Uranium in Lake Sediments of Humid Zone: A Case Study in the Southeast Fennoscandia (Karelia, Russia)

: The article presents data on the analysis of U accumulation in recent sediments of lakes in the territory of the Southeast Fennoscandia. The research was carried out in the study area of the Republic of Karelia. It has been established that the content of U in sediments varies from 0.1 to 42.3 mg/kg (median is 0.91 mg/kg). In general, the studied sediments of the region had low concentrations of U in comparison with the average content of this element in the upper part of the Earth’s crust. In some areas associated with deposits or ore occurrences of U, an increased content of U in lake sediments was revealed. The highest U accumulation level was found in the lake sediments, which are under the inﬂuence of the North Onega ore-geochemical region, where V deposits and ore occurrences that contain U, Fe, Mo and Cu are widespread. In the sediments of some studied lakes, Th anomalies were found, which often accompany U in ore geological formations. The analysis of uranium fractions in the sediments of some lakes in Karelia revealed the key role of the mineral (insoluble) phase in the accumulation of U, up to 64–68% of the total U content. The share of the organic fraction in the accumulation of U in the studied sediments of the lakes is small and ranges from 7 to 15% with respect to the total concentration of the metal.


Introduction
The study of lake sediments is an important part of environmental sciences.Recent lake sediments can indicate various processes occurring in the catchment area [1].The chemical composition of lake sediments reflects the features of rocks located near the water object.For example, studies of North American lakes (Ontario, Canada) have shown that organic sediments are enriched in nickel and copper near rocks with established mineralization of these metals [2].The study of lake sediments can be an important element of geological exploration, especially exploration of mineral deposits.Finding geochemical anomalies in various elements in sediments significantly narrows the search for minerals over large areas.In addition, lake sediments are used to assess the anthropogenic impact on a water body and the area around it.For example, geochemical studies of modern lake sediments in Florida (USA) have shown increased accumulation of lead due to emissions from vehicles that run on ethylene gasoline, as well as due to the use of lead arsenate for pest control on golf courses [3].In the Russian Arctic, recent sediments analyze from a mountain lake turned out to be enriched in phosphorus and rare earth metals due to the production of apatite-nepheline ore, as well as antimony, nickel and vanadium due to the operation of a boiler house and a thermal power plant [4].In general, with regard to geochemistry in lake sediments, the distribution of chemical elements is most often studied in order to establish the influence of industry or transport on the aquatic ecosystems.A recent review of paleolimnological studies in the territory of Russia identified it as one of the great lacustrine countries [5].The influence of natural, primarily geological, factors on the accumulation of heavy metals and other elements in deposits is usually considered in the context of studying the geochemical background of any territory [6][7][8].However, there are unique studies of lake sediments that allow marking catastrophic natural phenomena.In a study in Northwest Russia on the increased concentrations of nickel, copper, chromium, iridium and platinum in the sediments of lakes, the influence of a meteorite fall on the geochemistry of the sediments was found.The researchers also suggested the influence of the volcanic eruption 13,000 years ago on the anomalous accumulation of rare earth elements and strontium in the studied sediments [9].
In addition to heavy metals, an important role in the environmental sciences is occupied by studies of the behavior of other elements that are interesting from the point of view of their effect on environment.One of these elements is uranium (U).This is a chemical element of the third group of the seventh period of the periodic system of chemical elements of D.I. Mendeleev, with atomic number 92; it is a very heavy, slightly radioactive, glossy silver-white metal.Among all elements that naturally occur, U is the highest numbered.The content in the Earth's crust is, according to various estimates, from 0.9 to 2.8 mg/kg [10][11][12].The bulk of U is found in acidic rocks with a high silicon content.A significant mass of U is concentrated in sedimentary rocks, especially those rich in organic matter [12,13].For example, U is found in fossil coals at a concentration of 6.1 mg/kg [14].In the soils of the world, U concentration varies between 0.8 and 11 mg/kg [12]; the average content is 3 mg/kg in the soils of the world and 2.4 mg/kg in the soils of Europe [15].
