# Long-range transport and deposition on the Arctic snowpack of nuclear contaminated particulate matter

David Cappelletti, Žilvinas Ežerinskis, Justina Šapolaitė, Laurynas Bučinskas, Bartlomiej Luks, Adam Nawrot, Catherine Larose, Paolo Tuccella, Jean Charles Gallet, Stefano Crocchianti, et al.

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## ► To cite this version:

David Cappelletti, Žilvinas Ežerinskis, Justina Šapolaitė, Laurynas Bučinskas, Bartlomiej Luks, et al. Long-range transport and deposition on the Arctic snowpack of nuclear contaminated particulate matter. Journal of Hazardous Materials, 2023, 452, pp.131317. 10.1016/j.jhazmat.2023.131317. hal-04291445

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# Long-range transport and deposition on the Arctic snowpack of nuclear contaminated particulate matter

David Cappelletti<sup>a, h, *</sup>, Žilvinas Ežerinskis<sup>b</sup>, Justina Šapolaitė<sup>b</sup>, Laurynas Bučinskas<sup>b</sup>, Bartłomiej Luks<sup>c</sup>, Adam Nawrot<sup>c</sup>, Catherine Larose<sup>d</sup>, Paolo Tuccella<sup>e, f</sup>, Jean Charles Gallet<sup>g</sup>, Stefano Crocchianti<sup>a</sup>, Federica Bruschi<sup>a</sup>, Beatrice Moroni<sup>a</sup>, Andrea Spolaor<sup>h, i</sup>

## HIGHLIGHTS

* Atmospheric transport of 14C to the Arctic during warm intrusion events.

* Deposition of nuclear waste residuals onto the Arctic snowpack.

* Rain on snow event affected the spatial distribution of deposition.

* Air mass back trajectories tracked the source in central Europe.

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

Editor: Danmeng Shuai

**Keywords:**
Nuclear waste dispersion
Arctic
Rain-on-snow

## ABSTRACT

The primary environmental concern related to nuclear power is the production of radioactive waste hazardous to humans and the environment. The main scientific and technological problems to address this are related to the storage and disposal of the nuclear waste and monitoring the dispersion of radioactive species into the environment. In this work, we determined an anomalously high ¹⁴C activity, well above the modern natural background, on surface and seasonal snow sampled in early May 2019 on glaciers in the Hornsund fjord area (Svalbard). Due to the lack of local sources, the high snow concentrations of ¹⁴C suggest long-range atmospheric

<footnote><sup>a</sup> Department of Chemistry, Biology and Biotechnology, University of Perugia, Perugia, Italy</footnote>

<footnote><sup>b</sup> Department of Nuclear Research, Accelerator Mass Spectrometry Laboratory, Center for Physical Sciences and Technology, Vilnius, Lithuania</footnote>

<footnote><sup>c</sup> Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland</footnote>

<footnote><sup>d</sup> Univ Lyon, CNRS, INSA Lyon, Université Claude Bernard Lyon 1, Ecole Centrale de Lyon, Ampère, UMR5005, 69134 Ecully cedex, France</footnote>

<footnote><sup>e</sup> Department of Physical and Chemical Sciences, University of L'Aquila, Via Vetoio 46, 67100 Coppito, L'Aquila, Italy</footnote>

<footnote><sup>f</sup> Center of Excellence Telesensing of Environment and Model Prediction of Severe Events (CETEMPS), Via Vetoio, 67100 Coppito, L'Aquila, Italy</footnote>

<footnote><sup>g</sup> Norwegian Polar Institute, Tromsø, Norway</footnote>

<footnote><sup>h</sup> ISP-CNR, Istituto di Scienze Polari, del Consiglio Nazionale delle Ricerche, Venezia, Italy</footnote>

<footnote><sup>i</sup> Ca'Foscari University of Venice, Department of Environmental Sciences, Informatics and Statistics, Via Torino 155, 30172 Venice Mestre, Italy</footnote>

<footnote>* Corresponding author at: Department of Chemistry, Biology and Biotechnology, University of Perugia, Perugia, Italy.
E-mail address: david.cappelletti@unipg.it (D. Cappelletti).</footnote>
Warm and humid events
Svalbard

transport of nuclear waste particles from lower latitudes, where nuclear power plants and treatment stations are located. The analysis of the synoptic and local meteorological data allowed us to associate the long-range transport of this anomalous ¹⁴C concentration to an intrusion event of a warm and humid air mass that likely brought pollutants from Central Europe to the Arctic in late April 2019. Elemental and organic carbon, trace element concentration data, and scanning electron microscopy morphological analysis were performed on the same snow samples to better constrain the transport process that might have led to the high ¹⁴C radionuclide concentrations in Svalbard. In particular, the highest ¹⁴C values found in the snowpack (> 200 percent of Modern Carbon, pMC) were associated with the lowest OC/EC ratios (< 4), an indication of an anthropogenic industrial source, and with the presence of spherical particles rich in iron, zirconium, and titanium which, altogether, suggest an origin related to nuclear waste reprocessing plants. This study highlights the role of long-range transport in exposing Arctic environments to human pollution. Given that the frequency and intensity of these atmospheric warming events are predicted to increase due to ongoing climate change, improving our knowledge of their possible impact to Arctic pollution is becoming urgent.

