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Elemental and radiometrical characterization of surface water
Physico-chemical parameters of surface water
The physico-chemical parameters (i.e. pH, SC, ORP and DO) in surficial water samples from the 23 sampling sites are shown in Fig. 2 (Raw data in Table S2 from supplementary material). Results show low SC values of 47–597 µS/cm (average value of 260 µS/cm). The maximum SC values stemmed from Site 4 (due probably to the presence of sulfide-bearing schists in bedrocks outcropping in the drainage area according to the SGU local bedrock map30. Average pH values of 7.6 were observed in the studied lakes, with an interquartile between 7.1 and 8.2. Such high values could be due to the Swedish liming program initiated in 1977 to counteract the anthropogenic acidification observed in Swedish surface waters31. However, the minimum value of 4.9 was observed at Site 14, due to sulfide mining activities in this site, a derelict silver mine operated discontinuously from 1,483 to 1,900, leading the formation of a pit lake of around 240 m depth. The oxidation of galena and other sulfides found originally in this site may have caused such low pH values. The average ORP value in lake waters was 95 mV (interquartile range of 41 to 100 mV; Fig. 2), although a maximum value of 300 mV was observed at Site 4, where sulfide-bearing schists appear to outcrop in the drainage basin. Water samples were well oxygenated with DO values from 7.8 to 12 mg/L (65–99% of saturation) and average 9.9 mg/L. Such high concentrations of dissolved oxygen together with the low values of total phosphorous (average values of 2.5 mg/L, interquartile range of 1.3 to 3.8; Fig. 3), seem to indicate an oligotrophic nature of lakes studied.
Figure 2

Box-and-whisker plots of physico-chemical parameters (i.e. pH, specific conductivity (SC), oxidation–reduction potential (ORP) and dissolved oxygen (DO)) in pit lake surface water samples. The height of the box shows the interquartile range, which contains 50% of the values, while the horizontal line inside the box shows the median value and the red cross denotes the mean value. The whiskers are lines that extend from the box to the highest and lowest values excluding outliers (o) and extremes (*). In this sense, outliers represent those values being between 1.5 and 3 times larger than the length of the box from its upper or lower border while extreme are those greater than 3 times such value.

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Figure 3

(a) Concentration of elements in mg/kg and (b) in µg/kg of surficial water samples from the studied pit lakes, shown as box-and-whisker plots (refer to Fig. 2 for explanation) and sorted out by mean values.

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Major elements and trace elements in surface water
The content of dissolved elements in lakes is primarily controlled by rock weathering, atmospheric precipitation and evaporation-precipitation processes32. Ca and S are the main elements in disolution; in studied pit lakes average values of 140 and 280 mg/L of Ca and S (interquartile range of 50–220 mg/L and 110–420 mg/L), respectively, were recorded (Fig. 3a). Raw data to produce Fig. 3a,b) can be found as Table S3 in supplementary material. The main sources of S may be dry deposition and the acid rain in industrialized and urban areas; i.e. most lakes studies are found close to large cities (Stockholm, Örebro, Norköpping, etc.) and to a lesser extent the oxidation of minor amounts of sulfides present in the drainage catchment, while the high concentrations of Ca found in these lakes should be related to liming and to a lesser extent, to the dissolution of carbonates in old marble quarries. Compared to these elements, the concentration of others such as Na, Mg or K is noticeably lower; around 20 mg/L were observed (Fig. 3a). However, maximum values exceeding 100 mg/L of Na and Mg were found in Site 4 and Site 11, respectively. The latter site was a former quarry where marble (calcium-magnesium carbonate) was exploited. The intense water–rock interaction after flooding may have released significant concentrations of Mg to the water column.
Concerning trace elements, the most abundant is Fe with average values of 1,200 µg/L, followed by Zn (860 µg/L), Sr (120 µg/L), Mn (100 µg/L), Pb (38 µg/L), Cu (23 µg/L), Ba (22 µg/L), Cr (4.3 µg/L) and As (1.5 µg/L). The maximum values of Fe and Mn were observed at Site 13 (13,700 and 1,460 mg/L, respectively; Fig. 3b), probably due to the presence of colloidal material rich in Fe and Mn passing through the pore filter. This fact would also explain the high concentration of other trace metals such as Zn (8,400 mg/L). On the other hand, the average concentration of Th and U are 85 ng/L and 14 µg/L, respectively, although maximum values of 750 ng/L and 68 µg/L were observed. Such different values between U and Th may be related to differences in mobility of U and Th species despite that granites, the most abundant rock in the studied area, are mostly enriched in Th over U33. This enrichment of Th over U also applies to alkaline rocks (K or Na  >   > Ca) according to Dill34.
Alpha spectrometry of surface water
Uranium radioisotopes in surface water
A range from 0.3 to 1,183 mBq/kg with a mean value of 156 ± 272 mBq/kg (mean ± standard deviation) was found for 238U isotopes (Fig. 4a). Raw data to produce Fig. 4a,b and Table 1 can be found as Table S4 in supplementary material. These values could be compared with the geochemical background values of 238U for continental surface waters ranging from 0.02 to 266 mBq/kg and a mean value of 11 ± 21 mBq/kg18. The 72% of the sites had levels above the mean background value, so there is in general an enhancement of 238U level in pit lakes in southern Sweden. Furthermore, one natural lake was sampled at the beginning of each sampling campaign to be used as “reference value” in comparison with pit lakes. The 238U in a subset of 3 natural lakes ranged from 0.34 to 11 mBq/kg with mean 5.0 ± 5.2 mBq/kg), which is in the order of the environmental background value.
Figure 4