A significant proportion of U concentration in soil and sediment is associated with primary minerals (for example, uraninyl) of igneous or volcanic origin [16,17].However, U also forms strong compounds with organic substances [18].In addition, it is known that U can be sorbed by particles of clay minerals, iron hydroxides, and carbonates [19,20].Due to the described properties of U, this element can form surface deposits associated with peatlands and organic deposits of lacustrine-glacial origin in the areas with developed ore mineralization of U [21,22].In Europe, examples of these objects are found in Sweden, Finland and the UK.For example, in the Masugnsbyn peatland located in the north of Sweden, the U content in the peat ashes ranges from 400 to 31,000 mg/kg, with an average value of 2000 mg/kg [21].In the south of Great Britain, in the area of the Mountain Cellington Brooke, small peat deposits are enriched in U; the maximum U content in the peat can reach 580 mg/kg [13].In the south of the Czech Republic, U is enriched (up to 1127 mg/kg) in the organic sediments of Lake Pleshne, located near a granite massif with U mineralization.Moreover, the maximum concentrations of U are established in the layers of sediments of the Holocene age [23].In Canada (province of Saskatchewan), recent sediments of lakes located near the mining area of uranium-containing rocks were shown to be enriched with U. Since the operation of the mines, the flow of U into the lakes has increased up to 23 times in comparison the background [22].
The Republic of Karelia is the region in northwestern Russia almost entirely located on the Fennoscandian Shield, also covering the territory of Murmansk region, Finland, Sweden and Norway.The predominantly Karelian part of the crystalline shield consists of Proterozoic and Archean rocks aged from 0.54 to 3.6 billion years.Among the rocks of the Archean age, granites, tonalites, andesites, gabbro and basalts stand out.The geology of the Proterozoic is represented by granites, basalts, gabbro, quartzites, sandstones, dolomites, tuffs, carbonaceous shales, gneisses and syenites.The Phanerozoic rocks are few and predominantly represented by clays, siltstones, sands, conglomerates and dolomites [24,25].The quaternary sediments of Karelia are thin (on average 7-12 m) and mainly represented by the glacial and water-glacial formations of the last glacial period.Large areas of the region are generally devoid of quaternary sediments.In these places, magmatic rocks are shown directly on the surface [26].In this regard, the ore potential (Fe, Cr, Ti, V, Cu, Ni, Pb, Zn, Mo, W, Sn, Au, U, Mo, W, Sn, Au, U and rare earth elements) of the rocks of Karelia [27] can play an important role in the migration and accumulation of various chemical elements in the recent geological formations of the region, including soil, peat, and sediments of lakes and rivers.In this article, the main emphasis will be on the lake sediments of Karelia, because there are more than 60,000 small and large lakes in the territory of Karelia, and organic-rich lake sediments are widespread [28][29][30].Moreover, studying ecologically important elements is particularly interesting for environmental sciences.It is known that U in drinking water poses a risk to human health, especially children, due to its toxic potential [31,32].Studies have shown that uranium consumed by humans and animals can have a toxic effect, manifested by kidney damage [33].Studies of U in drinking water in Germany and human diseases have shown that U contamination may cause a small risk of leukemia in men and lung and kidney cancer in women [34].It is also statistically proven that in the United States, areas with human use of groundwater rich in uranium have a higher risk of colorectal cancer, breast cancer, kidney cancer, prostate cancer and total cancer compared with control areas [35].
Though the deposits and ore occurrences of U are widespread in the regions located on the Fennoscandian Shield, described and published findings of the U anomalies in the lakes are rare [4,18,36].Thus, based on a large amount of geochemical data obtained by the author from 2016 to 2020, the main goal of this work was to assess the level of U accumulation in modern sediments of lakes in the Republic of Karelia as part of the Southeast Fennoscandia, taking into account regional geological and geochemical features.

Materials and Methods
The data were obtained within the framework of different scientific projects.About 30 small lakes of the northern and southern parts of the Republic of Karelia, as well as Vygozero Reservoir (or Vygozero), were studied (Figure 1).Freshwater sediments were sampled from both ice of the water bodies and open water using an inflatable boat.To collect the sediments, a Limnos sampler (for the most recent layers of the sediments up to the depth of 60 cm from the border water-bottom) and a manual 'Russian' drill (for the sediments deeper than 60 cm from the border water-bottom) were used.After sampling the selected cores of the sediments, they were divided into 1-50 cm layers, depending on investigation purposes.Plastic containers were used for storing the samples before analytical studies and delivering the samples to the laboratory.