# 1. Introduction

The impact of nuclear waste on the Arctic initially focused on ocean transport [1,26,32] and has only recently expanded towards atmospheric transport [34,35]. ¹⁴C is an important, globally dispersed, long-lived radionuclide produced and released by the nuclear power industry [28,55,60] that often dominates the radiological exposure of humans. The distribution and impact of this radionuclide on the environment can be observed in various environmental media. Many biosphere components can efficiently capture ¹⁴C, whose activity can be used to track, predict and evaluate the transport pathways of pollution from nuclear facilities [4,14,28]. According to the latest research, ¹⁴CO₂ emissions from anthropogenic sources are about 135 TBq per year [65]. ¹⁴C is not directly formed in spent nuclear fuel reprocessing facilities, but is accumulated in the released fuel. A fraction of ¹⁴C remains in the reactor construction materials and radioactive waste, which are also important sources. ¹⁴C is produced in all nuclear reactors by the interaction of neutrons with materials containing carbon, nitrogen, and oxygen. Radiocarbon formed in solid materials (reactor structures) due to the activation of impurities usually cannot enter the environment [21]. However, radiocarbon produced during nuclear reactions when neutrons interact with impurities in the nuclear fuel or coolant can enter the environment through cracks in the fuel cladding in the case of leaky fuel assemblies. Additional releases are possible through reprocessing spent nuclear fuel or from the cooling systems of nuclear reactors. Radioactive wastes are produced in spent nuclear fuel reprocessing facilities, discharged to the atmosphere and sea, and disposed of as solid materials [42]. There are no special regulations concerning the permissible ¹⁴C emissions from nuclear installations at the European level. Every country establishes its own guidelines considering the site-specific peculiarities of the objects. The levels are derived based on the nationally established dose constraints, ensuring the adequate radioprotection of the public. The International Atomic Energy Agency (IAEA) recommends a dose constraint value of 0.3 mSv/year for every nuclear installation. However, it can vary from country to country, e.g., in Lithuania, the 0.2 mSv/year dose constraint was adopted, and the established maximal permissible levels of ¹⁴C released into the environment from the Ignalina Nuclear Power Plant are: 230 TBq/year for atmospheric releases and 33 MBq/year for liquid effluences (set by Lithuanian normative document LAND 42-2007). A well-known example of such pollution is the Sellafield nuclear complex in the United Kingdom. The atmospheric ¹⁴C discharges from Sellafield nuclear complex between 1951 and 1999 were estimated to be 259.2 TBq [28]. An average annual discharge of ¹⁴C from Sellafield to the Irish Sea between 1994 and 2014 was 7.9 TBq [42].

Scientific data on ¹⁴C atmospheric release are often limited to areas near nuclear fuel reprocessing plants [55,33]. Studies on the long-range transport of ¹⁴C, critical for understanding the potential for nuclear emissions to influence continental-scale gradients, are rare [18]. The carbonaceous aerosol released in the atmosphere and containing a certain amount of ¹⁴C and other radionuclide can be transported to

mountain glaciers or polar ice caps [11,13,30], where it may be deposited by dry and wet processes. Therefore, ¹⁴C can be used for dating snow and ice [38,62], but the measurement is complex, due to the low amount of extractable carbon and possible contamination by gaseous compounds. Moreover, it is known that ¹⁴C can be produced by cosmic rays directly in snow and glacial ice from oxygen-16, even if in low amounts [49,48]. In general contemporary sources dominate the carbonaceous aerosol and radiocarbon in the Arctic atmosphere and snow [40,54].

In this work, we present data demonstrating the atmospheric transport of reprocessed nuclear waste from northern Eurasia and central Europe to the Arctic stimulated by the changing Arctic climate. Changes in the Arctic are connected to hemispheric weather patterns in a complex way and have yet to be fully constrained [7,24,43,44,46]. The episodic and intense injections of humid and warm air masses from lower latitudes [10,17,22,53] are examples of this connection. These events are stimulated by various weather systems, which can also provide new effective routes for pollutant transport from lower latitudes into the Arctic region [5,9,27,36,39,63]. The frequency and intensity of these events are increasing [17] due to climate change which may, therefore, significantly affect pollutant dispersion in the Arctic atmosphere. A particular focus has been on winter warming events, which are more evident and constantly increasing in the last decades. The winter warming events are often associated with rain-on-snow (ROS) events that can significantly modify the structure of the annual snow pack, which, in turn, could affect the whole cryospheric environment, for example the foraging capability of herbivores. However, warming events also occur in the rest of the year and the underlying mechanisms need to be fully addressed to better understand their possible impacts in the Arctic region.

The present work is based on the determined anomalous ¹⁴C abundance, up to three times the natural background, in surface and seasonal snow sampled in May 2019 in two glaciers of southern Svalbard, a few weeks after an intense episode of warm and humid air intrusion arriving from lower latitudes. Due to the lack of local sources and the unambiguous signal of ¹⁴C, our results indicate that atmospheric transport of nuclear wastes can be very effective and rapid from Europe to the Arctic. Our study proposes that these warm and humid air masses can potentially deposit ¹⁴C and other residual nuclear waste particulates by associated snow deposition or, in specific cases, by ROS events thus demonstrating the impact that such events might have on Arctic pollution.

