Anaelle Magre

Beatrice Boulet

Anne De

Vismes Ott

Olivier Evrard

Laurent Pourcelot
email: laurent.pourcelot@irsn.fr

Anne De Vismes

Identification of the origin of radiocesium released into the environment in areas remote from nuclear accident and military test sites using the 135 Cs/ 137 Cs isotopic signature

Keywords: Environmental monitoring, radioactive contamination, isotopic signatures, ICP-MS/MS

Introduction

Since the 1950s, nuclear activities have been undertaken to develop medical, industrial, energy or military applications [START_REF] Snow | Method for Ultra-Trace Cesium Isotope Ratio Measurements from Environmental Samples Using Thermal Ionization Mass Spectrometry[END_REF] . These activities resulted in radioactive releases of about 8.1x10 [START_REF] Taylor | Preliminary Evaluation of (135)Cs/(137)Cs as a Forensic Tool for Identifying Source of Radioactive Contamination[END_REF] Bq 2,3 . Radiocesium isotopes including 134 Cs (T1/2 = 2.1 y), 135 Cs (T1/2 = 2.3.10 6 y) and 137 Cs (T1/2 = 30.2 y) rank among the radionuclides that were released in highest quantities [START_REF]Ionizing Radiation: Sources and Biological Effects[END_REF][START_REF] Unscear | Levels and effects of radiation exposure due to the accident at the Fukushima Daiichi nuclear power station: implications of information published since the UNSCEAR[END_REF] . With more than 1.4x10 [START_REF] Aoyama | Long-Range Transport of Radiocaesium Derived from Global Fallout and the Fukushima Accident in the Pacific Ocean since 1953 through 2017-Part I: Source Term and Surface Transport[END_REF] Bq dispersed into the environment and its relatively long half-life, 137 Cs is considered as the main source of long-term artificial radiation to the public [START_REF]Ionizing Radiation: Sources and Biological Effects[END_REF][START_REF] Unscear | Levels and effects of radiation exposure due to the accident at the Fukushima Daiichi nuclear power station: implications of information published since the UNSCEAR[END_REF][START_REF] Dunne | Spatial Pattern of Plutonium and Radiocaesium Contamination Released during the Fukushima Daiichi Nuclear Power Plant Disaster[END_REF][START_REF] Mahmood | Marine Radioactivity of Cs-134 and Cs-137 in the Malaysian Economic Exclusive Zone after the Fukushima Accident[END_REF][START_REF] Aoyama | Long-Range Transport of Radiocaesium Derived from Global Fallout and the Fukushima Accident in the Pacific Ocean since 1953 through 2017-Part I: Source Term and Surface Transport[END_REF] . Its monitoring in the environment is therefore of major interest to evaluate the risks incurred by the population and ecosystems [START_REF] Brimo | Long Term Decrease of 137Cs Bioavailability in French Pastures: Results from 25 Years of Monitoring[END_REF][START_REF] Chao | Determination of 135Cs by Neutron Activation Analysis[END_REF][START_REF] Qiao | Speciation Analysis of 129I, 137Cs, 232Th, 238U, 239Pu and 240Pu in Environmental Soil and Sediment[END_REF][START_REF] Zok | Determination of Characteristic vs Anomalous 135Cs/137Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples[END_REF] . It is therefore essential to be able to identify unambiguously the origin of radioactive contamination and to anticipate their potential impacts. Three main radioactive contamination sources of 137 Cs are distinguished in the environment, even if the proportion of each varies from an environmental sample to another. Thus 137 Cs may result from nuclear weapons testing (NWT), nuclear power plant accidents (Chernobyl in 1986 and Fukushima in 2011) and authorized discharges from nuclear facilities 2, [START_REF] Bérard | + Baryum 137m[END_REF][START_REF] Gray | Discharges to the Environment from the Sellafield Site, 1951-1992[END_REF][START_REF] Buesseler | Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity[END_REF] . 137 Cs is easily measured by gamma-ray spectrometry [START_REF] Mahmood | Marine Radioactivity of Cs-134 and Cs-137 in the Malaysian Economic Exclusive Zone after the Fukushima Accident[END_REF][START_REF] Colle | Fiche Radionucléide -Césium 137 et Environnement[END_REF][START_REF] De Vismes Ott | Fission Product Activity Ratios Measured at Trace Level over France during the Fukushima Accident[END_REF] . However, due to its potential supply from contrasted sources and its accumulation in the environment for several decades, its measurement alone does not allow to identify the origin of this contamination. To discriminate it, a method has been proposed based on the occurrence of contrasted isotopic signatures between the different radiocesium sources [START_REF] Zettwoog | Utilisation de La Signature Isotopique Des Radionucléides Relâchés Par Les Mines et Les Usines de Traitement de Minerais d'uranium Pour Discriminer Aux Bas Niveaux Leur Impact Environnemental de Celui de La Radioactivité Tellurique Naturelle[END_REF][START_REF] Moreno | Determination of Caesium and Its Isotopic Composition in Nuclear Samples Using Isotope Dilution-Ion Chromatography-Inductively Coupled Plasma Mass Spectrometry[END_REF][START_REF] Taylor | Preliminary Evaluation of (135)Cs/(137)Cs as a Forensic Tool for Identifying Source of Radioactive Contamination[END_REF] . The isotopic ratios are commonly employed to understand the mechanisms of release, transport, or redistribution of radionuclides in the environment.

Knowledge of radionuclide signatures is also of major interest for characterizing the radiological background to assess the specific contribution of potential future contamination events. In this context, several ratios have been employed to identify and characterize 137 Cs sources in the environment such as the 239+240 Pu/ 137 Cs activity ratio [START_REF] Meusburger | Plutonium Aided Reconstruction of Caesium Atmospheric Fallout in European Topsoils[END_REF][START_REF] Alewell | Suitability of 239+240Pu and 137Cs as Tracers for Soil Erosion Assessment in Mountain Grasslands[END_REF][23][START_REF] Cizdziel | Resolving Nevada Test Site and Global Fallout Plutonium in Attic Dust and Soils Using 137Cs/239+240Pu Activity Ratios[END_REF][START_REF] Hodge | Radiocesium and Plutonium: Still Together in "Background" Soils after More than Thirty Years[END_REF][START_REF] Bunzl | Cumulative Deposition of 137Cs, 238Pu, 239+240Pu and 241Am from Global Fallout in Soils from Forest, Grassland and Arable Land in Bavaria (FRG)[END_REF] . Nevertheless, the hypothesis of fractionation of two elements (Pu and Cs) with different bio-geochemical properties could not be excluded. As an alternative, the radiocesium isotopic signature was also used for many years by directly measuring the 134 Cs/ 137 Cs ratio by gamma-ray spectrometry [START_REF] Buesseler | Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity[END_REF][START_REF] De Vismes Ott | Fission Product Activity Ratios Measured at Trace Level over France during the Fukushima Accident[END_REF][START_REF] Carbol | Deposition and Distribution of Chernobyl Fallout Fission Products and Actinides in a Russian Soil Profile[END_REF][START_REF] Chino | Utilization of 134Cs/137Cs in the Environment to Identify the Reactor Units That Caused Atmospheric Releases during the Fukushima Daiichi Accident[END_REF][START_REF] Shibahara | Analysis of Cesium Isotope Compositions in Environmental Samples by Thermal Ionization Mass Spectrometry -1. A Preliminary Study for Source Analysis of Radioactive Contamination in Fukushima Prefecture[END_REF][START_REF] Thomas | First Assessment of Chernobyl Radioactive Plume over Paris[END_REF] . However, the use of this ratio is limited to a period of maximum ten years following the release due to the relatively short half-life of 134 Cs, a radionuclide that is no longer detectable in areas lowly affected by Fukushima fallout [START_REF] Zok | Determination of Characteristic vs Anomalous 135Cs/137Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples[END_REF][START_REF] Russell | Determination of Precise 135Cs/137Cs Ratio in Environmental Samples Using Sector Field Inductively Coupled Plasma Mass Spectrometry[END_REF][START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF] . In this context, an alternative was recently proposed by analyzing the much longer-lived 135 Cs (T1/2 = 2.3.10 6 y). The discriminating power of the 135 Cs/ 137 Cs isotopic ratio was demonstrated in several studies, which aims at identifying nuclear contamination sources and for improving our understanding of the dispersion patterns of anthropogenic radionuclides [START_REF] Snow | Method for Ultra-Trace Cesium Isotope Ratio Measurements from Environmental Samples Using Thermal Ionization Mass Spectrometry[END_REF][START_REF] Taylor | Preliminary Evaluation of (135)Cs/(137)Cs as a Forensic Tool for Identifying Source of Radioactive Contamination[END_REF][START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Yang | 135Cs Activity and 135Cs/137Cs Atom Ratio in Environmental Samples before and after the Fukushima Daiichi Nuclear Power Plant Accident[END_REF][START_REF] Bu | Ultra-Trace Determination of the 135Cs/137Cs Isotopic Ratio by Thermal Ionization Mass Spectrometry with Application to Fukushima Marine Sediment Samples[END_REF] .