The lake sediment samples were further dried to an air-dry condition at room temperature and then to an absolutely dry condition in an oven at the temperature of 100 • C. Laboratory tests were conducted at the Analytical Center of the Institute of Geology at the Karelian Research Center, Russian Academy of Sciences (Petrozavodsk, Karelia), and at the Institute of Chemistry and Technology of Rare Elements and Minerals at the Kola Research Center, Russian Academy of Sciences (Apatity, Murmansk Region).
To determine the loss of ignition (LOI) by the weight method, samples were heated at the temperature of 550 • C to constant weight.The content of the main elements in the lake sediments was assessed using an ARL ADVANT'X X-ray fluorescence spectrometer (Ther-moFisher Scientific, Waltham, MA, USA).The concentration of trace elements, including U, in the sediment samples was determined using an XSeries-2 ICP-MS mass spectrometer (Thermo Fisher Scientific).The analytical methods were described in detail in the previous works of the author [37,38].
To separate various fractions of chemical elements in the lake sediment samples, the method of sequential extraction [39] was applied for the following fractions:  The lake sediment samples were further dried to an air-dry condition at room temperature and then to an absolutely dry condition in an oven at the temperature of 100 °C.Laboratory tests were conducted at the Analytical Center of the Institute of Geology at the Karelian Research Center, Russian Academy of Sciences (Petrozavodsk, Karelia), and at the Institute of Chemistry and Technology of Rare Elements and Minerals at the Kola Research Center, Russian Academy of Sciences (Apatity, Murmansk Region).
To determine the loss of ignition (LOI) by the weight method, samples were heated at the temperature of 550 °C to constant weight.The content of the main elements in the lake sediments was assessed using an ARL ADVANT'X X-ray fluorescence spectrometer (ThermoFisher Scientific, Waltham, USA).The concentration of trace elements, including U, in the sediment samples was determined using an XSeries-2 ICP-MS mass spectrometer (Thermo Fisher Scientific).The analytical methods were described in detail in the previous works of the author [37,38].
To separate various fractions of chemical elements in the lake sediment samples, the method of sequential extraction [39] was applied for the following fractions: (a) water-soluble fractions (reagent H2O), (b) available (exchangeable) fractions (reagent NH4CH3COO), (c) fractions associated with Fe and Mn hydroxides (reagents 0.04 M NH2OH*HCl in 25% CH3COOH), (d) fractions associated with organic matter (reagents 0.02 M HNO3 + 30% H2O2 and 3.2 M NH4CH4COO in 20% HNO3), (e) acid-soluble (residual) fractions (reagent HNO3), To measure the total concentration of trace elements, the sediment samples were dissolved in HF, HNO 3 , and HCl in an open system.To ensure quality control and quality assurance, simultaneous measurements of certified reference materials (a sediment sample from Lake Baikal BIL-1-GSO 7126-94) were conducted.The results of the analysis of the collected samples and the reference sample showed that the measured concentrations in mg/kg are characterized by a relative standard deviation (RSD) of 6.7-10.6%(Table 1).Thus, the relative measurement error did not exceed the permissible values (15%) for the trace elements identified in the study [37].
Table 1.The results of measurement of trace element content (mg/kg) in the certified reference standard BIL-1 (sediment sample from Lake Baikal).Note.LOD-limit of detection; BIL-1 std .-thestandard value in the reference sample BIL-1; BIL-1 meas .-themeasured value in the reference sample BIL-1; ±-the certified range of measurement error; RSD abs .-theabsolute measurement error; RSD rel .-therelative measurement error; n/r-the measurement error range not calculated.The results were statistically processed.The arithmetic mean, standard deviation, median, and Spearman correlation coefficients were calculated using Microsoft Excel 2010.Inkscape 0.48 software was used to graphically illustrate the results.