# 2. Material and methods

## 2.1. *Sampling sites and climate in the study area*

The sampling campaign was conducted between the 6th and 10th of May 2019 in the Hornsund fjord area (Svalbard) in two nearby glaciers, Hansbreen (HSB) and Werenskioldbreen (WSB). A map of the sampling sites is shown in Fig. 1, together with a picture of the field sampling. The
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Map of the sampling locations. HSB1, ..., WSB5 = surface snow samples; SP=snowpit; S1, ..., S3 = soil samples. Field sampling at HSB2 is shown in the lower panel.</figcaption></figure>
climate on the shores of Hornsundfjorden is influenced by the warm West Spitsbergen Current. For this reason, this part of the coast of Svalbard coast has a milder climate with relatively higher air temperatures and yearly precipitation. Both parameters have a strong negative gradient toward the east coast [47]. The meteorological observations in the Polish Polar Station Hornsund (PPSH, WMO 01003) established since 1977, show 1.14 °C and 61.6 mm for the annual mean temperature and annual sums of precipitation, respectively, over a decade. The mean annual air temperature for the period 1979–2018 was – 3.7 °C, with March as the coldest month (– 10.2 °C) and July as the warmest month (4.6 °C) [64]. The annual sum of precipitation varies between years. The minimum amount of 230 mm was recorded in 1987, and the maximum of 805.5 mm in 2016. The driest months are April (average 23 mm) and May (average 24 mm). The highest sum of precipitation occurs in September, reaching, on average, 75 mm [64].

## 2.2. Sampling campaign and sample preprocessing

A total of ten surface snow samples (first layer, approx. 5 cm) were collected along the central transects (from lower to higher altitudes) of the HSB (n = 5) and WSB (n = 5) glaciers from the bottom, near sea level, to the top, around 500 m above sea level (a.s.l.), in both cases. The surface samples constitute the last significant snowfall before the sampling. The sample locations were chosen equally spaced distances at approximately 3 km and 1.7 km in HSB and WSB, respectively.

Snow samples were also collected from a 3 m deep snow pit (SP in Fig. 1) dug [58] above the HSB glacier equilibrium line between the HSB4 and HSB5 sampling sites (see Table 1 and Fig. 1). The snowpit samples represent the accumulated annual snowfalls and can be used to distinguish the most recent and the earlier precipitation events.

Snow samples were collected in sterile plastic bags (Whirl-Pak, Nasco Fort Atkinson) by an operator wearing a clean suit (see Fig. 1 lower panel) and kept frozen until pre-processing in the PPSH clean lab. The snow sampling procedure is described in Gallet et al., 2018. Snow samples were melted and then filtered (2 L of water equivalent per sample) both on pre-fired quartz (Whatman) and polycarbonate (Pall) 47 mm filters. The quartz filters were used for the transect and snow pits samples for elemental carbon (EC) and organic carbon (OC) quantification and ¹⁴C determination. Blanks were produced by filtering 500 mL of ultrapure water. The polycarbonate filters were used for SEM and trace element determination on the transect filters. All the logistic activities were based at PPSH under the Interact access project BC-HOR (Spatial Distributions of Black Carbon and Mineral Dust in Air and Snow Surface Layers on the Hornsund Glaciers). The BC-HOR project aimed to characterize black carbon and mineral dust in the Hornsund area. In the framework of the project, we collected soil samples from

<caption>**Table 1**</caption>

Percent modern carbon (pMC), elemental carbon (EC), organic carbon (OC) and OC/EC ratios in surface snow samples collect in Hansbreen (HSB) and Wernerskioldbreen (WSB). OC (µg L⁻¹) and EC (µg L⁻¹) are expressed as concentration (mass of carbon per water equivalent volume). H (m, asl) is the site elevation. Uncertainty on pMC is of the order of ± 1 %.

<table><thead><tr><th>sample</th><th>H</th><th>OC/EC</th><th>OC</th><th>EC</th><th>pMC</th></tr></thead><tbody><tr><td>HSB1</td><td>81</td><td>3.2</td><td>119</td><td>37</td><td>120.1 ± 0.5</td></tr><tr><td>HSB2</td><td>163</td><td>3.5</td><td>46</td><td>13</td><td><strong>211.3 ± 0.5</strong></td></tr><tr><td>HSB3</td><td>262</td><td>9.4</td><td>48</td><td>5.1</td><td>78.8 ± 0.2</td></tr><tr><td>HSB4 + HSB5</td><td></td><td></td><td></td><td></td><td>74.8 ± 0.4</td></tr><tr><td>HSB4</td><td>350</td><td>9.5</td><td>38</td><td>4.0</td><td>nd</td></tr><tr><td>HSB5</td><td>500</td><td>lod</td><td>24</td><td>lod</td><td>nd</td></tr><tr><td>WSB1</td><td>43</td><td>12</td><td>63</td><td>5.4</td><td>53.5 ± 0.2</td></tr><tr><td>WSB2</td><td>176</td><td>19</td><td>48</td><td>2.5</td><td>64.3 ± 1.3</td></tr><tr><td>WSB3 + WSB4 + WSB5</td><td></td><td></td><td></td><td></td><td><strong>314.0 ± 2.1</strong></td></tr><tr><td>WSB3</td><td>294</td><td>4.4</td><td>20</td><td>4.5</td><td>nd</td></tr><tr><td>WSB4</td><td>400</td><td>13.2</td><td>23</td><td>1.8</td><td>nd</td></tr><tr><td>WSB5</td><td>500</td><td>lod</td><td>9.7</td><td>lod</td><td>nd</td></tr></tbody></table>