The characteristic 135 Cs/ 137 Cs atomic ratios reported in the literature for fallout from the NWT, the Chernobyl accident and the Fukushima accident were 3.70 ± 0.68 [START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Lee | First Detection of Fallout Cs-135 and Potential Applications of 137Cs /135Cs Ratios[END_REF] , 0.66 ± 0.03 [START_REF] Snow | Method for Ultra-Trace Cesium Isotope Ratio Measurements from Environmental Samples Using Thermal Ionization Mass Spectrometry[END_REF][START_REF] Taylor | Preliminary Evaluation of (135)Cs/(137)Cs as a Forensic Tool for Identifying Source of Radioactive Contamination[END_REF][START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Zheng | Determination of 135Cs and 135Cs/137Cs Atomic Ratio in Environmental Samples by Combining Ammonium Molybdophosphate (AMP)-Selective Cs Adsorption and Ion-Exchange Chromatographic Separation to Triple-Quadrupole Inductively Coupled Plasma-Mass Spectrometry[END_REF][START_REF] Dunne | Procedures for Precise Measurements of 135Cs/137Cs Atom Ratios in Environmental Samples at Extreme Dynamic Ranges and Ultra-Trace Levels by Thermal Ionization Mass Spectrometry[END_REF] and 0.45 ± 0.02 [START_REF] Zok | Determination of Characteristic vs Anomalous 135Cs/137Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples[END_REF][START_REF] Yang | 135Cs Activity and 135Cs/137Cs Atom Ratio in Environmental Samples before and after the Fukushima Daiichi Nuclear Power Plant Accident[END_REF][START_REF] Zheng | Determination of 135Cs and 135Cs/137Cs Atomic Ratio in Environmental Samples by Combining Ammonium Molybdophosphate (AMP)-Selective Cs Adsorption and Ion-Exchange Chromatographic Separation to Triple-Quadrupole Inductively Coupled Plasma-Mass Spectrometry[END_REF][START_REF] Ohno | Determination of Radioactive Cesium Isotope Ratios by Triple Quadrupole ICP-MS and Its Application to Rainwater Following the Fukushima Daiichi Nuclear Power Plant Accident[END_REF] (decay-corrected to January 1 st , 2022), respectively. Nevertheless, it is noteworthy that very few data were reported for the NWT.

Despite the potential usefulness of the 135 Cs/ 137 Cs ratio, few data are available in environmental samples collected away from exclusion zones and former nuclear testing areas, due to several analytical challenges associated with the 135 Cs quantification [START_REF] Song | Rapid and Sensitive Determination of Radiocesium (Cs-135, Cs-137) in the Presence of Excess Barium by Electrothermal Vaporization-Inductively Coupled Plasma-Mass Spectrometry (ETV-ICP-MS) with Potassium Thiocyanate as Modifier[END_REF][START_REF] Russell | Determination of 135Cs and 137Cs in Environmental Samples: A Review[END_REF] . Its measurement was previously extremely difficult, if not impossible, by the use of radiometric methods because of the 135 Cs low-beta emissions (β -, Emax 268.9 keV) [START_REF] Yang | 135Cs Activity and 135Cs/137Cs Atom Ratio in Environmental Samples before and after the Fukushima Daiichi Nuclear Power Plant Accident[END_REF][START_REF] Liezers | Low Level Detection of 135Cs and 137Cs in Environmental Samples by ICP-MS[END_REF] . The most suitable measurement technique for 135 Cs is therefore mass spectrometry [START_REF] Lehto | Chemistry and Analysis of Radionuclides: Laboratory Techniques and Methodology[END_REF] such as ICP-MS/MS, first commercialized in 2013 by Agilent, requiring a specific sample treatment [START_REF] Zok | Determination of Characteristic vs Anomalous 135Cs/137Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples[END_REF][START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Magre | Improved Radiocesium Purification in Low-Level Radioactive Soil and Sediment Samples Prior to 135Cs/137Cs Ratio Measurement by ICP-MS/MS[END_REF] . However, in the environment and in zones that were not exposed to accidental or military radionuclide releases, only a low amount of radiocesium is available, while much higher levels of several elements causing spectral interferences at m/z 135 and 137 ( 135 Ba, 137 Ba, 95 Mo 40 Ar, 97 Mo 40 Ar, 119 Sn [START_REF] Colle | Fiche Radionucléide -Césium 137 et Environnement[END_REF] O, 121 Sb 16 O) are found [START_REF] Epov | Collision Cell Chemistry for the Analysis of Radioisotopes by Inductively Coupled Plasma Mass Spectrometry[END_REF] . To overcome these difficulties, a high Cs-selective radiochemistry procedure must be applied to a large quantity of sample (resulting in complex chemistry) and it must be combined with an efficient mass spectrometry measurement to remove any trace of interferents before the quantification of the 135 Cs/ 137 Cs isotopic ratio [START_REF] Zok | Determination of Characteristic vs Anomalous 135Cs/137Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples[END_REF][START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Zheng | Determination of 135Cs and 135Cs/137Cs Atomic Ratio in Environmental Samples by Combining Ammonium Molybdophosphate (AMP)-Selective Cs Adsorption and Ion-Exchange Chromatographic Separation to Triple-Quadrupole Inductively Coupled Plasma-Mass Spectrometry[END_REF][START_REF] Dunne | Procedures for Precise Measurements of 135Cs/137Cs Atom Ratios in Environmental Samples at Extreme Dynamic Ranges and Ultra-Trace Levels by Thermal Ionization Mass Spectrometry[END_REF] .

The goal of the current research was therefore to investigate the applicability of the 135 Cs/ 137 Cs isotopic signature for identifying contrasted 137 Cs sources and estimating their respective contributions to radiocesium found in environmental samples collected in an area remote from nuclear accident and test sites (i.e., mainland France). To achieve this objective, twentyone soil or sediment samples (matrices known to strongly absorb 137 Cs by clay minerals) were collected in two main regions preferentially impacted by either fallout from atmospheric nuclear weapons tests, the Chernobyl accident or both sources [START_REF] Meusburger | Plutonium Aided Reconstruction of Caesium Atmospheric Fallout in European Topsoils[END_REF]23 . Indeed, 137 Cs deposited in France soon after the releases from the Fukushima accident is considered to be negligible in comparison with 137 Cs from Chernobyl accident (500 to more than 1000 lower levels) [START_REF] Evrard | Evidence of the Radioactive Fallout in France Due to the Fukushima Nuclear Accident[END_REF] . The 135 Cs/ 137 Cs isotopic ratio were then quantified by ICP-MS/MS after extracting and purifying the cesium from the solid matrix according to the method recently proposed by Magré et al. (2022) [START_REF] Magre | Improved Radiocesium Purification in Low-Level Radioactive Soil and Sediment Samples Prior to 135Cs/137Cs Ratio Measurement by ICP-MS/MS[END_REF] .

