Thi-Hong-Hanh Le

Hervé Michel

Julie Champion

Affiliation

210 Po sequential extraction applied to wetland soils at uranium mining sites

Keywords: Polonium-210, Sequential extraction, Soils, Uranium mining

Former uranium mining activities have led to the presence of naturally occurring nuclides embedded in soil. Such activities have also modified the secular equilibrium between radionuclides in 238 U decay series. The objective of this paper is to quantify the long-term effect of former uranium mining activities on the behavior of the final radionuclide in the 238 U-series, i.e. polonium-210 ( 210 Po), present in soils. Soil samples are extracted from two uranium sites in France, specifically a quarried site and a natural site. The polonium distribution is studied within the various soil fractions, namely: water soluble, exchangeable, bound to carbonates, bound to iron/manganese oxides, bound to organic matter, and residual. 210 Po is mainly found in the residual fraction of both study sites (87-90%), followed by the carbonates fraction (5-9%). The 210 Po activity in the other fractions is very small in comparison with total activity.

2

Introduction

Over 200 uranium mines sites exist in France, some of which had been in operations until the beginning of the 1990's [START_REF] Ielsch | Estimation and mapping of uranium content of geological units in France[END_REF]. Their existence has led to the presence of naturally occurring nuclides embedded in soil. Given society's persistent concern about radioactivity in the environment, assessing the impact of radioactive metals on environmental compartments is necessary in order to adopt appropriate precautions. To quantify this impact and understand the behavior of radionuclides in the environment, the main scientific challenge calls for identifying the chemical mechanisms that control radionuclide mobility, bioavailability and ecotoxicity. The major scientific milestone consists of quantifying labile species in the ecosystem and then describing their speciation in terrestrial and aquatic environments.

Discovered in 1898 by Pierre and Marie Curie, 210 Po is a natural radionuclide occurring in the environment, with its precursors, i.e. radon-222 and lead-210, being the penultimate daughter of natural uranium-238. Of the 33 polonium isotopes, 210 Po is one of the most abundant. Due to its very high specific activity (1.66 10 14 Bq.g -1 ), 210 Po is a substance extremely toxic to humans. Just a few micrograms are sufficient to cause death. This metallic substance is some one million times more toxic than sodium or potassium cyanide [START_REF] Al-Masri | Determination of 210Po in tea, maté and their infusions and its annual intake by Syrians[END_REF][START_REF] Ansoborlo | Review of Chemical and Radiotoxicological Properties of Polonium for Internal Contamination Purposes[END_REF]. The issue of its bioavailability is directly correlated with the species' chemical form, which depends on environmental characteristics (pH, Eh, organic carbon content, etc.). This correlation between speciation and bioavailability remains puzzling given the current state of knowledge on polonium behavior (notably as regards the limited thermodynamic databases describing surface complexation reactions).