lod = limit of detection; nd = not determined.

deglaciated spots along the coast and also in the glacial moraine. The soil samples were gently scraped from the upper (few cm) loose soil layer. Sea water samples were also collected and filtered with the same protocol as melted snow samples.

## 2.3. Synoptic analysis

The synoptic analysis was conducted by ERA5 reanalysis [25] relative to the period 13–19 April 2019, when the warm and humid transport event took place. Upper air geopotential height, temperature, and mean sea level pressure anomalies were calculated with data covering 1991–2020 and averaging the six-hourly data of our period of interest as reference climate. Hourly surface temperature and precipitation data were collected by the meteorological station at PPSH.

## 2.4. Organic and elemental carbon

Organic (OC) and elemental (EC) carbon were determined on pre-fired quartz filters using the semi-continuous thermo-optical analyzer (Sunset Laboratory Inc.) and following the EUSAAR2 protocol [6]. A 1 cm² punch was used and optical correction for charring was performed by transmittance laser (678 nm). The manufacturer-specified detection limits are 0.2 µg C cm⁻² of filter for both EC and OC. The mean blank value for OC, in the order of 1 µg C cm⁻², was obtained by averaging multiple measurements on filters using different funnels. A sucrose standard solution spiked on clean filters was used for calibration and for QA/QC controls.

## 2.5. Radiocarbon analysis

¹⁴C was measured on quartz filters and on soil samples using a single-stage accelerator mass spectrometer (SSAMS, NEC, USA). Samples were prepared using an automated graphitization system consisting of an elemental analyser (EA, Vario Isotope Select, Elementar, GmbH) connected to an Automated Graphitization Equipment AGE-3 (IonPlus AG). Since the carbon content on the snow filter was low, we used several punches (from 3 to 25) of each snow filter to collect > 0.2 mg carbon (the lowest limit of AGE-3). Filter sample punches were burned in the EA and the CO₂ produced was collected in the trap of the AGE-3 system. When sufficient carbon mass was collected on the trap, CO₂ was released and graphitized. Soil samples underwent similar processing. The SSAMS was used for the ¹⁴C/¹²C ratio measurements of the graphitized samples. The accuracy for the ¹⁴C/¹²C ratio was 0.3 %. Radiocarbon dates are reported as percent modern carbon (pMC). Phthalic anhydride was used to estimate the processed background, and was determined to be 0.24 pMC. Alfa Aesar Black Carbon powder was used to test the SSAMS background, and was found to be 0.066 pMC. NIST-OXII standard (134.06 pMC) was used as the reference material. The ¹³C/¹²C ratio was used for the isotopic fractionation correction. More details of the method are provided in [16].

## 2.6. Trace elements analysis

Trace elements present in the insoluble component of the impurities in the snow samples were determined on half of the polycarbonate filters after acid digestion assisted with microwaves (MARS 5) using high-purity acids and reagents (Suprapur® - Supelco) and ultrapure water (18.2 MΩ). 8 mL of HNO₃ ultrapure 65 % and 2 mL of H₂O₂ ultrapure 30 % were added to each sample and the mixture was heated to 170 °C in the microwave system. Thirty-four trace elements (Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Ag, Cd, Sb, Rb, Ir, Sr, Zr, Cs, Ba, Hg, Pb) were quantified using a tandem mass spectrometer with a collision reaction cell and an inductively coupled plasma source (ICP-MS-QQQ Agilent 8900). The acquisition was performed at 1550 W of plasma RF power in kinetic energy discrimination mode, using He as the collision gas (5.5 mL min⁻¹). Instrument
parameters were optimized for the best sensitivity in the whole mass range, and minimum oxides (< 1.5 %) and double charges (< 1.5 %). Quality assurance was provided by determining the elemental concentrations for duplicate samples and one reference material (Certified Reference material SS-1-Contaminated Soil). The recovery of total metal concentration varied between 79% and 130% among the different analytes. The Limit of Detection (LOD) of the methods ranged from 0.01 to 1 ppb, depending on the element (Table SM2). Details on the procedure for trace element characterization are reported in [19].