General Patterns of U Accumulation
The median U content in the sediments of the Karelian lakes was 0.91 mg/kg (the values varied between 0.14 and 42.33 mg/kg) (Figure 2).The large scale of the U content values was also noted when studying the small lakes of the Southern Urals (Russia), where the U concentrations ranged from 1.15 to 88.16 mg/kg [40,41].In the previously studied pre-industrial (background) layers of the sediments of the small lakes in Karelia, the U content was 1.5 mg/kg [37].Thus, the background layers of the sediments of the Karelian lakes were richer in the studied metal.

Geological Interpretation of U Anomalies
These high U concentrations in the sediments of Lake Gryaznoe are mainly caused In general, the U concentration in the freshwater sediments of the region (in the background and recent sediment layers) was lower than the average U content in the Earth's crust, which is 1.7 mg/kg [10].However, in individual lakes (Gryaznoe, Plotichie, Kitaiskoe, Chetyrekhverstnoe, Airanne and Raivattalanlampi), elevated concentration of this metal relative to the Earth's crust were detected.For example, in the sediments of Lake Gryaznoe (Medvezhiegorsk district, the Republic of Karelia) the U concentration varied between 5.7 and 42.3 mg/kg, increasing from the upper (most modern) layers of the deposits to the lower (Holocene) ones (Figure 3).Consequently, a 25-fold U content in comparison to the Earth's crust was revealed in the lowest layers of Lake Gryaznoe.The median content of this metal in the sediments of the studied lake was 8.4 mg/kg, which is five times higher than the Earth's crust.

Geological Interpretation of U Anomalies
These high U concentrations in the sediments of Lake Gryaznoe are mainly caused by natural processes, since the water body is located in the zone of the North Onega ore geochemical district, where mineral deposits and occurrences of vanadium ores with high U concentration were revealed [42].The ores of this area are mainly localized in terrigenous-carbonate rocks of the Fennoscandian Shield.The U minerals are represented by uraninite, pitchblende, coffinite and, less often, brannerite.In the Srednyaya Padma field (Figure 4), U-V-ores contain on average 610-740 mg/kg of U [43].In addition to U, these ores have an increased content of Fe, Mo, Cu and Au.The pairs of concentration U-Cu and U-Mo from the sediments of Lake Gryaznoe showed a high level of correlation (Figure 5).This indicates a single source of these metals in the water and the sediments of the water body as well as the single mechanism of their accumulation in the lake sediments.The sediments of the lakes in the city of Medvezhegorsk (Plotichie and Kitaiskoe), also located in the zone of North Onega ore geochemical district, were studied as well.In the sediments

Geological Interpretation of U Anomalies
These high U concentrations in the sediments of Lake Gryaznoe are mainly caused by natural processes, since the water body is located in the zone of the North Onega ore geochemical district, where mineral deposits and occurrences of vanadium ores with high U concentration were revealed [42].The ores of this area are mainly localized in terrigenouscarbonate rocks of the Fennoscandian Shield.The U minerals are represented by uraninite, pitchblende, coffinite and, less often, brannerite.In the Srednyaya Padma field (Figure 4), U-V-ores contain on average 610-740 mg/kg of U [43].In addition to U, these ores have an increased content of Fe, Mo, Cu and Au.The pairs of concentration U-Cu and U-Mo from the sediments of Lake Gryaznoe showed a high level of correlation (Figure 5).This indicates a single source of these metals in the water and the sediments of the water body as well as the single mechanism of their accumulation in the lake sediments.The sediments of the lakes in the city of Medvezhegorsk (Plotichie and Kitaiskoe), also located in the zone of North Onega ore geochemical district, were studied as well.In the sediments of Lake Plotichie, the U concentration varied between 1.7 and 12.5 mg/kg (the median was 2.7 mg/kg), and in the sediments of Lake Kitaiskoe, the U content varied between 1.1 and 5.4 mg/kg (the median was 2.6 mg/kg).In both cases, there was a slight enrichment over the Earth's crust, which confirms the natural origin of U anomalies in the lake sediments of southeastern Karelia.Furthermore, in the sediment core of Lake Plotichie, an increase in U concentration from the upper layers to the lower ones was observed.