In the literature, many studies [START_REF] Al-Masri | Speciation of 210Po and 210Pb in air particulates determined by sequential extraction[END_REF][START_REF] Ekdal | 210Po and 210Pb in soils and vegetables in Kucuk Menderes basin of Turkey[END_REF][START_REF] Karunakara | Distribution and enrichment of in the environment of Kaiga in South India[END_REF][START_REF] Özden | Assessment of the effects of physical-chemical parameters on 210Po and 210Pb concentrations in cultivated and uncultivated soil from different areas[END_REF][START_REF] Strumińska-Parulska | Is ecological food also radioecological? -210Po and 210Pb studies[END_REF] demonstrate that anthropic activities, such as phosphate industries, heavy metal mining and oil drilling, can lead to a local 'overconcentration' of 210 Po. The correlation with uranium mining (i.e. ore extraction) is most obvious, over the range of 0.014 to 22 kBq.kg -1 on surface soil [START_REF] Lapham | Health implications of radionuclide levels in cattle raised near U mining and milling facilities in Ambrosia lake, New Mexico[END_REF][START_REF] Sethy | Assessment of naturally occurring radioactive materials in the surface soil of uranium mining area of Jharkhand, India[END_REF][START_REF] Thomas | Radionuclides in the terrestrial ecosystem near a canadian uranium mill -Part II: Small mammal food chains and bioavailability[END_REF]. None of these authors could determine whether the polonium present is labile. The fate of polonium in the environment will differ because its behavior and mobility in the soil depend on its physicochemical forms as well as soil characteristics [START_REF] Blanco | Sequential extraction for radionuclide fractionation in soil samples: a comparative study[END_REF][START_REF] Jia | The fractionation and determination procedures for the speciation of 210Pb and 210Po in soil samples[END_REF][START_REF] Prakash | Distribution of 210Po in soils of Virajpet taluk, Coorg District, Karnataka[END_REF][START_REF] Žemberyová | The utilization of modified BCR threestep sequential extraction procedure for the fractionation of Cd, Cr, Cu, Ni, Pb and Zn in soil reference materials of different origins[END_REF]. metals or radionuclides within the various constituent solid phases of soils or sediments. This methodology is based on a successive treatment of specific solutions, each designed to solubilize a targeted mineralogical fraction [START_REF] Vandenhove | Comparison of two sequential extraction procedures for uranium fractionation in contaminated soils[END_REF]. Despite problems such as re-adsorption, non-selectivity of extraction reagents and an incomplete dissolution of target phases [START_REF] Schultz | Evaluation of a sequential extraction method for determining actinide fractionation in soils and sediments[END_REF], sequential extraction yields key information on the original properties of radionuclides. 210 Po in mineral samples was speciated by applying sequential extraction in order to identify their sources [START_REF] Al-Masri | Speciation of 210Po and 210Pb in air particulates determined by sequential extraction[END_REF][START_REF] Jia | The fractionation and determination procedures for the speciation of 210Pb and 210Po in soil samples[END_REF][START_REF] Štrok | Partitioning of natural radionuclides in sediments around a former uranium mine and mill[END_REF].

The aim of this study is to determine the distribution of 210 Po in the various geochemical fractions of soil samples from two uranium sites in France, specifically an operational site (Rophin) and a natural site (Granges du Colonel). For this purpose, a six-step sequential extraction method has been applied. This methodology was validated using certified reference materials from the IAEA Agency.

Materials and methods

Study sites

The two selected sites are in two enriched uranium zones in France (see Fig. 1). The first, named 'Rophin', is located in the Puy-de-Dôme Department about 3.4 km northwest of the city of Lachaux. The surrounding landscape is hilly and composed of forests and meadows.

The soil at the Rophin site is qualified as wet to marshy. Quarried from 1948 to 1957, the Rophin site produced 0.4 tons of uranium for 2,000 tons of ore actually mined. This site now serves as a storage depot for uranium waste and has not been remodeled since its inception in the 1950's.

A mechanical or washing plant was built in 1948 on the mine itself in order to treat the extracted material. The objective of this facility was to concentrate uranium ores from local mining operations before processing.

The Rophin site is currently used to store ore processing residues and dismantling products from laundries. The actual storage location lies on the former mining tile of the Rophin mine and contains 30,000 tons of waste. Moreover, since this site has not been remodeled since its initial construction in the 1950's, it is now naturally vegetated.

The second site, named 'Granges du Colonel', lies roughly 7 km northeast of Belvédère in the Alpes-Maritimes Department. As part of the search for uranium resources in France, the granitic massif in this department was prospected during the years 1956-58. The occurrences of uranium here are mainly confined to the Permian lands, with the Granges du Colonel site offering some of the highest indices. Yet this site has never been quarried, due perhaps to its remote location and lack of accessibility or to local opposition. Samples from this site were collected in order to compare the impact of human activities on the behavior of polonium in the soil.