## 2.7. Scanning electron microscopy (SEM)

Individual particle analyses were performed by SEM coupled to energy-dispersive X-ray microanalysis. The aerosol samples were prepared by cutting single portions (~ 10 mm × 10 mm) from the central part of the polycarbonate filters and mounting them onto SEM aluminum stubs using double-sided carbon tape. The samples were coated with a 100–150 Å carbon film to provide electrical conductivity and prevent charge build-up during exposure to the electron beam. SEM imaging was performed using a ZEISS Supra 25 microscope equipped with a field emission gun and a GEMINI column employed at a variable voltage (15–20 kV) and magnification (500–350,000 ×) to distinguish particle types and textural details. The instrument is also equipped with an X-ray dispersive spectrometer (QUANTAX EDS microanalysis system coupled with ESPRIT software for data analysis). EDS spectra (spot size

5 nm², working distance 8.5 mm) were collected for 90 s and the elemental composition was obtained after standardless matrix correction provided by the ESPRIT software. Values lower than 0.1 wt% (SEM detection limit) were omitted. Selected particles in the digitalized images underwent image analysis (IA) using the software Image Tool 3.0 (http://compdent.uthscsa.edu/dig/) to obtain a set of morphological parameters for each particle grain. In particular, we considered the Feret diameter (FD), which is the diameter of a circle having the same area as the object. FD, thus, provides a measurement of the mean grain size. The analytical precision of FD, obtained from triplicate measurements of selected particles at different magnifications (5000, 10,000, and 20,000 ×), is better than 2 %. Details on the methodology are reported in Moroni et al. [41].

## 2.8. Back trajectories analysis

Hourly air mass back trajectories (BT) were computed using HYSPLIT 4 [59] for ten days prior to the period of interest (April 2019), considering three endpoints located in Hornsund at 50, 500, and 1000 m agl. The endpoint heights provided a reasonable coverage of the lower troposphere in the Hornsund area, considering that the mountain tops are in the 600–900 m range. We exploited meteorological fields from GDAS (Global Data Assimilation Service) with 0.5-degree spatial resolution [51,50].

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Synoptic conditions in 13–19 April 2019 in the European Arctic from ERA5. (a) 500 hPa geopotential height (GPH) and mean sea level pressure anomalies; (b) 850 hPa temperature and GPH anomaly (c) Svalbard, maximum surface temperatures on an hourly basis and 2 m temperature anomaly (d) Svalbard, cumulated precipitations and rain fraction.</figcaption></figure>
# 3. Results and discussion

## 3.1. Synoptic analysis

Air mass composition can changes rapidly and closely associated with synoptic atmospheric circulation, particularly when long-range transport occur [12,15,20]. In this context, the dispersion, atmospheric transformation, dry and wet deposition processes of radioactive material are relevant and can be modeled when measurements are available (see for instance the case of Fukushima accident [31]). Fig. 2 reports the average synoptic pattern in the time span 13–19 April 2019. This period was characterized by a significant undulation of the Atlantic Ocean due to a deep trough exiting from Greenland. Warm and humid air on it’s Eastern boundary was flowing toward the Arctic latitudes, where it rotated around a high-pressure system centered in the north of Scandinavia (Fig. 2a). As a result, an extended temperature anomaly dominated the Arctic region from Western Greenland up to the Barents Sea, with values up to 16 °C above the climatological values in the Greenland Sea (Fig. 2b). Fig. 2c displays the maximum surface temperatures on an hourly basis and the temperature anomaly recorded in the analyzed period on Svalbard. According to ERA5 data, the maximum temperatures reached were above 0 °C in many areas of Svalbard, with values up to 4–5 °C. Consequently, a consistent precipitation fraction observed was rain, especially in the lower elevations on the Southern part of Spitsbergen (Fig. 2d).

Elevated air temperatures and days with liquid precipitation were observed in April 2019 at the PPSH. In total, eight melt days (MD, days with temperature above 0 °C) and five melt and precipitation days (MPD, which are MD with precipitation $P > 0$ mm) were recorded. The sum of Positive Degree-Days (PDD, the sum of mean daily temperatures for all days where the temperature is above 0 °C) for this period equaled 9.9 °C. All MPDs mentioned above occurred between the 14th and 20th of April, with maximum daily precipitation on the 19th of April (19.5 mm, $T_{mean} = 1.8$ C, $T_{max} = 2.6$ °C). A comparison of observed temperature and precipitation data recorded at the Hornsund station with ERA5 reanalysis daily data is reported in Fig. 3. The datasets show consistent trends with temperatures above 0 °C and intense precipitation between the 16th and the 20th of April.

## 3.2. <sup>14</sup>C activity, composition and morphology of deposited particulate matter

The snow sampling campaign occurred between May 6th and May 10th, just after the above-mentioned event. Results for OC, EC, and <sup>14</sup>C are reported in Table 1 for the snow samples of interest. An anomalous high <sup>14</sup>C activity, reported as pMC value, was recorded in the surface snow at the HSB lower sampling sites (HSB1 & HSB2), especially at HSB2 (pMC = 211). A very high pMC value (= 314) was also measured in the composite WSB3 +WSB4 +WSB5 sample. Samples needed to be pooled due to their low total carbon content (< 100 µg), which was below the detection limit for <sup>14</sup>C determination in the present setup. Since the value determined for the composite sample is an average, we expect pMC to be much higher for one of the specific subsamples.