The data obtained on the soils and sediments samples taken in France were then used to estimate the contribution of each source by comparing the original 135 Cs/ 137 Cs ratios determined in this research with the characteristic ratios of 137 Cs sources reported in the literature. In addition, the proportion of each source estimated with this new tracer ( 135 Cs/ 137 Cs) was compared with those given by other more commonly used diagnostic ratios such as 239+240 Pu/ 137 Cs.

Experimental section

Environmental samples.

Soils and sediments collected in France. The selection of environmental samples was based on previous work namely the cartographic reconstruction of the 137 Cs theorical deposits in France following NWT or the Chernobyl accident. To characterize the fallout from each of these 137 Cs sources, samples from two regions preferentially impacted by either NWT or Chernobyl fallout were studied. The investigated areas are respectively located in the mountainous areas of Pyrenees and in the Mercantour (southern French Alps), both regions being devoid of any nuclear facility, so that NWT and Chernobyl fallout provide the only sources of artificial radioactivity. The environmental matrices analyzed were soils and lake sediments, and they were chosen because of their strong capacity to fix radiocesium onto clay minerals. Indeed, in these matrices, 137 Cs activities ranged from a few Bq.kg -1 to a few tens of Bq.kg -1 and could reach several thousands of Bq.kg -1 in mountainous areas [START_REF] Pourcelot | Formation of Radioactivity Enriched Soils in Mountain Areas[END_REF][START_REF] Roussel-Debel | 137Cs in French Soils: Deposition Patterns and 15-Year Evolution[END_REF] .

In this study, ten lake sediment, four surface soil (0-5 cm depth) and three subsurface soil (5-10 cm depth) samples collected in the Mercantour massif between 2013 and 2021 were analyzed (Figure 1). Four soil samples collected in 2014 in the Pyrenees were also investigated, including three surface soils (0-10 cm depth) and one subsurface soil (10-20 cm depth). Comprehensive data on the sampling of these sediment and soil samples is provided in Table S1 in the supplementary material section.

Accuracy verification of the applied method. Due to the lack of certified reference samples in 135 Cs, the validity of the results on the collected soils and sediments was ensured by analyzing, two types of materials. The first was IAEA-330 and IAEA-375 certified reference materials (CRM) from the International Atomic Energy Agency (IAEA). Both CRMs were selected because of their frequent use in the validation of 135 Cs/ 137 Cs analysis protocols [START_REF] Snow | Method for Ultra-Trace Cesium Isotope Ratio Measurements from Environmental Samples Using Thermal Ionization Mass Spectrometry[END_REF][START_REF] Zok | Determination of Characteristic vs Anomalous 135Cs/137Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples[END_REF][START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Zheng | Determination of 135Cs and 135Cs/137Cs Atomic Ratio in Environmental Samples by Combining Ammonium Molybdophosphate (AMP)-Selective Cs Adsorption and Ion-Exchange Chromatographic Separation to Triple-Quadrupole Inductively Coupled Plasma-Mass Spectrometry[END_REF][START_REF] Dunne | Procedures for Precise Measurements of 135Cs/137Cs Atom Ratios in Environmental Samples at Extreme Dynamic Ranges and Ultra-Trace Levels by Thermal Ionization Mass Spectrometry[END_REF][START_REF] Epov | Collision Cell Chemistry for the Analysis of Radioisotopes by Inductively Coupled Plasma Mass Spectrometry[END_REF] . These samples of spinach (IAEA-330) and soil (IAEA-375) were respectively collected in Ukraine and Russia in 1990 and they show a 135 Cs/ 137 Cs ratio characteristic of Chernobyl fallout. In addition, three sediment samples exposed to high levels of Fukushima fallout were used. These sediment samples were collected in 2012, 2015 and 2020 in the Niida River catchment, which drains a part of the main radioactive plume in the Fukushima Prefecture, Northeastern Japan [START_REF] Evrard | Radionuclide Contamination in Flood Sediment Deposits in the Coastal Rivers Draining the Main Radioactive Pollution Plume of Fukushima Prefecture, Japan (2011-2020[END_REF] . Validity of the analytical method was evaluated by comparing the 135 Cs/ 137 Cs ratios measured in the IAEA and Niida samples with the published 135 Cs/ 137 Cs isotopic signature for the Chernobyl and Fukushima accidents, respectively 1,12,20,32,33,36- 38 .

Figure 1: Location of sampling sites in France

Estimated contribution from each source of 137 Cs

Using bi-elemental isotopic signature: 239+240 Pu/ 137 Cs. Nuclear Weapons Tests is considered the only source of airborne Pu in France [START_REF] Duffa | Répartition du Plutonium et de l'Américium dans l'environnement terrestre de la basse vallée du Rhône[END_REF][START_REF] Duffa | Activités de 238Pu et de 239+240Pu dans les sols cultivés de la basse vallée du Rhône[END_REF] . Indeed after 1 st may 1986, the contaminated air masses following Chernobyl accident contained mostly volatile fission products with negligible activity of plutonium [START_REF] Ballestra | Fallout Deposition at Monaco Following the Chernobyl Accident[END_REF] . Thus in the current work, assuming transuranics solely derived from NWT, we have used 239+240 Pu/ 137 Cs previously derived for NWT [START_REF] Hodge | Radiocesium and Plutonium: Still Together in "Background" Soils after More than Thirty Years[END_REF] , for which an activity 239+240 Pu/ 137 Cs ratio of 0.04 ± 0.01 was found (decay-corrected to January 1 st , 2022) [START_REF] Hodge | Radiocesium and Plutonium: Still Together in "Background" Soils after More than Thirty Years[END_REF] and the 239+340 Pu activity concentration (Bq.kg -1 ) of the samples (namely 239+340 Pusamp) to deduced the 137 ) 𝑁𝑊𝑇 atomic ratios corresponded to the characteristic atomic ratios of both sources reported in the literature: 0.66 ± 0.03 and 3.70 ± 0.68 (decay-corrected to January 1 st , 2022). It should be noted that the 135 Cs/ 137 Cs characteristic ratio of NWT reported was a "simple" average ratio considering the number of years since the period of testing (1950s to 1980s), assuming a 135 Cs/ 137 Cs atomic ratio of 1 at the date of each detonation [START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Lee | First Detection of Fallout Cs-135 and Potential Applications of 137Cs /135Cs Ratios[END_REF] .

The associated uncertainty was therefore significant compared, for example, to that of Chernobyl accident, which occurred at a specific date.

For each analyzed sample, the proportion of 137 Cs resulting from the Chernobyl accident (𝑥 𝐶ℎ𝑒𝑟𝑛 )

was estimated from equation (3) using a simple two end-member un-mixing model (Eq. 4).

Methods applied to determine 137 Cs and 241 Am activities

Gamma-ray spectrometry measurements. Once collected, the soil and sediment samples were dried, crushed, sieved (< 2 mm) and then calcined at 500°C overnight in a muffle oven. The resulting ash was packed into a 60 mL Petri box (Øint=71 mm; hint=15 mm) and measured by gamma-ray spectrometry. This first measurement aimed to assess the initial 137 Cs activity to determine the amount of sample to be analyzed to quantify the 135 Cs/ 137 Cs isotopic ratio and evaluate the chemical recoveries. The 241 Am activity was also measured by gamma-ray spectrometry.

The two gamma detectors employed were High-Purity Germanium (HPGe) distributed by Ortec with a crystal volume about 180 cm 3 . For the first detector, the relative detection efficiency and resolution on the peak corresponding to 60 Co at 1332 keV were respectively 53% and 1.7 keV (IRSN/LMRE). The corresponding values amounted to 48% and 1.9 keV for the second detector (LSCE). The HPGe were previously calibrated in energy, resolution, and detection efficiency on the 40-1700 keV energy range using certified multigamma sources prepared by IRSN (IRSN/LEI, France).

The activities of 137 Cs and 241 Am were respectively quantified using their characteristic emissions at 661.6 keV and 59.5 keV with long counting times (up to 24h) to improve the measurement statistics.