Samples, sampling protocol, pretreatment

Certified material reference: IAEA-385

The protocol validation and reproducibility steps were performed using the certified material standard IAEA-385 with a given value of 210 Po. The material was collected in the Irish Sea by the Centre for the Environment, Fisheries and Aquaculture Science (CEFAS), Lowestoft, U.K. in 1995. It was then dried, sieved through a 250-µm mesh and homogenized by mixing in a nitrogen atmosphere. This sample received International Atomic Energy Agency (IAEA) certification [START_REF] Pham | Report on the worldwide Intercomparison Exercise IAEA-385[END_REF]. The IAEA-385 sediment was subjected to the recognized analysis procedure before application to our environmental samples.

Gamma Cartography

Gamma cartography was conducted at the study sites in order to identify areas of high activity. Two gamma ray maps of the sites were derived from a gamma ray sensor with a Colibri device (Canberra, Inc.). The data were acquired automatically every five seconds.

These measurements were recorded in May 2017 for the 'Granges du Colonel' site and in November 2017 for 'Rophin'. The gamma mapping images and photographs were superimposed using the QGIS (Quantum Geographic Information System) software.

Sampling and Sample Preparation

Soil samples were collected in wetlands (denoted RW and GW for Rophin Wetland and Granges du Colonel Wetland, respectively) with soil traps placed at a depth of 15 cm. After sampling and storage in a plastic bag, the samples were dried in the oven at 50 °C for 10 days followed by cooling at room temperature. Stones, pebbles, plant materials and roots were manually removed. The samples were then sieved through a 2-mm screen sieve before being crushed in a ceramic pestle and mortar to obtain a homogeneous mixture that was stored until analysis.

Analysis of soil properties

Soil pH

The soil pH was measured by a method specified in NF ISO 10390 (AFNOR 1994b).

Measurements were carried out in two different media, i.e. distilled water or a 1 M KCl solution (the cations in these solutions displace the protons absorbed on the soil ion exchangers with as much quantitative precision as possible). Five grams of dry soil samples were vigorously shaken with 25 ml of the corresponding solution for 5 minutes. The suspension was then left to rest at least 4 hours before measurement with a pH meter (SevenCompact TM METTLER TOLEDO).

Carbonate and organic matter quantification

The organic and carbonate matter contents in the soils were determined according to the LOI (Loss On Ignition) method [START_REF] Heiri | Loss on ignition as a method for estimating organic and carbonate content in sediments: reproducibility and comparability of results[END_REF]. Approximately 1 g of dried soil sample was introduced into the oven at 50 °C overnight. Next, the soil samples were heated at 550 °C for 5 h in the muffle furnace (Nabertherm) to burn the organic matter. In a subsequent step, they were heated to 950 °C for 2 h to burn the carbonate matter. Between each step, the samples were cooled in a desiccator and weighed.

The organic content (%OM) was calculated using the following equation:

% = × 100 Eq. 1
For carbonate content (%CO ), the calculation proceeded as follows:

%CO = × 100 × 1.36 Eq. 2
where denotes the dry weight of the sample before combustion, the dry weight of the sample after heating at 550°C, and the dry weight of the sample after heating at 950°C (all in g). The weight loss at 950°C multiplied by 1.36 (ratio of carbonate to carbon dioxide mass) equals the weight of the carbonate in the original sample.

210 Po analysis

Determination of total 210 Po activity in the soil 0.5 g of soil with 40 mBq of an added 209 Po (Eckert Ziegler) tracer was digested with 10 ml of hydrofluoric acid (39%) in a microwave (Ethos Easy). The microwave cycle was set with two steps: the first one raised the temperature to 200 °C for 20 minutes, while the second step stabilized this temperature for 120 minutes. After cooling at room temperature, the hydrofluoric acid was evaporated to dryness at 50 °C. Then, 10 ml of concentrated nitric acid (65%) were added and then heated again with the same microwave cycle of mineralization.