To put these pMC values in perspective, the reference radiocarbon values for 100 % biogenic carbon, based on modern atmospheric <sup>14</sup>CO<sub>2</sub>, are in the order of 100 pMC [45]. Values of <sup>14</sup>C found in seasonal snow and ice samples are typically lower than 100 pMC [48,62], with the exception of ice formed in the bomb period [38]. Therefore, since there are no known local sources of <sup>14</sup>C, the observed exceptionally high pMC values could only be attributed to the transport of radioactive material from a nuclear power plant or nuclear waste reprocessing site.

Based on the data presented in Table 1, there is no correlation between the values of OC, EC, and those of <sup>14</sup>C. OC, in fact, is dominant at the lower altitudes, closer to the sea level, while <sup>14</sup>C reaches the highest values at intermediate altitudes (between 150 and 300 m. asl, depending on the glacier). This piece of evidence suggests that the deposition event was probably connected with orographic clouds extending over both glaciers. The OC/EC parameter, on turn, can discriminate between particulate matter sources. This ratio varies from high values (up to 15) in the cleanest remote locations where EC is very low, to small values in polluted environments, where the primary sources (industrial and vehicular traffic) of both OC and EC are prevailing [57]. We recorded low OC/EC ratios in snow samples showing high pMC values, suggesting a common anthropogenic source. In turn the low pMC samples showed significant higher OC/EC values.

Data obtained for the snow pit (elevation 405 m. asl) sample and pooled snow pit samples, as well as two soil and one seawater sample are reported in Table 2. A very high pMC value (= 197) was recorded in the

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Precipitation (bars) and mean air temperature (lines) at the PPSH station in April and May 2019. Measured values are reported in black while values from ERA5 reanalysis are shown in red.</figcaption></figure>
<caption>**Table 2**</caption>

Percent modern carbon (pMC), elemental carbon (EC), organic carbon (OC) and OC/EC ratios in snow pit samples collect in Hansbreen (HSB) glacier, soils and sea water. OC (µg L⁻¹) and EC (µg L⁻¹) are expressed as concentration (mass of carbon per water equivalent volume). Depth (cm) is the layer depth in the snowpit. Uncertainty on pMC is of the order of ± 1 %. SP_SUB1 (= SP1 + SP2 + SP3) and SP_SUB2 (= SP4 + SP5 + SP6 + SP7) are composite samples from the snowpit.

<table><thead><tr><th>Sample</th><th>Depth</th><th>OC/EC</th><th>OC</th><th>EC</th><th>pMC</th></tr></thead><tbody><tr><td>SP1</td><td>288-280</td><td>5.5</td><td>11.4</td><td>2.1</td><td>nd</td></tr><tr><td>SP2</td><td>280-250</td><td>2.2</td><td>12.2</td><td>5.5</td><td>nd</td></tr><tr><td>SP3</td><td>250-200</td><td>1.3</td><td>2.1</td><td>1.7</td><td>nd</td></tr><tr><td>SP_SUB1</td><td>288-200</td><td></td><td></td><td></td><td>197.2 ± 0.9</td></tr><tr><td>SP4</td><td>200-150</td><td>6.9</td><td>20</td><td>3.0</td><td>nd</td></tr><tr><td>SP5</td><td>150-100</td><td>11.5</td><td>23</td><td>2.0</td><td>nd</td></tr><tr><td>SP6</td><td>100-50</td><td>lod</td><td>19</td><td>lod</td><td>nd</td></tr><tr><td>SP7</td><td>50-0</td><td>lod</td><td>5.0</td><td>lod</td><td>nd</td></tr><tr><td>SP_SUB2</td><td>200-0</td><td></td><td></td><td></td><td>74.3 ± 0.4</td></tr><tr><td>SOIL1</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>6.0 ± 0.05</td></tr><tr><td>SOIL2</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>1.8 ± 0.03</td></tr><tr><td>SOIL 3</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>66 ± 0.3</td></tr><tr><td>Sea Water</td><td>0</td><td>73</td><td>392</td><td>5.3</td><td>91.3 ± 0.4</td></tr></tbody></table>

lod = limit of detection; nd = not determined.

first pooled sample of the snow pit (corresponding to the upper first meter). The low values of pMC (< 80) recorded in the surface snow at the nearest sites (HSB3 and HSB4 + HSB5) suggested that the ¹⁴C contamination at the snow pit site was located under the first snow layer (10 cm) and within the first meter of the snowpack. The OC/EC trend is consistent with this hypothesis. Soil and seawater samples showed low, close to natural, pMC values. Indeed, there wasn't much snowfall on Hansbreen between April 1st and the first week of May, as indicated by the snow depth rise recorded on stakes. On the lower elevation stakes (below 363 m a.s.l.), there was snowpack compaction (due to melting and densification). On the upper elevation stakes, in the vicinity of the snow pit location, there was a slight increase in the depth at the end of April after the warm event. In the snow pit microphysical profile, we found visible melt forms and icing at a depth of 30 cm, which suggested percolating water which then refroze and was covered with a small

amount of fresh snow. Therefore, most of the radionuclide's deposition was probably associated with liquid precipitation at the lower coastal sites. However, at 500 m a.s.l., the deposition was still solid; hence, a succession of melting refreezing and snowfall events insulated and covered the radiogenic layer.