The spectra analysis was performed with the GENIE 2000 software operated by Mirion Tech. Inc.

and the "home-made" MadOni software. Then, corrections were applied to consider the selfattenuation effect according to the density and the matrix of the analyzed sample, and the used geometry.

In five of the samples in which 137 Cs contents exceeded 2x10 4 Bq.kg -1 , the 241 Am activity could not be quantified by gamma-ray spectrometry due to the very high Compton effect caused by the significant presence of 137 Cs making difficult the discrimination of the peaks at low energy. In these samples, the 241 Am activity was measured by alpha spectrometry according to the analytical procedure described by Goutelard et al. (1998) [START_REF] Goutelard | Alpha-Spectrometry Measurement of Am and Cm at Trace Levels in Environmental Samples Using Extraction Chromatography[END_REF] (see below) and used routinely in the laboratory (IRSN/LMRE) under COFRAC accreditation.

Alpha spectrometry for 241 Am activity measurements. For each sample, 1 to 2 grams of ash were introduced into a beaker and spiked with 243 Am to determine the chemical yield and the 241 Am activity. The samples were then leached at 150°C during 24h with concentrated HNO3 and HCl in presence of hydrogen peroxide (H2O2). The solid residue was separated from the supernatant by centrifugation, leached again for 2h at 150 °C with concentrated HNO3 and rinsed with ultrapure water. The supernatants and rinse were combined. 15 g of oxalic acid were added to the solution with 5 mL of CaCl2 at 100 mg.g -1 and the mixture was adjusted to pH 1.5 to form a calcium oxalate precipitate. After centrifugation, the precipitate was dried and destroyed by heating with 10 mL of concentrated HNO3 and 5 mL of H2O2. The mixture was evaporated to dryness and the residue was dissolved in 40 mL of 8M HNO3 with 0.2 g of NaNO2. The americium was then purified using a series of four ion exchange resins. In detail, Am was separated from Pu(IV) using 6.4 g of AG®1-X8 resin (100 -200 mesh, Cl -form, Biorad Tech.) conditioned with 30 mL of 8M HNO3. The sample was loaded and then the resin rinsed with 40 mL of 8M HNO3. The sample eluate and rinses were combined, evaporated to dryness and the residue was dissolved in 10 mL of 9M HCl. Am was separated from Fe and U using a double column containing 6.5 g of AG®1-X8 resin and 2 g of AG®50W-X8 resin (100 -200 mesh, H + form, Biorad Tech.). The sample loading and rinsing fractions (45 mL of 9M HCl) were again combined and evaporated. The residue was dissolved in 10 mL of 2M HNO3/0.5M Al(NO3)3. The solution was purified of metals (Cu, Ni) with 0.7 g of TRU resin (100-150 μm, Triskem Int.). Once the sample was loaded and the resin rinsed (10 mL of 2M HNO3), Am was eluted with 25 mL of 0.03M HNO3. The eluate was evaporated to dryness and then redissolved in 20 mL of 1M HNO3/93% CH3OH mixture. Finally, Am was separated from Rare Earth Elements using 4 g of AG®1-X4 resin (100 -200 mesh, Cl -form, Biorad Tech.). The sample was loaded onto the resin and rinsed successively with 15 mL of 1M HNO3/93% CH3OH, 80 mL of 0.1M HCl/0.5M NH4SCN/80% CH3OH and 22 mL of 1M HNO3/93%. Am was eluted from the AG®1-X4 resin with 75 mL of 1.5M HCl/86% CH3OH mixture. The eluate was evaporated and dissolved was concentrated HNO3, ultrapure H2O and Na2SO4. The solution was adjusted to pH 1.90-1.95 and Am was electroplated for 2h at 1A on a stainless-steel disk. The disk was counted for 5 days using a lowlevel background α-spectrometer Alpha-Analyst (Canberra). The activity of 241 Am were quantified using its emitted-alpha particles at 5,485 keV.

It should be noted that all activities presented in this work were given with an uncertainty at k=2.

For the 137 Cs and 241 Am measured activities, this uncertainty considered those related to counting statistics and detector calibration. Regarding the uncertainties of the 137 CsNWT and 239+240 Pu activities deduced by equation 1 and 2, they included the 241 Am measurement uncertainty and those associated with the 239+240 Pu/ 137 Cs and 241 Am/ 239+240 Pu reference ratios.

Method applied for 135 Cs/ 137 Cs isotopic ratio determination

Cs extraction and purification from solid matrices. The procedure of Cs extraction and purification from the environmental matrix was based on the analytical method described in our previous work [START_REF] Magre | Improved Radiocesium Purification in Low-Level Radioactive Soil and Sediment Samples Prior to 135Cs/137Cs Ratio Measurement by ICP-MS/MS[END_REF] . 1 to 100 g of sample ash (depending on the measured 137 Cs activity) were introduced into a beaker. The radiocesium was extracted from the solid matrix by a succession of four acid leaching steps of 8 hours on a hot plate with temperature set at 180°C. The first leaching was performed with 69.5% HNO3 in presence of 30% H2O2 provided by Carlo Erba. The following leaching were carried out with a 69.5% HNO3/37.5% HCl mixture (1:2, v/v) still in the presence of H2O2. Between successive leaching steps, the solid residue was separated from the supernatant by centrifugation.

The supernatants were combined, evaporated to near dryness. The pH of the solution was adjusted to pH 1 -2 by diluting the residue with ultrapure H2O milli-Q H2O (with resistivity 18.2 MΩ.cm -1 )

produced by a Merck Millipore system. 120 mg of ammonium molybdophosphate (AMP) powder (purchased from Alfa Aesar) were added to the solution to selectively complex Cs. The solution was then stirred for 30 minutes at room temperature and centrifuged for 15 minutes at 3500 rpm. After centrifugation, the supernatant was removed and the yellow precipitate containing Cs was filtered on a polyvinylidene fluoride membrane filter (PVDF, 0.45 µm pore size, Merck Millipore). The precipitate was washed with 5 mL of milli-Q H2O and dissolved with 24 mL of 2M NH4OH resulting in the complete decomposition of the AMP molecule and consequently, the release of Cs. Cs was then purified of all its interferents by combining the chemical properties of two extraction resins provided by Biorad Technologies. First, 3 mL of AG®MP-1M anion exchange resin (100 -200 mesh, Cl -form) conditioned with 1.5M NH4OH were used to separate Cs from polyatomic interferents such as Mo, Sb and Sn. Under these conditions, Cs + ions were not adsorbed on the resin and were collected in the loading and rinsing fractions (2 x 3 mL of 1.5M NH4OH), whereas Mo, Sb and Sn, present in oxide form, remained on it. Finally, Cs was separated from its isobaric interferent (Ba) using 8 mL of AG®50W-X8 cationic resin (100 -200 mesh, H + form) conditioned with 20 mL of milli-Q H2O followed by 20 mL of 1.5M NH4OH. After the loading of the sample, the resin was rinsed with 50 mL of 1.5M NH4OH and converted by 20 mL of milli-Q water followed by 20 mL of ultrapure 1.5M HNO3. Cs was selectively eluted with 50 mL of ultra-pure 1.5M HNO3 while Ba remained fixed on the AG®50W-X8 resin. The eluate was measured by gamma-ray spectrometry, to estimate the chemical yield by monitoring 137 Cs. Then, it was evaporated to dryness and dissolved in 5 mL of ultrapure 0.5M HNO3 to be measured by mass spectrometry. flow rate. The sensitivity and oxide rate settings were optimized using a multi-element solution concentrated to 2.5 ng.g -1 in Cs, Ce and Ba prepared from certified solutions (Carlo Erba). Each measurement was corrected with an acid blank. All the experimental conditions are listed in Table S2.

ICP-MS

The interferent (Ba, Mo, Sb, Sn) contents were monitored in single quadrupole mode without reaction gas and calculated by an external calibration method. The standards were prepared from ultra-pure milli-Q water and certified mono-elemental solutions provided by Carlo Erba. The matrix effect was corrected using indium as internal standard (concentrated to 3 ng.g -1 ).