Then, it is evaporated to dryness at 50 °C. For spontaneous polonium deposition onto a silver disc, the residue was dissolved in a 0.5 M HCl solution before adding ascorbic acid (1 g for 10 ml HCl 0.5 M) [START_REF] Nelson | Polonium-210 accumulates in a lake receiving coal mine dischargesanthropogenic or natural?[END_REF][START_REF] Özden | Assessment of the effects of physical-chemical parameters on 210Po and 210Pb concentrations in cultivated and uncultivated soil from different areas[END_REF]. The contact time was set for 24 h at room temperature. The deposition was carried out in a nearly enclosed environment in order to reduce the loss by evaporation [START_REF] Jia | The fractionation and determination procedures for the speciation of 210Pb and 210Po in soil samples[END_REF]. The disc was then washed using demineralized water and air-dried. Lastly, the polonium activity was measured by means of alpha spectrometry. Triplicate samples were prepared to ensure the quality of the analysis performed.

Sequential Extraction Procedure

The distribution of 210 Po in soil fractions was studied using a sequential extraction protocol developed by Tessier et al. [START_REF] Tessier | Sequential extraction procedure for the speciation of particulate trace metals[END_REF] and Al-Masri et al. [START_REF] Al-Masri | Speciation of 210Po and 210Pb in air particulates determined by sequential extraction[END_REF].

This six-step procedure isolates the following fractions: water soluble, exchangeable, boundto-carbonates, bound-to-iron/manganese oxides, bound-to-organic matter, and residual fraction. Triplicate samples were prepared in order to ensure analytical quality.

One gram of each soil sample was introduced into centrifuge cones (50 ml) and stirred at room temperature for the first five steps. After each step, the suspension was centrifuged (Sigma TM 10223) at 4,000 rev/min for 20 min. The supernatant was removed and filtered through a 0.45-µm pore size cellulose filter. The extracted solutions were evaporated to dryness at 50 °C. The residue in the centrifuge tube was then extracted by the reagent from the next steps according to the following procedure.

Fraction 1: Water soluble

The samples were first extracted using 20 ml of demineralized water for 24 h in order to release water-soluble 210 Po.

Fraction 2: Exchangeable

The residue from Fraction 1 was leached with 20 ml of 1 M NH4OAC for 24 h. The 210 Po adsorbed on mineral surfaces, capable of exchanging with an ion widely introduced into the solution (NH4 + ), is released.

Fraction 3: Bound-to-Carbonates

The residue from Fraction 2 was contacted with a 20 ml buffered NaOAc/HOAc solution (pH 5.2 ± 0.1) for 6 h. At this pH, elements precipitated or co-precipitated with the carbonates, as well as those that bind to the surfaces of solids by pH-dependent bonds, are released in solution [START_REF] Cornu | Extractions séquentielles et spéciation des éléments trace métalliques dans les sols naturels[END_REF].

Fraction 4: Bound-to-iron and manganese oxides

The residue from Fraction 3 was contacted with 20 ml of 0.04 M NH2OH.HCl for 24 h in order to free 210 Po, which had been bound to iron and manganese oxides.

Fraction 5: Bound-to-organic matter

The residue from Fraction 4 was contacted with 15 ml of 30% H2O2 acidified to pH 2 by 0.02 M HNO3 for 24 h; afterwards, 5 ml of 3.2 M NH4OAc was added before separation so as to avoid sorption. This extraction step destroys the organic materials in order to release the complex or adsorbed metals.

Fraction 6: Residual fraction

Once the first five fractions had been removed, the residue consisted of the primary and secondary minerals, i.e. the most difficult to extract [START_REF] Rodgers | The Potential of Sequential Extraction in the Characterisation and Management of Wastes from Steel Processing: A Prospective Review[END_REF]. The residue from

Fraction 5 was transferred to a Teflon tube and mineralized by the same procedure to determine the total 210 Po activity concentration under the section 'Determination of total 210 Po activity in the soil'. 209 Po was added to the filtrate resulting from each leaching step before evaporating to dryness at 50 °C. The residue obtained was then dissolved using 0.5 M hydrochloric acid. The 210 Po activity concentration in each fraction could be determined by alpha spectrometry after the spontaneous deposition of polonium on the sliver disc.