The deposition of ¹⁴C was associated with particulate matter, whose nature was further investigated by SEM. The snow sample filtrates were characterized by high abundances of the following silicate minerals: biotite, muscovite, chlorite, quartz, alkali feldspar, plagioclase, and actinolite. Minor and accessory phases are ilmenite, rutile, pyrite, monazite, zircon, and native Bi (Fig. 4). The mineralogical assembly reflects the geological nature of the Caledonian crystalline basement outcropping in the Hornsund area [52]. Particle size was variable and independent of the nature of the mineralogical species and the location of the sampling sites. The medium to low sorting, and the angular morphology of the grains indicated limited aeolian transport of the particles from the site of origin to the site of settling, suggesting a local source.

Significant amounts of Fe-bearing particles were also present in all the samples. They were easily recognizable due to their high contrast in the back-scattered electron images. Two distinct particle shapes, i.e. angular and spherical, were distinguished. Angular particles were found in all the samples. Spherical particles, instead, were primarily found in samples HSB2 and WSB3, they were sporadic in samples HSB4 and WSB5, and absent in the rest of the samples. Angular particles exhibited roughly geometrical to irregular shapes and varying grain boundary complexity (Fig. 5a). Spherical particles, in turn, were characterized by smooth to gently sutured surfaces and rough surfaces in very few cases (Fig. 5b). Both particle types contained Fe or Si and Al oxides as the main constituents, along with alkali (Mg, Ca, Na, K) and metal (Zn, Ti, Mn, Cu) impurities. While the Fe-oxide content dominated the chemical composition of angular particles (see Figure SM6) the composition of spherical particles showed two composition patterns, the first with Fe amounts higher than 90 %, and the latter with Si:Al:Fe ratios ranging from 60:20:20 to 30:50:20 (Fig. 6). No apparent correlation between the size, composition and location of the sampling sites could explain the distribution of both particle types. One particle with a peculiar

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Scanning electron microscopy micrographs of the snow sample filtrates with representative mineral phases: (a) ilmenite (ilm), (b) rutile (ru), (c) pyrite (py) inclusions, (d) metallic bismuth (Bi), (e) monazite (mnz), and (f) zircon (zrc) grains. The red crosses mark the spot analyses.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Scanning electron microscopy micrographs of Fe-bearing angular (a) and spherical (b) particles, and a Zr-bearing (Ti-ox) particle (c) with composition (d). The red crosses mark the spot analyses.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Ternary diagram Si-Al-Fe of spherical particles, distinguished by size (Feret diameter, FD).</figcaption></figure>

composition was found in sample HSB2. This particle had a prismatic, quite elongated shape and showed a composition dominated by Zr and oxygen with minor amounts of Fe, Si, Cu, Al and Ti, and traces of K, Ca, Mg (Fig. 5d). This is compatible with crystalline zirconolite (likely

CaZrTi₂O₇), a mineral phase particularly resistant to weathering, which makes it an ideal waste form phase for immobilizing plutonium and other artificial actinides during nuclear waste thermal treatment [37].

The bulk geochemical features of the samples (data presented in table SM1) support this evidence resulting from phase analysis. In particular the concentrations of Zr, Ti and Sb in HSB2 and WSB3 samples are much higher compared to the rest of the samples and are consistent with the composition of radioactive nuclear wastes [3,29].

To summarize, the SEM and ICP-MS analyses identified particles of local and non-local origins. The latter were essentially found at HSB2 and WSB3 sites where the ¹⁴C activity was the largest. Finally, the snow pit ¹⁴C results are supported by high-resolution trace element analysis on the same snow pit [58] that showed clear peaks of many chemical tracers, including rare earth elements (see Figures SM2) just below the surface snow layer.

### 3.3. Particulate matter transport and nuclear waste dispersion dynamics

The motion of the air masses associated with the weather pattern that generated the warm air intrusion of mid April 2019 can be visualized by looking at the BT calculated with the HYSPLIT model. Fig. 7 shows four daily examples: Fig. 7a illustrates the situation of April 10, before the beginning of the event; Figs. 7b and 7c show the apex of the transport between the 16 and the 21 of April 2019; Fig. 7d shows the case of April 22, after the end of the event. The complete set of daily calculations between April 10 and April 22 is reported in Supplementary material
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Air masses back trajectories (BT) calculated for the receptor site of Hornsund (red star) at three different endpoints (50, 500, 1000 m above ground) every 6 h for (a) April 10, (b) April 16, (c) April 21 and (d) April 22. The color gradient of the BT indicate the elevation above ground. The velocity of the BTs is shown in Figures SM1. The red circles indicate the location of the main ¹⁴C release reactors and spent fuels reprocessing sites [18].</figcaption></figure>