The 135 Cs/ 137 Cs isotopic ratios were quantified in triple quadrupole mode by setting Q1 and Q2 to the 135 and 137 m/z. To effectively remove the last traces of interferences, a mixture of N2O (99.998% purity, Messer), He (99.9992% purity, Air product) and NH3/He (10/90 m/m, 99.98% purity, Air product) was added in the CRC with flow rates of about 0.55, 1 and 4.79 mL.min -1 , respectively.

Isotopic ratios were corrected for mass bias by the sample standard bracketing approach with a solution with a known 135 Cs/ 137 Cs ratio delivered by DES-SEARS (CEA). The uncertainty associated with the corrected ratio were also given at k=2. It considered the variation of the ICP-MS/MS signal during the measurement of the sample and the uncertainty of the mass bias correction.

Evaluation of the chemical performance. The chemical performances (recovery yield and eluate purity) achieved for each sample are detailed in Table S3. Recovery yields were estimated by monitoring 137 Cs by gamma-ray spectrometry in both the solid and purified liquid samples. They ranged from 57% to 93%. Regarding potential spectral interferences during the analyses, the radiochemical treatment of the samples allowed to significantly reduce the levels of Ba, Mo, Sb and Sn in the measurement fraction, with contents ranging from 0.1 to 5.4, 0.13 to 17.5, 0.03 to 0.46, and 0.2 to 12.2 ng.g -1 , respectively. To accurately measure the 135 Cs/ 137 Cs ratio, these traces of interferents that remained present in solution were effectively removed by adding a mixture of reaction gases (N2O, NH3, He) in the CRC of the mass spectrometer.

Results and Discussion

Tracing the origin of radiocesium contamination by measuring 137 Cs and 241 Am activities by nuclear counting -Use of bi-elemental isotopic signatures.

The activities of 137 Cs and 241 Am in lake sediment, surface soil and subsurface soil samples collected in France are presented in Table 1. Regarding 137 Cs, a significant heterogeneity of activities was found between the two main sampling regions (Mercantour Massif or the Pyrenees) as well as, within the same region, between the analyzed matrices (sediment or soil). The highest variability was obtained in the Mercantour soils (including surface and subsurface soils) with values ranging from a few tens (165 ± 11) to several tens of thousands (103,308 ± 113) of Bq.kg -1 . Such a strong heterogeneity is consistent with "enrichment spots", i.e. surfaces of a few dm² strongly concentrating 137 Cs from Chernobyl fallout [START_REF] Pourcelot | Formation of Radioactivity Enriched Soils in Mountain Areas[END_REF] . In contrast, 137 Cs activities measured in the soils of the Pyrenees exhibited low heterogeneity and were about 2 to 7 times lower than in the Mercantour, with levels ranging between 23 ± 4 and 93 ± 28 Bq.kg -1 . An explanation to this lower heterogeneity is that the nuclear weapon tests occurred over a longer period (between the 1950s and 1980s).

Overall, for both regions, 137 Cs activity decreased with increasing sampling depths. For the sediments from the Mercantour lakes, the 137 Cs concentrations were significant and relatively homogeneous compared to the soils, with activities varying from 249 ± 19 to 5,274 ± 350 Bq.kg -1 .

For each site and each matrix studied, the distribution of 137 Cs activities was in good agreement with that reported in the literature [START_REF] Meusburger | Plutonium Aided Reconstruction of Caesium Atmospheric Fallout in European Topsoils[END_REF][START_REF] Pourcelot | Formation of Radioactivity Enriched Soils in Mountain Areas[END_REF][START_REF] Roussel-Debel | 137Cs in French Soils: Deposition Patterns and 15-Year Evolution[END_REF] . High to very high levels of 137 Cs were expected in the Mercantour resulting from a superposition of deposition from NWT and the Chernobyl accident. In contrast, the Pyrenees soils showed lower activities, which was expected as they were hypothesized to have been mainly supplied by the NWT. These preliminary interpretations were confirmed with the assessment of the 137 Cs contributions of each source using the 241 Am/ 239+240 Pu and then the 239+240 Pu/ 137 Cs ratios provided for the NWT (see experimental section) (Table 1). They show, overall, that the proportion of 137 Cs from Chernobyl fallout increases significantly with the total 137 Cs activity while the 137 Cs activity resulting from the NWT remains relatively homogenous (28 to 606 Bq.kg -1 )

for both studied mountainous areas and both environmental matrix (cf. Figure 2). Thus, the use of bi-elemental signatures ( 241 Am/ 239+240 Pu and 239+240 Pu/ 137 Cs) allowed, in a first step, to estimate the contributions of each 137 Cs source. Nevertheless, the hypothesis of the fractionation of two elements with different bio-geochemical properties (i.e., Am vs. Pu, Pu vs. Cs) cannot be fully excluded, although they were described in the literature to be relatively similar [START_REF] Chawla | Atmospheric Deposition and Migration of Artificial Radionuclides in Alpine Soils (Val Piora, Switzerland) Compared to the Distribution of Selected Major and Trace Elements[END_REF][START_REF] Bundt | Impact of Preferential Flow on Radionuclide Distribution in Soil[END_REF] . Tracing the origin of radiocesium contamination using the 135 Cs/ 137 Cs isotopic ratio measured by ICP-MS/MS -Use of mono-elemental isotopic signature.

The analytical procedure to quantify the 135 Cs/ 137 Cs isotopic ratio in environmental samples was first applied to samples known to be contaminated with Chernobyl (IAEA-330 and IAEA-375) [START_REF] Snow | Method for Ultra-Trace Cesium Isotope Ratio Measurements from Environmental Samples Using Thermal Ionization Mass Spectrometry[END_REF][START_REF] Taylor | Preliminary Evaluation of (135)Cs/(137)Cs as a Forensic Tool for Identifying Source of Radioactive Contamination[END_REF][START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Zheng | Determination of 135Cs and 135Cs/137Cs Atomic Ratio in Environmental Samples by Combining Ammonium Molybdophosphate (AMP)-Selective Cs Adsorption and Ion-Exchange Chromatographic Separation to Triple-Quadrupole Inductively Coupled Plasma-Mass Spectrometry[END_REF][START_REF] Dunne | Procedures for Precise Measurements of 135Cs/137Cs Atom Ratios in Environmental Samples at Extreme Dynamic Ranges and Ultra-Trace Levels by Thermal Ionization Mass Spectrometry[END_REF] or Fukushima fallout (Niida_2012, Niida_2015 and Niida_2020) [START_REF] Evrard | Radionuclide Contamination in Flood Sediment Deposits in the Coastal Rivers Draining the Main Radioactive Pollution Plume of Fukushima Prefecture, Japan (2011-2020[END_REF] . The analytical results are presented in Table 2 and they were compared with the few data available in the literature for both sources of accidental contamination. To facilitate the comparison of results, all isotopic ratios were decay-corrected to January 1 st , 2022. For both IAEA reference samples, which are generally used in the literature to validate the 135 Cs/ 137 Cs ratio analysis methods, the measured ratios (0.66 ± 0.06 and 0.67 ± 0.02) agreed well with those reported in the literature [START_REF] Snow | Method for Ultra-Trace Cesium Isotope Ratio Measurements from Environmental Samples Using Thermal Ionization Mass Spectrometry[END_REF][START_REF] Taylor | Preliminary Evaluation of (135)Cs/(137)Cs as a Forensic Tool for Identifying Source of Radioactive Contamination[END_REF][START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Zheng | Determination of 135Cs and 135Cs/137Cs Atomic Ratio in Environmental Samples by Combining Ammonium Molybdophosphate (AMP)-Selective Cs Adsorption and Ion-Exchange Chromatographic Separation to Triple-Quadrupole Inductively Coupled Plasma-Mass Spectrometry[END_REF][START_REF] Dunne | Procedures for Precise Measurements of 135Cs/137Cs Atom Ratios in Environmental Samples at Extreme Dynamic Ranges and Ultra-Trace Levels by Thermal Ionization Mass Spectrometry[END_REF] (ranging from 0.66 ± 0.06 to 0.68 ± 0.13). In the absence of certified reference materials characteristic of Fukushima fallout, the 135 Cs/ 137 Cs isotopic ratios obtained in the sediments collected in the Niida river catchment were compared with the ratios published by studies characterizing the Fukushima fallout. For these sediments, the average measured 135 Cs/ 137 Cs ratios of 0.47 ± 0.02 was fully consistent with the ratios reported in the literature [START_REF] Zok | Determination of Characteristic vs Anomalous 135Cs/137Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples[END_REF][START_REF] Yang | 135Cs Activity and 135Cs/137Cs Atom Ratio in Environmental Samples before and after the Fukushima Daiichi Nuclear Power Plant Accident[END_REF][START_REF] Zheng | Determination of 135Cs and 135Cs/137Cs Atomic Ratio in Environmental Samples by Combining Ammonium Molybdophosphate (AMP)-Selective Cs Adsorption and Ion-Exchange Chromatographic Separation to Triple-Quadrupole Inductively Coupled Plasma-Mass Spectrometry[END_REF][START_REF] Ohno | Determination of Radioactive Cesium Isotope Ratios by Triple Quadrupole ICP-MS and Its Application to Rainwater Following the Fukushima Daiichi Nuclear Power Plant Accident[END_REF] and ranging from 0.45 ± 0.03 to 0.47 ± 0.04. This comparison demonstrated the reliability of the analytical procedure. It is noteworthy that soils from this area show nearly homogenous isotopic ratios, ranging from 3.94 ± 0.89 to 4.29 ± 0.21. Compared with the isotopic signatures reported for each source in the literature or in Table 2 of the current research (NWT, Chernobyl, and Fukushima accidents), such a low 135 Cs/ 137 Cs was consistent with the Chernobyl fallout signature, while the highest ratio lied within the range of values attributed to the NWT (Figure 3).

