Pit lakes from Southern Sweden: natural radioactivity and elementary characterization

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.

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.

Sorting out ²³⁸U activity concentration in pit lake waters, the higher values correspond to sites (mBq/kg):

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.

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 low^38,39 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.

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%).

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 (K_d) have been performed.

Distribution coefficient (K_d) 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 K_d analysis. Some clear trends can be observed (Fig. 7): S (associated with sulfides) with low K_d can easily move to the aqueous phase, while the opposite behavior was found for Th with the highest K_d showing a clear tendency to remain in the solid phase. Intermediate values were obtained for the other trace elements having U with ranges of K_d values similar to Cr, Cu, Zn, As, Sr, Ba or Pb. Based on K_d 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:

Nguyen et al.⁴⁴ reported K_d values for various natural aquatic systems (Australian coastal and estuaries, six estuaries in Texas and in several other natural lakes), with log K_d 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 K_d 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 K_d 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 C_f and C_d 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 C_d value. Attending to calculated values (Table 3), only one site (Site 14) was found with a very high C_d value, mainly due to a very high Pb C_f, together with considerable C_f 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).

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).

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.

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 K_d values

In order to assess the K_d, 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 K_d values.

Using the described methodology, K_d 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 (k_d average 3,500) than ²³⁸U (k_d 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 (k_d average 2.9·10⁵) has a higher trend to incorporate into the aqueous solution in comparison with ²³²Th (k_d 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, K_d 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 K_d was observed than for U isotopes (impliying lower fractionation into aqueous phase than U), but lower K_d than Th isotopes, implying a higher fractionation into water than Th.

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

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):

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.

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.

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.

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