the colloidal fraction (>80%) and probably associated with organic matter. A significant amount of colloidal 9°Sr was detected in organic-rich soil solutions. Our results suggest that binding to organic colloids in the soil solutions plays a key role with respect to the mobility of plutonium in natural alpine soils and, to a lesser extent, to the mobility of 9°Sr.

Atmospheric deposition and migration of artificial radionuclides in Alpine soils (Val Piora, Switzerland) compared to the distribution of selected major and trace elements.

Artificial radionuclides ((137)Cs, (90)Sr, Pu, and (241)Am) are present in soils because of Nuclear Weapon Tests and accidents in nuclear facilities. Their distribution in soil depth varies according to soil characteristics, their own chemical properties, and their deposition history. For this project, we studied the atmospheric deposition of (137)Cs, (90)Sr, Pu, (241)Am, (210)Pb, and stable Pb. We compared the distribution of these elements in soil profiles from different soil types from an alpine Valley (Val Piora, Switzerland) with the distribution of selected major and trace elements in the same soils. Our goals were to explain the distribution of the radioisotopes as a function of soil parameters and to identify stable elements with analogous behaviors. We found that Pu and (241)Am are relatively immobile and accumulate in the topsoil. In all soils, (90)Sr is more mobile and shows some accumulations at depth into Fe-Al rich horizons. This behavior is also observed for Cu and Zn, indicating that these elements may be used as chemical analogues for the migration of (90)Sr into the soil.