Polonium quantification

209 Po and 210 Po were quantified using dual alpha spectrometers EG&G Ortec 576A equipped with boron-implanted silicon detectors offering a 450 mm 2 active area and an alpha resolution (FWHM) of 20 keV at 5.47 keV. Pulses were analyzed with a multichannel buffer analyzer (Spectrum Master Ortec 919) and spectra with the Vision software. The efficiency was determined by means of calibrated discs electroplated with known amounts of tracer, while the background noise was being controlled regularly. The average efficiency of the detector was 22 ± 2%. The lower limit of detection for the method employed was 0.2 mBq [START_REF] Leclercq | Behaviors and transfers of anthropogenic radionuclides (137Cs, 239+240Pu and 241Am) in a protected alpine wetland (France)[END_REF].

Results and discussion

Total 210 Po activity in soil

The soil mineralization procedure using the microwave oven was performed with the IAEA-385 sediment standard in order to validate this protocol. The recommended specific activity of 210 Po for IAEA-385 was 28.0 Bq.kg -1 , with a confidence interval ranging from 23.8 to 36.6

Bq.kg -1 dry weight. Table 1 shows the results of the total 210 Po activity concentration for the standard IAEA-385 obtained from this work. The average activity found equals: 27 ± 3

Bq.kg -1 dry weight. This value lies within the confidence interval reported by IAEA, thus

showing good agreement between the determined values and IAEA values (28.0 Bq.kg -1 ). The results of replicated measurements attest to the reproducibility of the procedure used.

Consequently, analyses of the standard IAEA-385 confirm that the recommended mineralization procedure to determine total 210 Po amount yields reliable results. Gamma radiation activity in this area reaches 3500 c.s -1 , which is 7 times higher than the site background level. The gamma irradiation levels at the Granges du Colonel site are much lower than at Rophin.

A soil sample denoted RW (for Rophin Wetland) was extracted from a wetland where radionuclide activity was locally at its highest level. A Granges du Colonel wetland soil sample (GW) was also extracted for comparative purposes. Both samples were taken at depths of 15 cm. This mineralization protocol was then applied to the RW and GW samples. Table 1 reports their respective 210 Po activity concentration (Bq.kg -1 dry weight) levels. In the RW sample, the 210 Po level was more than 40 times greater than that of GW, i.e. the 210 Po activity concentrations in the GW and RW soil samples are 82 ± 4 (Bq.kg -1 ) and 3,461 ± 148 (Bq.kg - 1 ), respectively. These results are consistent with output from the gamma maps (Fig. 2). The much higher 210 Po activity concentration of the Rophin sample can be explained by the presence of a high radiation background emanating from the uranium storage facility at Rophin.

Sequential Extraction Studies

According to Tessier et al. [START_REF] Tessier | Sequential extraction procedure for the speciation of particulate trace metals[END_REF] This extraction procedure was then applied to GW and RW samples. The 210 Po activity concentrations in the different soil fractions are presented in Table 2. Sequential extraction results from the GW and RW samples are similar to those of the IAEA-385 sample. 210 Po was mainly found in the residual fraction, followed by the carbonate fraction. The sum of 210 Po activity in the four other fractions is relatively low, not exceeding 8% of total activity.

In a water-soluble and exchangeable fraction, the quantity of 210 Po was very small in comparison with the other fractions. It can be concluded that polonium mostly attaches to insoluble compounds in the atmosphere after the decay of radon gas [START_REF] Al-Masri | Speciation of 210Po and 210Pb in air particulates determined by sequential extraction[END_REF].

The 210 Po was then deposited onto the ground as dry and wet deposition, which constitutes their main source in surface soil [START_REF] Karunakara | Distribution and enrichment of in the environment of Kaiga in South India[END_REF].

Apart from the residual fraction, 210 Po is mainly present in the carbonate fraction. However, the percentage of 210 Po in the carbonate fraction of RW is much higher than that of GW, i.e.