(SM1). The figures clearly show that the air masses passed over Europe in an anticyclone-like vortex due to the high-pressure system centered in the north from Scandinavia. The planetary boundary layer (PBL) height over central Europe (UK, France, Sweden) was above 400 m a.s.l. for the central hours of the day. Contour plots of the PBL height over Europe are shown in Supplementary material (SM5) for the 20 of April. Similar values were found for the other days. A comparison of BT and PBL heights suggests that air masses entrained the PBL over the land in a sequence that involved Finland, Sweden, Northern France, and central UK, prior to moving to the Arctic. The first BTs reached the south of Svalbard on April 14. The transport accelerated and reached the maximum intensity around April 16–17 when the air masses passing over Europe reached Svalbard in less than 48 h. After the apex of the event, air mass velocity slowed until April 21. On April 22, there was a mutation of the weather pattern, which produced more stable conditions over Svalbard for the next 2–3 weeks and a drastic change in air mass trajectories (Fig. 7d). The red circles in Fig. 7 indicate the location of the main ¹⁴C release reactors and spent fuels reprocessing sites [18] which are located along the pathway of the BTs, most of which were traveling near the ground in the proximity of the nuclear sites.

The characteristics of the particles found in the snowpack point to natural sources for the angular particles, and anthropogenic source/s for spherical particles. The characteristics of the spherical particles, in particular, recall those of fly ash, i.e. fine powder mainly consisting of spherical, glassy particles produced as a by-product in coal-fired power stations [61]. The particular nature of these particles and the elevation

at which they were found in the snow cover suggest that they were long-range transported before being deposited during the same ROS or snowfall event. Fly ash powders are used to encapsulate of Intermediate Level Radioactive Waste (ILRW) in European sites [2,8,23,56,66]. The pattern of back-trajectories and the presence of zirconolite in one of the samples, mostly enriched in fly ash, support this hypothesis.

The scenario that emerges from the above results is consistent with a LRT of nuclear waste particulate matter, characterized by a substantial amount of ¹⁴C, in the context of an intense warm and humid air mass. For most of the period from April 10th to 21st, 2019, clouds covered the entire sky in the Hornsund area, and the cloud base did not exceed 400 m a.s.l. (see Fig. SM3 and SM4). In this period winds blew from S through W to NNW, with the prevailing wind from a westerly direction. The anomalous concentration of ¹⁴C was found on WSB between 290 and 500 m a.s.l., and on HSB, only 163 m a.s.l. It can be assumed that low clouds, along with westerly winds, entered over the WSB and made contact with the snow surface around 300 m a.s.l. Further on, the clouds rolled over the Kossibapasset and descended lower over the HSB, with associated precipitations near HSB2. At the upper elevations (i.e. snowpit) the contaminated layer was covered with successive clean snow strata.

## 4. Conclusions

In this work, we demonstrated the occurrence of an atmospheric LRT event of nuclear waste in the Arctic which substantially contaminated
the annual snowpack in Svalbard Islands. We recorded a very high ¹⁴C activity (> 200 pMC) in several spots of two glaciers located at low and medium altitudes (< 500 m asl) near the PPSH. The highest concentration of ¹⁴C in the snowpack was associated with insoluble particulate matter of spherical shape, distinctive to industrial processes such as those produced in spent nuclear fuel reprocessing facilities. Elemental analysis consistently shows concentration peaks of elements such as Ti, Zr, and Sb typically generated in these processes. At the synoptic scale, the LRT was associated with an intense and prolonged intrusion of warm and humid air masses from Northern and Central Europe. The scenario that emerges from our results is consistent with a LRT from Northern Europe (Russia, Sweden, France, UK) associated with an intense ROS event that deposited a substantial amounts of radioactive nuclear waste. The deposition event was probably also associated with clouds, given that the samples with detectable concentrations were localized between 80 and 500 m asl.

Such events are becoming more and more frequent due to the current climate change. Indeed, as we demonstrated, they impact the Arctic atmosphere and cryosphere by transporting and depositing significant amount of environmental pollutants, including radioactive nuclear wastes.

## Environmental Implications

We discovered anomalously high 14C activity in Arctic snow sampled in spring 2019. We attributed the contamination to an intense episode of warm and humid air intrusion from lower latitudes were the air masses have uptake residuals from nuclear waste reprocessing materials. Elemental and organic carbon and trace element concentration data and SEM morphological analysis of the filtered particulate matter from snow samples support the results.

In a scenario of a warming Arctic these intrusions events are expected to increase in frequency and intensity therefore posing concerns about the contamination of a pristine environment with hazardous materials such as nuclear wastes and pollutants in general.

## CRediT authorship contribution statement

DC, JCG, AS: Conceptualization, Methodology, Writing – review & editing. ZE, JS, LB, BL, AN, CL, PT, SC, FB, BM: Investigation, Data curation, Writing – review & editing. DC: Writing – original draft. DC, AS, BM, CL: Data curation, Writing – review & editing.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data Availability

Data will be made available on request.

## Acknowledgments

All the logistic activities were based at the Polish Polar Station Hornsund (WMO 01003) under the Interact access project BC-HOR. The Research Council of Norway (RCN) that funds the Svalbard Strategic Grant BC-3D (project n. 283466) is also acknowledged. All the data produced in this work are available upon request to the authors.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jhazmat.2023.131317.

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