(a) 238U activity concentration levels in surface water samples from the studied pit lakes in southern Sweden. The horizontal line shows the mean geochemical background18. (b) 210Po activity concentration in surface water samples from pit lakes, presented together with its normal distribution and a box-and-whisker plot. Raw data can be found in Table S4.

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Table 1 Summary of activity concentration of U, Th and Po isotopes from 238U and 232Th series in pit lake surface water samples (n = 34).
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Sorting out 238U activity concentration in pit lake waters, the higher values correspond to sites (mBq/kg):

$${text{S}}21 , left( {1183} right) , > {text{ S}}15 , left( {735} right) , > {text{ S}}4 , left( {680} right) , > {text{ S}}3 , left( {609} right) , > {text{ S}}8 , left( {236} right) , > {text{ S}}2 , left( {154} right)$$

Enhanced levels of 238U were found and it is well known the potential risk to population due to the chemical and radiological toxicity of 238U, with the two important target organs being the kidneys and the lungs.
In this sense, it is worth mentioning that one of the pit lakes is nowadays used as tap water reservoir (Site 2). According to WHO35 there is a guidance level of 10 Bq/L in drinking water for 238U from the radiotoxic perspective, which is by far one order of magnitude higher than the maximum 238U level found in this sampling. However, due to the chemotoxicity of U a more restricted threshold of 30 µg/kg (370 mBq/kg) was defined by WHO in 2011. In our case, the 238U activity concentration at Site 2 was 150 mBq/kg which is 2.4 times lower than the threshold, so the chemotoxicity of U is not relevant for local inhabitants.
On the other hand, most of the lakes studied are used for recreational purposes (i.e. fishing, swimming, diving). Although dermal contact is considered a relatively unimportant path of exposure due to the limited transfer from skin to the blood, another possible routes of radionuclide incorporation or impact should be considered from the dose assessment perspective.
Regarding 234U, most of the samples had higher values than 238U, ranging from 0.3 to 1,700 mBq/kg with a mean of 210 ± 375 mBq/kg. The 234U/238U activity concentration ratios of pit lake water samples (Fig. 5) were all above unity except for one site. The 234U to 238U ratio should be 1 in case of secular equilibrium but it is well known that there may exist a disequilibrium in water with 234U/238U ratios above unity, due to a selective leaching, alpha-recoil transfer of 234Th directly into the aqueous phase and the combination of the two processes36. The ratio between these radionuclides in surficial water is typically 1.1 to 1.3, with higher values related to a major input of underground water into the water body. The present results are consistent with the expected fractionations based on the greater mobility of U and particularly 234U37.
Figure 5

Isotopic ratios showing disequilibrium in 238U series for pit lakes water samples. Sampling sites were sorted out attending to an increasing 230Th/234U ratio. Raw data can be found in Table S4.