Accordingly, it can be deduced that the 137 Cs radioactive contamination of the 21_PNM24_1

Mercantour soil was almost exclusively due to the Chernobyl accident fallout while that of the Pyrenees soils was almost exclusively due to the fallout of nuclear weapons testing. Nevertheless, it should be noted that the 137 Cs contributions of Chernobyl fallout calculated for soil samples from the Pyrenees (Table 3) were slightly negative, due to the uncertainty of NWT signature, with, as underlined in the introduction, only two values of 135 Cs/ 137 Cs available for this source in the literature up to now [START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Lee | First Detection of Fallout Cs-135 and Potential Applications of 137Cs /135Cs Ratios[END_REF] . This could be also explained by the fact that these estimates relied on a 135 Cs/ 137 Cs ratio value characteristic of NWT averaged over the entire testing period (1950s to 1980s) assuming a 135 Cs/ 137 Cs ratio of 1 at the date of detonation [START_REF] Lee | First Detection of Fallout Cs-135 and Potential Applications of 137Cs /135Cs Ratios[END_REF] . However, the value of this ratio could be refined based on the proportion of 137 Cs deposited during each year due to fallout from NWT, while still assuming a 135 Cs/ 137 Cs ratio of 1 at the time of testing. Considering the proportion of annual deposition of 137 Cs between 1958 and 1980 given by UNSCEAR [START_REF]Ionizing Radiation: Sources and Biological Effects[END_REF] and assuming the accumulation of cesium with time in the soil or the sediments, the average 135 Cs/ 137 Cs ratio of the resulting deposit would be about 3.94 to the reference time of January 1 st , 2022, close to the average value reported for NWT is 3.70 [START_REF] Zhu | Determination of Ultralow Level 135Cs and 135Cs/137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS[END_REF][START_REF] Lee | First Detection of Fallout Cs-135 and Potential Applications of 137Cs /135Cs Ratios[END_REF] (Table S4). According to experimental 135

Conclusions

The main objective of the current research was to improve our knowledge of the radiological background in environments remote from nuclear tests or nuclear accident sites by analysing an isotopic signature little exploited to date, i.e., the 135 Cs/ 137 Cs ratio. We have recently developed an analytical method to measure this ratio in solid environmental matrices and have applied it to soil and sediment samples collected in two areas of interest in France (Pyrenees and Mercantour massif).

135 Cs/ 137 Cs ratios ranging from 0.66 ± 0.04 in Mercantour soils to 4.29 ± 0.21 in Pyrenees soils were measured corresponding to typical ratios reported in the literature for Chernobyl accident fallout and from nuclear weapons tests, respectively. From these results, the contribution of each of these sources could be estimated and successfully compared with those calculated from more commonly used diagnostic ratios ( 241 Am/ 239+240 Pu and 239+240 Pu/ 137 Cs). Accordingly, the current work demonstrates the strength of the 135 Cs/ 137 Cs isotopic ratio as a novel fingerprint for identifying the radioactive contamination sources and estimating their contribution in the environment. Moreover, the monitoring of this isotopic ratio in the areas that were the most affected by the Chernobyl and Fukushima accidents could provide a surrogate for the 134 Cs/ 137 Cs ratio, as the use of this ratio is

  /MS for135 Cs/137 Cs isotopic ratio and interferent concentration measurements. Isotopic ratio and interferent content measurements were performed with the Agilent 8900#100 triplequadrupole ICP-MS coupled with the SPS4 auto-sampler and controlled by the MassHunter 4.6 software. The formed ions in the hot plasma were extracted and focused with a Ni skimmer cone and a x-lens block. The instrument is equipped with an octupole reaction-collision cell (CRC) between two quadrupole mass filters (Q1 and Q2) allowing an efficient removal of interferences. All measured solutions (blanks, standards, and samples) were prepared in 0.5M ultra-pure nitric acid and introduced into the ICP-MS by a MicroMist nebulizer with a peristaltic pump at 0.35 mL.min-1

Figure 2 :

 2 Figure 2: Distribution of the estimated 137 Cs activity resulting from the Chernobyl accident (a) and nuclear weapons testing (b) as a function of total 137 Cs activity measured by gamma-ray spectrometry in soil and sediment samples collected in France. All activities are decay-corrected to January 1, 2022, and the error bars correspond to a measurement uncertainty at k = 2. 137 Cs/ 239+240 Pu is used here to estimate the respective sources of 137 Cs. (see experimental section)

  ± 0.02 0.46 ± 0.01 Ohno et al. (2014) Zheng et al. (2014) Yang et al. (2016) Zok et al. (2021) Niida_2015 0.46 ± 0.01 (n=3) Niida_2020 0.49 ± 0.04 (n=1) Decay-corrected to January 1 st , 2022. The isotopic ratios are provided with uncertainty at k = 2.

Figure 3 :

 3 Figure 3: 135 Cs/ 137 Cs atomic ratios measured by ICP-MS/MS (decay-corrected to January 1 st , 2022) in the samples used for method validation (IAEA and Niida) and in French soils and sediments collected in the two investigated areas. The error bars correspond to a measurement uncertainty at k = 2 and the colored bands represent the typical 135 Cs/ 137 Cs ratios of the three main fallout sources.

Figure 4 :

 4 Figure 4: Relationship between 135 Cs/ 137 Cs isotopic ratios and 137 Cs activity in sediments and soils collected in the Mercantour massif and the Pyrenees, France. The dark blue and orange bands indicate the typical 135 Cs/ 137 Cs ratios reported in the literature for fallout from nuclear weapons testing (NWT) and the Chernobyl accident. All data were decay-corrected to January 1 st , 2022.

Table 1 :

 1 137 Cs and 241 Am activities measured by gamma-ray spectrometry in sediments and soils of the investigated sites as decay-corrected to January 1 st , 2022, and with 326 uncertainty at k=2. 239+240 Pu,137 Cs activities resulting from nuclear weapons testing (NWT) and137 Cs from Chernobyl (Chern) deduced from 241 Am/ 239+240 Pu and 239+240 Pu/ 137 Cs 327 activity ratios reported in the literature for NWT contribution (0.40 ± 0.06 and 0.04 ± 0.01 respectively, and decay-corrected to January 1 st , 2022) (see experimental section).