In the sediments of Lake Chetyrekhverstnoe (Petrozavodsk), the U content also reached concentrations exceeding Earth's crust (up to 8.1 mg/kg).In this case, this is due to the proximity of the Ptitsefabrika U deposit, situated in the south of Petrozavodsk (Figure 4) [43].This deposit is localized in feldspars-quartz sandstones and siltstones of the Fennoscandian Shield.The maximum U content in the rocks of the deposit reaches 2200 mg/kg.As in the case of Lake Gryaznoe, Lake Chetyrekhverstnoe had the maximum U concentration in the lower (Holocene) sediment layers.Notably, in the Ptitsefabrika mineral deposit, U is associated with Mo, Ag and rare metals.This is reflected in the high level of correlation between U and Mo, as well as U and the rare-earth elements in the studied sediments.This confirms the geological effect on the elevated U level in the sediments of Lake Chetyrekhverstnoe.At the same time, the trend of increasing U concentrations in the most recent layers of Lake Chetyrekhverstnoe sediments should be noted (Figure 6).
of Lake Plotichie, the U concentration varied between 1.7 and 12.5 mg/kg (the median wa 2.7 mg/kg), and in the sediments of Lake Kitaiskoe, the U content varied between 1.1 an 5.4 mg/kg (the median was 2.6 mg/kg).In both cases, there was a slight enrichment ove the Earth's crust, which confirms the natural origin of U anomalies in the lake sediment of southeastern Karelia.Furthermore, in the sediment core of Lake Plotichie, an increas in U concentration from the upper layers to the lower ones was observed.In the sediments of Lake Chetyrekhverstnoe (Petrozavodsk), the U content als reached concentrations exceeding Earth's crust (up to 8.1 mg/kg).In this case, this is du to the proximity of the Ptitsefabrika U deposit, situated in the south of Petrozavods of Lake Plotichie, the U concentration varied between 1.7 and 12.5 mg/kg (the median wa 2.7 mg/kg), and in the sediments of Lake Kitaiskoe, the U content varied between 1.1 and 5.4 mg/kg (the median was 2.6 mg/kg).In both cases, there was a slight enrichment ove the Earth's crust, which confirms the natural origin of U anomalies in the lake sediment of southeastern Karelia.Furthermore, in the sediment core of Lake Plotichie, an increas in U concentration from the upper layers to the lower ones was observed.In the sediments of Lake Chetyrekhverstnoe (Petrozavodsk), the U content als reached concentrations exceeding Earth's crust (up to 8.1 mg/kg).In this case, this is du to the proximity of the Ptitsefabrika U deposit, situated in the south of Petrozavods U concentration in the lower (Holocene) sediment layers.Notably, in the Ptitsefabrika mineral deposit, U is associated with Mo, Ag and rare metals.This is reflected in the high level of correlation between U and Mo, as well as U and the rare-earth elements in the studied sediments.This confirms the geological effect on the elevated U level in the sediments of Lake Chetyrekhverstnoe.At the same time, the trend of increasing U concentrations in the most recent layers of Lake Chetyrekhverstnoe sediments should be noted (Figure 6).In the sediments of lakes Airanne and Raivattalanlampi located near the coastline of Lake Ladoga increased U concentration (Figure 6) were also revealed.In particular, in the sediments of Lake Airanne (the city of Sortavala), the concentration of this metal varied between 2.7 and 3.8 mg/kg (median is 3.5 mg/kg), which is 1.6-2.2times higher than the Earth's crust.In the sediments of Lake Raivattalanlampi, the U content ranged from 2.1 to 3 mg/kg (the median was 2.6 mg/kg), which is 1.2-1.8times higher than the Earth's crust.The northern part of Lake Ladoga is known for its occurrences and deposits of various metals, including U [43].For example, Lake Airanne is located in the zone of the Sortavala ore-geochemical cluster, where one U deposit is situated (Figure 4).The average U content in the basic magmatic rocks of the ore-geochemical cluster is 1.7 mg/kg.In addition to U, these rocks are also enriched in W, Mo, Th and other metals [42].The increased U concentrations in the sediments of Lake Raivattalanlampi may be associated with biotite and grenade-biotite plagiogneiss and plagioshales of the Fennoscandian Shield, widespread in the Lakhdenpokhsky district of the Republic of Karelia (Figure 4).The average U content in these rocks is 26 mg/kg [42].In addition to U, there are elevated concentrations of Mo, Be and some rare-earth elements in these rocks.