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Thorium radioisotopes in surface water
The activity concentration average of 230Th (belonging to 238U series) was of 3.1 mBq/kg ranging from MDA (~ 0.1) to 26 ± 3 mBq/kg. The range of values found belongs to the typical environmental values for this isotope. Thorium concentration in water is expected to be very low38,39 although it can increase due to soluble complexes with humic material or carbonates40. These low values compared with those of 238U (and also compared with 234U, the 230Th direct ancestor) can be explained by a very low mobilization of Th and its tendency to be fixed to the solid phase. As a consequence, 230Th/234U activity ratios were all found lower than unity (Table 1 and Fig. 5) with a minimum value of 8·10–4.
Regarding 232Th and its daughters, the activity concentration levels were even lower than 230Th (Table 1 and Fig. 5), in accordance with Jia et al.41. The MDA for 232Th was 0.5 mBq/kg, and 46% of the samples had levels below MDA. 232Th activity concentration varied from MDA to 8.8 ± 2.7 mBq/kg with an average of 0.9 ± 1.6 mBq/kg. In continental waters 232Th ranges from less than 0.008 to 1.50 mBq/kg with mean 0.10 ± 0.16 mBq/kg18. In a direct comparison between Th isotopes, the ratio 230Th/232Th is mostly higher than 1 (Fig. 5), pointing out the different origin of these two radioisotopes.
Due to the low activity concentration of Th isotopes, these radionuclides will have a negligible radiological impact on exposed individuals.
210Po in surface water
Average activity concentration of 210Po, also belonging to 238U series, in surface water samples was 10.5 ± 17.9 mBq/kg with a variation from 0.8 to 95 ± 4 mBq/kg. The distribution of these data can be seen in Fig. 4b where 94% of the samples had values below 25 mBq/kg and 76% below 10 mBq, which is in agreement with environmental levels42.
From a dosimetric perspective, 210Po is the radionuclide with the highest ingestion dose coefficient what implies the higher radiological toxicity. As an example, in Site 2 (mentioned before) a pit lake used as water reservoir for human consumption, a straightforward assessment of the annual committed effective dose via ingestion is showed in Table 2. From this table, the multiplication of activity concentration, annual intake of water (assumed to be 2 kg/day in adults) and the effective dose coefficient by ingestion for adults, provides the annual dose by ingestión due to water including 238U, 234U and 210Po radionuclides (Th isotopes are neglected for being below MDA). Taking into account that the threshold for the effective dose in water is 0.1 mSv/y43, the total amount (0.0136 mSv/y) represents only 14% of this threshold.
Table 2 Annual committed effective doses (mSv/y) by ingestion in adults due to water consumption from pit lake site 2.
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Elemental and radiometrical characterization of sediments
Elementary characterization
The abundance of major and trace elements was determined in 14 sediment samples by XRF (Fig. 6), and the observed composition reflects accurately the lithological characteristics of the study area. Raw data to produce Fig. 6 can be found as Table S5 in supplementary material. The most abundant element is Si (62% of average), followed by Al (9.8%) present in aluminosilicate. The presence of Ca (average of 4.9%) suggests the influence of liming in the studied lakes, although the existence of derelict marble quarries among the sampling sites could also explain such values. Lower abundance of K, Mg, and Na was observed (3.2, 3.0 and 1.0%, respectively) related to the weathering of bedrock. The presence of oxides and hydroxides seems to be limited to Fe (average value of 4.8%), considering the low concentration of Mn in sediments (0.1%).
Figure 6

Concentration of major and trace metals in sediments (n = 14) from pit lakes, presented as box-and-whisker plots and sorted out by mean values. For explanation of the boxes and whiskers, refer to Fig. 2.

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Concerning trace metals, a crustal element like Ba was among the most abundant in sediments (309 ppm), followed by Pb (285 ppm), Zn (163 ppm), Sr (82 ppm), Cr (45 ppm), Cu (41 ppm), Th (18 ppm), As and Ni (14 ppm) or U (12 ppm). Liming of lake waters may have caused a net transference of metals from the water column to the lake sediments due to increase of pH values. In order to estimate the metal fluxes from the water column to the sediment and vice versa, assessment of distribution coefficients (Kd) have been performed.
Distribution coefficient (Kd) was estimated as the ratio between sediment and water concentration for an element. In some water and sediment samples, values below the detection limit were observed for some elements, and in such cases half of the detection limit was assumed in order to be considered in this Kd analysis. Some clear trends can be observed (Fig. 7): S (associated with sulfides) with low Kd can easily move to the aqueous phase, while the opposite behavior was found for Th with the highest Kd showing a clear tendency to remain in the solid phase. Intermediate values were obtained for the other trace elements having U with ranges of Kd values similar to Cr, Cu, Zn, As, Sr, Ba or Pb. Based on Kd average values and interquartile ranges, we can sort out the trend to be mobilized into the aqueous solution for the elements in pit lakes as follows:

$$left( {{text{Highest mobility}},{text{ i}}.{text{e}}.{text{ lowest K}}_{{text{d}}} } right){text{ S }} > {text{ Cu }}sim {text{ Zn}}sim {text{ P }} ge {text{ U }} ge {text{ As}}sim {text{ Cr}}sim {text{ Ba }} > {text{ Fe }} > {text{ Th }}left( {text{Lowest mobility}} right)$$

Figure 7

Distribution coefficient (Kd) for elements in pit lakes (n = 14) and sorted out by mean values. For explanation of the boxes and whiskers refer to Fig. 2.