	328
	Sample name	Matrix	Study area	Sampling site	Depth (cm)	137 Cs activity (Bq.kg -1 dry)	241 Am activity (Bq.kg -1 dry)	239+240 PuNWT activity (Bq.kg -1 )	137 CsNWT activity (Bq.kg -1 )	137 CsChern activity (Bq.kg -1 )
	20_PNM45_1			Main Terre Rouge lake	-	1234 ± 114	4.99 ± 0.71	12.48 ± 2.59	312 ± 101	922 ± 159
	20_PNM45_2			Temporary Terre rouge lake	-	2047 ± 193	5.98 ± 0.97	14.95 ± 3.30	374 ± 125	1673 ± 259
	20_PNM45_3			Superior Lausfer lake	-	513 ± 38	3.21 ± 0.48	8.02 ± 1.70	200 ± 66	313 ±
	20_PNM45_4			Inferior Lausfer lake	-	249 ± 19	1.50 ± 0.46	3.75 ± 1.27	94 ± 39	155 ±
	13-PNM4	Lake	Mercantour	Cerise lake	-	1375 ± 95	0.90 ± 0.36	2.25 ± 0.96	56 ± 28	1319 ± 99
	13-PNM7	sediment	mountain	Besson lake	-	2086 ± 155	2.05 ± 0.54	5.11 ± 1.56	128 ± 50	1958 ± 163
	13-PNM9			Vens lake	-	555 ± 40	1.63±0.42	4.08 ± 1.22	102 ± 40	453 ±
	13-PNM19			Lausfer lake	-	862 ± 66	3.82 ± 0.57	9.54 ± 2.02	239 ± 78	624 ± 102
	21_PNM24_5			Terre Rouge Main lake	-	659 ± 45	2.58 ± 0.33	6.46 ± 1.27	161 ± 51	497 ±
	21_PNM24_6			Terre Rouge Inferior lake	-	5274 ± 350	8.43 ± 0.99	21.07 ± 4.01	527 ± 166	4747 ± 387
	21_PNM24_1				0 -5 5 -10	89,894 ± 90 103,501 ± 97	6.15 ± 0.54* 3.08 ± 0.31*	15.37 ± 2.68 7.70 ± 1.39	384 ± 117 192 ± 59	89,510 ± 148 103,308 ± 113
	21_PNM24_2				0 -5	165 ± 11	1.34 ± 0.32	3.34 ± 0.95	84 ± 32	82 ± 34
	21_PNM24_3		Mercantour mountain	Restefond pass	0 -5 5 -10	31,214 ± 96 3169 ± 223	4.16 ± 0.49* 0.68 ± 0.10*	10.40 ± 1.99 1.69 ± 0.36	260 ± 82 45 ± 15	30,954 ± 126 3321 ± 224
	21_PNM24_4	Soil			0 -5 5 -10	16,066 ± 32 803 ± 53	9.69 ± 0.94* 2.38 ± 0.25	24.23 ± 4.32 5.95 ± 1.09	606 ± 186 149 ± 46	15,460 ± 189 654 ±
	14_PYR_2			Coumebière plateau	0 -10	75 ± 10	0.81 ± 0.31	2.02 ± 0.82	51 ± 24	25 ± 26
	14_PYR_3		Pyrenees	Labant pond	0 -10 10 -20	93 ± 28 23 ± 4	2.78 ± 1.08 < 0.36	6.95 ± 2.90 -	174 ± 84 -	< 1,03 -
	14_PYR_53			Besse pass	0 -10	40 ± 4	0.46 ± 0.35	1.14 ± 0.88	28 ± 23	12 ± 24
	* Alpha spectrometry measurement
	329

Table 2 :

 2 135 Cs/137 Cs isotopic ratios measured by ICP-MS/MS for materials impacted by Chernobyl (IAEA-330 and IAEA-375) and Fukushima (Niida_2012, Niida_2015 and Niida_2020) fallout. Comparison of these ratios with isotopic signatures reported in the literature for these contamination sources.

	Sample

name Main source of 137 Cs 135 Cs/ 137 Cs isotopic ratio

		Current work	Literature values
			0.66 ± 0.06
	IAEA-330	0.66 ± 0.06 (n=3)	0.66 ± 0.03 0.66 ± 0.06
	Chernobyl fallout		0.66 ± 0.13

(at/at)

Table 3 :

 3 135 Cs/ 137 Cs isotopic ratios in lake sediments and soils collected in France. 351Data are decay-corrected to January 1 st , 2022. All results presented are provided with an uncertainty at k = 2.352The analytical results show strongly variable135 Cs/ 137 Cs signatures with a minimum value of 0.66 ± 0.04 in a Mercantour soil and a maximum value of 4.29 ± 0.21 in a soil sample from the Pyrenees.

								Proportion
	Sample name	Matrix	Study area	Sampling site	Depth (cm)	137 Cs activity (Bq.kg -1 )	135 Cs/ 137 Cs atomic ratio (at/at)	of 137 Cs from Chernobyl
								accident
	20_PNM45_1			Main Terre Rouge lake	-	1234 ± 114	1.64 ± 0.05	68%
	20_PNM45_2			Temporary Terre Rouge lake	-	2047 ± 193	1.23 ± 0.10	81%
	20_PNM45_3			Superior Lausfer lake	-	513 ± 38	2.29 ± 0.05	46%
	20_PNM45_4			Inferior Lausfer lake	-	249 ± 19	2.18 ± 0.24	50%
	13-PNM4	Lake	Mercantour	Cerise lake	-	1375 ± 95	0.85 ± 0.08	94%
	13-PNM7	sediment	mountain	Besson lake	-	2086 ± 155	1.06 ± 0.09	87%
	13-PNM9			Vens lake	-	555 ± 40	1.41 ± 1.86	75%
	13-PNM19			Lausfer lake	-	862 ± 66	1.86 ± 0.13	61%
	21_PNM24_5			Terre Rouge Main lake	-	659 ± 45	1.46 ± 0.22	74%
	21_PNM24_6			Terre Rouge Inferior lake	-	5274 ± 350	1.07 ± 0.03	87%
					0 -5	89,894 ± 90	0.68 ± 0.01	99%
	21_PNM24_1
					5 -10	103,501 ± 97	0.66 ± 0.04	100%
	21_PNM24_2				0 -5	165 ± 11	2.27 ± 0.29	47%
	21_PNM24_3		Mercantour mountain	Restefond pass	0 -5	31,214 ± 96	0.71 ± 0.02	98%
					5 -10	3169 ± 223	0.71 ± 0.03	98%
		Soil			0 -5	16,066 ± 32	0.75 ± 0.02	97%
	21_PNM24_4
					5 -10	803 ± 53	1.00 ± 0.07	89%
	14_PYR_2			Coumebière plateau	0 -10	75 ± 10	4.29 ± 0.21	-19%
					0 -10	93 ± 28	4.26 ± 0.66	-18%
	14_PYR_3		Pyrenees	Labant pond
					10 -20	23 ± 4	3.94 ± 0.89	-8%
	14_PYR_53			Besse pass	0 -10	40 ± 4	4.04 ± 0.75	-11%