88% and 43% respectively, by comparing it with the sum of the first five fractions. While the carbonate content of both samples is equal (1.8% dry weight -Table 3). The uranium mining activities promoted the exhalation of radon. This phenomenon has consequently modified the secular equilibrium between radionuclides in the 238 U decay series. Radiochemical disequilibrium points to the radionuclide mobilization and transport processes [START_REF] Bister | Impact of former uranium mining activities on the floodplains of the Mulde River, Saxony, Germany[END_REF]. Elevated levels of 210 Po activity in the carbonates fraction can result from the decay of unsupported radon-222; therefore, uranium mining activities at the Rophin site may indeed influence the geochemical distribution of 210 Po. The increase in hydrogen ion concentration changes the mobilization intensity of heavy metals. A low soil pH value, in addition to mobilizing heavy metals, affects the activity of metal ions in the water contained in soil pores, thus also altering their bioavailability [START_REF] Fijałkowski | The influence of selected soil parameters on the mobility of heavy metals in soils[END_REF]. The pH value of RW is less than that of GW (Table 3), which may explain the higher portion of 210 Po found in the carbonate-bound fraction in the RW sample.

210 Po displays a low percentage (less than 1%) when NH2OH.HCl was introduced during the fourth step of the procedure, which would suggest that this element is only slightly associated with iron and manganese oxides.

As regards the organic fraction, numerous studies [START_REF] Avadhani | Distribution and behaviour of natural radionuclides in soil samples of Goa on the southwest coast of India[END_REF][START_REF] Narayana | Behavior of 210Po and 210Pb in high background areas of coastal Kerala on the south west coast of India[END_REF][START_REF] Özden | Assessment of the effects of physical-chemical parameters on 210Po and 210Pb concentrations in cultivated and uncultivated soil from different areas[END_REF] have shown a positive correlation between organic matter content and 210 Po activity: the higher the organic matter content, the greater the level of 210 Po activity.

In contrast with these observations, results from the sequential extraction in this study reveal that 210 Po activity concentration in the organic fraction remains very small, 0.16% and 3.43%

for RW and GW, respectively, whereas the organic matter content in RW and GW amounts to respectively 22.7% and 9.2%. Prakash et al. found that the lower the organic matter percentage in the soil samples, the higher the level of 210 Po activity and vice versa [START_REF] Prakash | Distribution of 210Po in soils of Virajpet taluk, Coorg District, Karnataka[END_REF]. The organic matter and 210 Po were detected in the same location, i.e. on the soil surface, yet they remain uncorrelated. According to Vaaramaa, 210 Po in the organic layer most probably originated in large part from atmospheric depositions, and no systematic correlation appeared between these two elements [START_REF] Vaaramaa | Distribution of 210Pb and 210Po in boreal forest soil[END_REF]. 210 Po can indeed stem from two activity sources: supported 210 Po from decay of the remaining radon-222 in the soil matrix, and unsupported 210 Po due to the deposition of radon decay products from the atmosphere [START_REF] Matthews | Determination of 210Po in environmental materials: A review of analytical methodology[END_REF][START_REF] Özden | Assessment of the effects of physical-chemical parameters on 210Po and 210Pb concentrations in cultivated and uncultivated soil from different areas[END_REF][START_REF] Persson | 210Po and 210Po in the Terrestrial Environment[END_REF]. Upon reaching the soil surface as fallout from the atmosphere, 210 Po is rapidly adsorbed by the surface soil, which accounts for the principal source of 210 Po in surface soil [START_REF] Avadhani | Distribution and behaviour of natural radionuclides in soil samples of Goa on the southwest coast of India[END_REF][START_REF] Jia | The fractionation and determination procedures for the speciation of 210Pb and 210Po in soil samples[END_REF]. On the other hand, the soil surface is also a place where organic substances are primarily found. This initial layer of soil is mainly composed of decomposed plant debris and humic substances. The surface soil layer is also known to be the zone in which most biological activity occurs.