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Nguyen et al.44 reported Kd values for various natural aquatic systems (Australian coastal and estuaries, six estuaries in Texas and in several other natural lakes), with log Kd ranges of 3.0–5.9, 3.8–6.7 and 3.8–7.2 for Cu, Zn and Pb, respectively. In our survey of pit lakes, we found log Kd ranges of 0.78–3.9, 1.0–3.4 and 2.1–3.5 for Cu, Zn and Pb, respectively. After comparison of these data sets, it is clear that the mobilization of these metals into the aqueous phase in the surveyed pit lakes is higher than in natural water environments (lower Kd values), once again demonstrating the need to study these special water bodies for the potential risk as a source of heavy metals in the surrounding environment.
As a tool to identify whether there exist or not a chemical pollution risk to the environment in a pit lake, sediment elementary composition can be used according to Håkanson29 proposal. In this model, after aplying Eq. (1), both Cf and Cd are classified in 4 levels: low, moderate, considerable and very high (Table 3). In the present work, Hg and PCB concentrations were not determined and Cd was found below detection limit (0.1 ppm) in all sediments. Thus 6 metals were included in the assessment what implies a conservative Cd value. Attending to calculated values (Table 3), only one site (Site 14) was found with a very high Cd value, mainly due to a very high Pb Cf, together with considerable Cf values of Zn and As. This lake is a well-known site for local people due to its “turquoise” water and commonly used for diving and swimming (see supplementary material file, picture 23).
Table 3 Contamination factor (Cf) and degree of contamination (Cd) in sediments from pit lakes for selected elements.
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Radiometrical characterization of sediments
In this section the concentration of NORM radionuclides, determined by means of alpha and gamma spectrometry in a set of 14 sediment samples is reported. U, Th and Po activity concentrations of radionuclides from 238U and 232Th series, determined by alpha spectrometry are shown in Table 4 (raw data to produce this table can be found as Table S6 in supplementary material).
Table 4 U, Th and Po activity concentrations of radionuclides from 238U series, 235U and 232Th in pit lake sediments (n = 12) determined by alpha spectrometry.
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These alpha spectrometry data were based on total digestion of the samples, and it should be noted that differences in activity concentration of U, Th and Po isotopes will depend on the digestion method used25. In that work the ratio was in general higher than unity (demonstrated in a subset of 10 sediment samples from Swedish pit lakes), and it was concluded that Th isotopes are more bound to the “undissolved fraction” than U and Po isotopes, which is in good agreement with the results presented above.
Gamma spectrometry results are presented in Table 5. Regarding the 238U series: 234Th, 226Ra (via 214Bi and 214Pb) and 210Pb, the activity concentrations agreed within 2-sigma criteria in most of the cases. Hence, we can conclude that there is practically secular equilibrium in sediments for the 238U series radionuclides (with only 210Pb in some cases having some excess, unsupported 210Pb). In connection with alpha measurements (Table S6 in supplementary material), 238U and 234U (via alpha) fit with 234Th (via gamma)25 and 210Po (via alpha) matches with 210Pb what proves the 210Pb-210Po secular equilibrium in the lower part of the 238U series in this set of sediments. It should be noted that the measured sediments originate from the shoreline and not from the deepest part of the lake. Additionally, due to the enhanced levels of 238U for some sediment samples values of 235U above gamma spectrometry MDA were found as well in 6 samples. In these samples the average 238U/235U ratio was 20.2 ± 4.0, which is in good agreement with the natural ratio 238U/235U of 21.4 in the environment36. Concerning alpha spectrometry, due to a lower MDA, 235U was measured in 11 samples and the average 238U/235U ratio was 22.4 ± 3.1, same accuracy but a more precise result than the obtained by gamma spectrometry.
Table 5 Radionuclide activity concentration in sediments (Bq/kg) based on gamma spectrometry measurements.
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A clear secular equilibrium was found for radionuclides in the 232Th series (228Ac, 212Pb, 212Bi and 208Tl), matching all radionuclides within k = 1 criteria uncertainty. For that reason, the reported value of 232Th-series refers only to 232Th. These values, as well as the corresponding values for 137Cs and 40K activity concentrations are shown in Table 5.
According to UNSCEAR45 the natural radionuclide content for soils in Sweden range (in Bq/kg): from 12–170, from 14–94 and from 560–1,150 for 226Ra, 232Th and 40K, respectively. The values obtained from the sediment samples are to a high extent within these ranges (Tables 4 and 5), except for sites S3, S21, S22 and S23, where isotopes from U, Th series and 40K (in some cases) levels exceed these values. It should then be noted that sites S3 and S21 also belong to those with the highest 238U activity concentrations in water samples.
Radionuclide Kd values
In order to assess the Kd, defined as the ratio between the activity concentration in sediment and the one in water, the data for U, Th and Po isotopes should in general be based on alpha spectrometry of sediments applied to total digestion and of water samples. However, to save time and resources, gamma spectrometry can be used for sediment samples when secular equilibrium in a decay series occurs25. Secular equilibrium between 238 and 234U was confirmed in sediments via alpha spectrometry, so 234Th may be analyzed instead. Furthermore, a comparison between 232Th via alpha and 228Ac via gamma spectrometry, confirmed a secular equilibrium, and hence the latter radionuclide can represent the activity concentration of the parent of the series. Regarding 230Th in sediments, 226Ra-234Th equilibrium was confirmed (with 230Th in between). Additionally, 210Po would require 210Pb-210Po equilibrium (to some extent also achieved within this set of sediments), so we can use 210Pb instead. If all these equilibria are fulfilled, alpha spectrometry in water and gamma spectrometry in sediment could then be used to obtain appropriate Kd values.
Using the described methodology, Kd values were assessed for each radionuclide (Table 6 and Fig. 8). Raw data can be found in Table S7, supplementary material. It can be observed that 238U and 234U have a similar behavior although 234U has a slight higher tendency (kd average 3,500) than 238U (kd average 4,700) to mobilize into the aqueous phase, confirming what was obtained through disequilibrium analyses of 238U series in water. And the same applies to 230Th and 232Th, i.e. similar behavior between isotopes although 230Th (kd average 2.9·105) has a higher trend to incorporate into the aqueous solution in comparison with 232Th (kd average 3.9·105). This behaviour can likely be connected with the alpha-recoil transfer of 234U directly into the aqueous phase, producing this preference to leach for 230Th compared with 232Th. Additionally, Kd ranges were in good agreement for U and Th isotopes (Fig. 8) and the ones for elemental U and Th (Fig. 7), implying a good performance between the 4 different techniques involved in this assessment (ICP-MS, XRF, alpha and gamma spectrometry). Concerning 210Po isotopes, a higher Kd was observed than for U isotopes (impliying lower fractionation into aqueous phase than U), but lower Kd than Th isotopes, implying a higher fractionation into water than Th.
Table 6 Distribution coefficient Kd for radionuclides in pit lakes (n = 14).
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Figure 8