  Cs/137 Cs ratios (3.94 ± 0.89 to 4.29 ± 0.21), actual deposition by NWT in the Pyrenees has likely mainly occurred in the early period of 1950s to 1980s. Nevertheless, due to the limited data on the impact of NWT fallout before the 1960s in the Pyrenees, it is difficult to determine an average value for the135 Cs/ 137 Cs ratio, specific for this region. The main limitation of this method is therefore the uncertainty in the regional variation of 135 Cs and 137 Cs deposits due to nuclear weapons testing.The other Mercantour soil and sediments samples with 135 Cs/ 137 Cs ranging between 0.68 ± 0.01 and 2.29 ± 0.05 showed radioactive contamination constituted of a mix of both sources in variable proportions. Thus, for each sample, the proportion of 137 Cs resulting from Chernobyl fallout was estimated using simple un-mixing models following methodology given in experimental section (Table3). In addition, the data highlighted a relationship between137 Cs activity estimated by gamma-ray spectrometry and the 135 Cs/ 137 Cs isotopic ratio measured by ICP-MS/MS. As shown in Figure4, when the 137 Cs activity increased, the 135 Cs/137 Cs ratio decreased until it reached the135 Cs/ 137 Cs isotopic signature of Chernobyl fallout. Similarly, to the trend of 135 Cs/ 137 Cs, a trend of134 Cs/ 137 Cs variations in soils of the Mercantour mountain has been shown on the one hand soils strongly enriched in137 Cs originating from Chernobyl and on the other hand other soils with low137 Cs contamination due to NWT fallout[START_REF] Pourcelot | Formation of Radioactivity Enriched Soils in Mountain Areas[END_REF] . Thus, both trends involving isotopes ratios of Cs suggest that soils and sediments contain the two sources (NWT and Chernobyl) in various proportions from one sample to another.All these observations agree with what was previously predicted and, therefore, demonstrate the applicability of the 135 Cs/ 137 Cs ratio to discriminate unambiguously the sources of radioactive contamination. Moreover, the use of a mono-elemental isotopic signature such as135 Cs/ 137 Cs ratio allows to get rid of any potential fractionation caused the potential difference of behavior between two elements (such as Am and Pu or Pu and Cs), even if they share very similar chemical properties.

Table S3 :

 S3 Analytical performance (Cs recovery and eluate purity) achieved for each sample analyzed

	Sample name	Depth (cm)	Analyzed mass (g)	137 Cs recovery (%)	Interferent concentration in the ICP-MS measurement fraction (ng.g -1 ) Ba Mo Sb Sn
	20_PNM45_1	-	63.142	83	1.09 ± 0.01	0.57 ± 0.02	0.11 ± 0.01	1.85 ± 0.02
	20_PNM45_2	-	61.519	73	1.69 ± 0.03	0.28 ± 0.02	0.08 ± 0.01	1.25 ± 0.03
	20_PNM45_3	-	94.853	78	1.52 ± 0.31	0.41 ± 0.09	0.08 ± 0.02	2.89 ± 0.59
	20_PNM45_4	-	102.059	62	1.39 ± 0.67	17.46 ± 2.29	0.04 ± 0.02	0.24 ± 0.14
	13_PNM4	-	113.369	76	1.96 ± 0.29	0.24 ± 0.03	0.06 ± 0.01	2.17 ± 0.28
	13_PNM7	-	84.482	84	1.20 ± 0.19	0.28 ± 0.04	0.46 ± 0.07	1.67 ± 0.27
	13_PNM9	-	60.080	84	1.15 ± 0.02	0.21 ± 0.02	0.05 ± 0.01	1.14 ± 0.01
	13_PNM19	-	80.360	80	1.02 ± 0.19	0.39 ± 0.07	0.21 ± 0.04	0.81 ± 0.15
	21_PNM24_5	-	99.030	77	1.16 ± 0.36	0.29 ± 0.09	0.08 ± 0.02	2.24 ± 0.66
	21_PNM24_6	-	49.671	82	2.10 ± 0.02	2.45 ± 0.04	0.07 ± 0.01	1.48 ± 0.02
	21_PNM24_1	0 -5 5 -10	5.131 6.326	82 85	0.24 ± 0.01 0.37 ± 0.02	1.88 ± 0.04 0.94 ± 0.11	0.04 ± 0.01 0.04 ± 0.01	3.01 ± 0.08 12.16 ± 0.63
	21_PNM24_2	0 -5	109.884	57	3.39 ± 0.11	11.43 ± 0.28	0.03 ± 0.01	0.48 ± 0.03
	21_PNM24_3	0 -5 5 -10	10.852 55.037	83 82	0.60 ± 0.01 5.45 ± 0.88	0.18 ± 0.02 0.26 ± 0.04	0.04 ± 0.01 0.05 ± 0.01	7.21 ± 0.84 0.47 ± 0.08
	21_PNM24_4	0 -5 5 -10	11.970 100.610	93 71	0.09 ± 0.01 1.03 ± 0.02	0.13 ± 0.01 0.33 ± 0.02	0.05 ± 0.01 0.07 ± 0.01	5.64 ± 0.12 1.04 ± 0.02
	14_PYR_2	0 -10	113.455	65	0.34 ± 0.01	0.31 ± 0.02	0.07 ± 0.01	0.62 ± 0.03
	14_PYR_3	0 -10 10 -20	34.039 103.584	72 61	0.52 ± 0.01 1.76 ± 0.03	0.38 ± 0.02 0.31 ± 0.01	0.04 ± 0.01 0.04 ± 0.01	1.48 ± 0.02 1.21 ± 0.04
	14_PYR_53	0 -10	110.342	68	3.39 ± 0.01	0.30 ± 0.01	0.03 ± 0.01	0.77 ± 0.02

Table S4 :

 S4 Estimation of the135 Cs/ 137 Cs ratio characterizing NWT fallout by weighting the proportions of annual deposits with the theorical isotopic ratio of 1 at the time of detonation. Annual 137 Cs deposition estimated from the annual 90 Sr deposition from NWT reported in the UNSCEAR report(1982) and the average 137 Cs/ 90 Sr activity ratio of 1.6.

		Annual deposition*	Proportion of	Theoretical 135 Cs/ 137 Cs isotopic
	Year	(10 16 Bq)	annual deposition	ratio per year**
		90 Sr	137 Cs	(%)	(decay corrected to January 1 st , 2022)
	Pre-1958	6.68	10.69	14.54	4.57
		2.33	3.73	5.07	4.37
		3.89	6.22	8.47	4.27
		0.97	1.55	2.11	4.17
		1.30	2.08	2.83	4.08
		5.34	8.54	11.62	3.98
		9.70	15.52	21.11	3.89
		6.13	9.81	13.34	3.80
		2.86	4.58	6.23	3.72
		1.21	1.94	2.63	3.63
		0.62	0.99	1.35	3.55
		0.72	1.15	1.57	3.47
		0.54	0.86	1.18	3.39
		0.76	1.22	1.65	3.31
		0.70	1.12	1.52	3.24
		0.32	0.51	0.70	3.16
		0.12	0.19	0.26	3.09
		0.45	0.72	0.98	3.02
		0.22	0.35	0.48	2.95
		0.10	0.16	0.22	2.89
		0.30	0.48	0.65	2.82
		0.37	0.59	0.81	2.76
		0.2	0.32	0.44	2.69
		0.11	0.18	0.24	2.63
	TOTAL	45.94	73.50	100	3.97

***

135

Cs/ 137 Cs isotopic ratio calculated per year assuming a 135 Cs/ 137 Cs isotopic ratio of 1 at the time of detonation and for decay-correction to January 1 st , 2022. The total corresponds to the 135 Cs/ 137 Cs characteristic of NWT weighted by the proportion of annual deposits.

Acknowledgement

G. Salaün is warmly acknowledged for his excellent field work in the Alps and Pyrenees mountains and his expertise for preparation of the samples in the laboratory.

We thank S. Aubry and S. Bonnot from the gamma team of the LMRE for the measurements of 137 Cs and 241 Am by gamma-ray spectrometry and also S. Thomas for her recommendations on the 241 Am measurement by alpha spectrometry.

We also warmly thank H. Isnard and S. Mialle from the Laboratory of Analytical development for Nuclear Isotopic and Elemental analysis (CEA) for the supply and qualification of the spent fuel sample used in this research to correct the 135 Cs/ 137 Cs isotopic ratios during the ICP-MS/MS measurements.

Once the analytical procedure to measure 135 Cs/ 137 Cs was validated, it was applied to lake sediment 349 and soil samples collected in France (Table 3). limited in time given the rapid decay of 134 Cs. In future research, it could be interesting to consider the potential occurrence of diagnostic signatures of radiocesium released by nuclear facilities.

SUPPORTING INFORMATION

Identification of the origin of radiocesium into the environment in areas remote