The highest 210 Po portions were dissolved in mineral acids, with observed values equaling 90% and 87% for the RW and GW samples, respectively. This finding reflects immobilization of the particular radionuclide, which is due to the fact that 210 Po has been incorporated into crystalline silicates. The low 210 Po availability leads to the conclusion that its presence in the environment is not hazardous for ingestion pathways, but only for inhalation of the particulate materials [START_REF] Desideri | Analytical methods for the characterization and the leachability evaluation of a solid waste generated in a phosphoric acid production plant[END_REF].

Conclusion

The total 210 Po activity concentration in soil samples extracted from two wetland areas near

Figure and Table captions

Fig. 1: Map of the mean uranium content of geological units in France from [START_REF] Ielsch | Estimation and mapping of uranium content of geological units in France[END_REF], with identification of our two experimental sites

Figure 2

 2 Figure 2 presents the gamma ray map for both the Rophin (A) and Granges du Colonel (B) sites. At Rophin, a stream flows down a gradient of the uranium mining zone and subsequently merges with a north-south watercourse. The combined outflow of the two currents is named the Terrasson stream. A radionuclide accumulation zone was identified through this gamma mapping adjacent to the Terrasson stream 150 m after the confluence.

French

  uranium mining sites has been determined. The sample taken near a uranium storage facility reveals a significant level of total 210 Po activity; this finding is correlated with the high radiation background. To study the distribution of 210 Po in soils, a sequential extraction procedure was developed, validated with the certified reference material (IAEA-385) and then applied to soil samples from the two study sites. From these sequential extraction analyses, the 210 Po found was almost entirely in the residual fraction of both sites. Apart from the residual fraction,210 Po is mainly present in the carbonate-bound fraction; however, the percentage of 210 Po in the RW sample exceeds that of GW even though the carbonate content of both samples is equal. Elevated levels of 210 Po activity in the carbonate-bound fraction may result from the decay of unsupported 222 Rn. Underground mining operations can thus provide drains for the circulation of radon gas. This phenomenon modifies the secular equilibrium between radionuclides in 238 U decay series.

Fig. 2 :Fig. 3 :

 23 Fig. 2: Gamma cartography of the "Rophin" site (A) and the "Granges du Colonel" site (B)

  [START_REF] Al-Masri | Speciation of 210Po and 210Pb in air particulates determined by sequential extraction[END_REF], a modified procedure was performed in order to detect the 210 Po distribution within the geochemical fraction. The IAEA-385 standard was studied for 210 Po speciation using the procedure of sequential extraction; results are shown in Table2. It can be noticed that the210 Po fraction proportion of the sample equals: 1.24% in water-soluble form, 1.71% in exchangeable form, 5.61% bound to carbonates, 2.92% bound to iron and manganese oxides, 1.09% bound to organic matter, and 87.4% bound to residue. In other words, 210 Po was almost entirely found in the last fraction, thus indicating an association with the insoluble mineral phases. The sum of 210 Po activity concentrations in these six each individual sequential extraction fractions is equal to 25 ± 3 Bq.kg -1 . This is in good agreement with the total 210 Po activity measured in the unfractionated sample of 27 ± 3 Bq.kg -1 . According to results obtained from the standard sediment IAEA-385, this sequential extraction procedure has successfully identified the 210 Po distribution in the various soil fractions.

Table 1 :

 1 Total 210 Po activity concentration (Bq.kg -1 dry weight)

Table 2 :

 2 Fractionation of 210 Po in the study samples (Bq.kg -1 ± SD)

Table 3 :

 3 Properties of the studied soil samples Information value of 210 Po: 28.0 Bq.kg -1 with a confidence interval from 23.8 to 36.6 Bq.kg -1 dry weight

	Fig. 3:

Acknowledgement

The authors thank the PACA Region for the doctoral grant awarded to Thi-Hong-Hanh Le, the POLLUSOLS program of Pays de la Loire Region for it support and the ZATU (Zone Atelier Térritoire Uranifère), Parc National du Mercantour for samplings.

List of figures and table