Distribution coefficient (Kd) for radionuclides (n = 14) for the studied pit lakes. Sediment samples were measured by gamma and water samples by alpha spectrometry, assuming secular equilibrium. For explanation of the boxes and whiskers refer to Fig. 2.

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The aforementioned results can be summarized by ranging the radionuclides mobility into the aqueus solution in pit lake waters in the following way:

$$left( {{text{Highest mobility}},{text{ i}}.{text{e}}.{text{ lowest K}}_{{text{d}}} } right)^{{{234}}} {text{U }} >^{{{238}}} {text{U }} >^{{{21}0}} {text{Po }} >^{{{23}0}} {text{Th }} >^{{{232}}} {text{Th }}left( {text{Lowest mobility}} right)$$

Ambient dose rate equivalent
In Fig. 9, the ambient dose rate equivalent is plotted in terms of ambient dose rate. The values range from 0.06 to 0.37 μSv h−1 with an average value of 0.14 ± 0.08 µSv h−1 that is a typical environmental value. Note that Sites 22 and 23 were not measured due to technical problems during the sampling campaign. Thus, several sites with enhanced ambient dose rate levels compared with environmental average values (0.14 μSv h−1) were identified, with the five sites with highest values (in μSv h−1):

$${text{S3 }}left( {0.{37}} right) , > {text{ S7 }}left( {0.{32}} right) , > {text{ S4 }}left( {0.{21}} right) , > {text{ S21 }}left( {0.{19}} right) , > {text{ S15 }}left( {0.{18}} right)$$

Figure 9

Ambient dose rate equivalent (μSv h−1) in pit lakes from southern Sweden.

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Rock samples
Rock samples were collected and measured by gamma and alpha spectrometry (using total digestion) from four out of the five sites with highest ambient dose rate equivalent (except site 4). Secular equilibrium was found in all cases within 1-k criteria uncertainty for activity concentrations of 238U, 235U and 232Th natural series (Fig. 10), demonstrating good agreement between gamma and alpha spectrometry results. Concerning 238U to 232Th series ratios, 238U series activity concentration was higher than that of the 232Th series at site S3, while at sites S7 and S21 the opposite was found. Site S15 shows this ratio compatible with unity.
Figure 10

Activity concentration of selected radionuclides in rock samples based on gamma and alpha spectrometry from four of the five pit lake sites with highest external gamma dose rates.

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Large heterogeneities regarding activity concentration of radionuclides were found in rocks from a single site, and a clear example occurs at site 3B (labelled as RS3B and RS3B-2). The RS3B-2 sample (from a former feldspar mine) exhibit activity concentrations of 513 ± 50, 22.6 ± 1.5 and 8.7 ± 1.4 Bq/g for 238U, 235U and 232Th, respectively, indicating an approximate 4% mass content of U. According to IAEA46, any material exceeding 1 Bq/g of 238U or 232Th will require further investigations in terms of its use or storage. The radiological assessment becomes a relevant study to address because this site is quite often visited by people for recreation. The activity ratio 238U/235U of 22.7 ± 2.7 is in good agreement with the natural ratio 21.4.
A detailed examination by SEM–EDX was applied to two rocks from sites S3B and S7, with the highest ambient dose rate equivalents, to determine the morphology and chemical composition of minerals forming the bedrock, which could explain the high dose rates found.
SEM–EDX for rocks from site S7
The bedrock outcropping in the surrounding of Site 7 is mainly composed of granite and pegmatite, formed by quartz, feldspar and mica, although other minor minerals can be found. Figure 11a,b shows SEM backscattered images (BES) and combined with the data presented in Table 7, a predominance of particles of aluminosilicate and Fe oxides can be detected. However, the presence of brighter particles denoting the presence of elements of higher atomic number is also observed. Semi-quantitative analyses on this sample show high concentrations of Y (up to 11%) and Th (up to 28%) according to Fig. 11c and Table 7. The presence of these elements may be related to the accessory mineral assemblage commonly found in granites, such as monazite, xenotime, Th-ortosilicate and uraninite47. The weathering of these minerals may lead to the release into the lake sediments although due to the topography of site S7, no sediments could be collected at this site.
Figure 11

(a) and (b) SEM Backscattered images of rock sample RS7 marking spots where six different spectra were recorded. c) SEM–EDX spectra from spot 6 (spectrum 6).

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Table 7 Semi-quantitative elementary composition (% weight) at 6 spots acquired from RS7.
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SEM–EDX for rocks from site S3
Different materials compose the bedrock surrounding site 3. The predominant rocks in the drainage area are granitoids and syenitoids, although the presence of volcanic acid rocks such as rhyolite and dacites are also observed. Figures 12a–c shows backscattered images of rock samples collected at site 3. The results from Table 8 show presence of aluminosilicates and Fe oxides, as well as tantalum and niobium. Ta and Nb mineralization is often associated with geochemically specialized granites which are characterized by enrichment in fluorine, and by the development of pervasive, postmagmatic alteration48. Similar to the case of site 7, the occurrence of accessory minerals (e.g. zircon, Th-orthosilicate and uraninite) in granite may explain the high concentrations of Th (up to 32%) and U (up to 8.9%) observed in this area. It is worth to point out the huge size of the heaviest particles in this sample (Fig. 12a). The presence of U and Th-enriched zircon as accessory mineral of pegmatites (similar composition as granites) caused enhanced levels of radionuclides in ground waters of SW Niger49. Therefore, water interacting with such materials need to be studied in the long term.
Figure 12

SEM Backscattered images of rock sample RS3B-2, and spots where the spectra were taken.

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Table 8 Semi-quantitative elementary composition (% weight) at 6 spots acquired from RS3B-2.
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