Studying the geology and geochemistry of the area showed that the rocks with a high U content have elevated concentrations of Th, which often accompanies U in minerals and rocks.For example, in the rocks of Lakhdenpokhsky and Sortavalsky districts, with U mineralization, the average content of Th varies from 6.6 to 26 mg/kg [42].Analyzing the accumulation of this element in the sediments of lakes Airanne and Raivattalanlampi showed an increased Th content relative to the Earth's crust (up to 12.4 mg/kg for Lake Airanne and up to 9.2 mg/kg for Lake Raivattalanlampi).In general, in the sediments of the studied Karelian lakes, the Th content varied between 0.34 and 16.38 mg/kg (Figure 7).The median content of this element was 2.39 mg/kg, which is lower than the Earth's crust (8.5 mg/kg) [10].In addition to the lakes of the Northern Lake Ladoga area, elevated (>8.5 mg/kg) Th concentration was revealed in the sediments of lakes Chetyrekhverstnoe (up to 12.1 mg/kg), Plotichie (up to 8.6 mg/kg) and No. 2 (up to 16.4 mg/kg).Only small ore In the sediments of lakes Airanne and Raivattalanlampi located near the coastline of Lake Ladoga increased U concentration (Figure 6) were also revealed.In particular, in the sediments of Lake Airanne (the city of Sortavala), the concentration of this metal varied between 2.7 and 3.8 mg/kg (median is 3.5 mg/kg), which is 1.6-2.2times higher than the Earth's crust.In the sediments of Lake Raivattalanlampi, the U content ranged from 2.1 to 3 mg/kg (the median was 2.6 mg/kg), which is 1.2-1.8times higher than the Earth's crust.The northern part of Lake Ladoga is known for its occurrences and deposits of various metals, including U [43].For example, Lake Airanne is located in the zone of the Sortavala ore-geochemical cluster, where one U deposit is situated (Figure 4).The average U content in the basic magmatic rocks of the ore-geochemical cluster is 1.7 mg/kg.In addition to U, these rocks are also enriched in W, Mo, Th and other metals [42].The increased U concentrations in the sediments of Lake Raivattalanlampi may be associated with biotite and grenade-biotite plagiogneiss and plagioshales of the Fennoscandian Shield, widespread in the Lakhdenpokhsky district of the Republic of Karelia (Figure 4).The average U content in these rocks is 26 mg/kg [42].In addition to U, there are elevated concentrations of Mo, Be and some rare-earth elements in these rocks.
Studying the geology and geochemistry of the area showed that the rocks with a high U content have elevated concentrations of Th, which often accompanies U in minerals and rocks.For example, in the rocks of Lakhdenpokhsky and Sortavalsky districts, with U mineralization, the average content of Th varies from 6.6 to 26 mg/kg [42].Analyzing the accumulation of this element in the sediments of lakes Airanne and Raivattalanlampi showed an increased Th content relative to the Earth's crust (up to 12.4 mg/kg for Lake Airanne and up to 9.2 mg/kg for Lake Raivattalanlampi).In general, in the sediments of the studied Karelian lakes, the Th content varied between 0.34 and 16.38 mg/kg (Figure 7).The median content of this element was 2.39 mg/kg, which is lower than the Earth's crust (8.5 mg/kg) [10].In addition to the lakes of the Northern Lake Ladoga area, elevated (>8.5 mg/kg) Th concentration was revealed in the sediments of lakes Chetyrekhverstnoe (up to 12.1 mg/kg), Plotichie (up to 8.6 mg/kg) and No. 2 (up to 16.4 mg/kg).Only small ore occurrences of Th, associated, as a rule, with the clastic rocks (quartz-pebble conglomerates) of the early Proteorozoic age having a U-thorium nature, are known from the territory of Karelia [43].The connection between Th and U is confirmed by their close correlation in the recent sediments of the lakes Chetyrekhverstnoe, Airanne and Raivattalanlampi, where high concentrations of Th were registered (Figure 8).However, in general, in all the studied lake sediments, significant correlations between Th and U were not detected.
ates) of the early Proteorozoic age having a U-thorium nature, are known from the terri tory of Karelia [43].The connection between Th and U is confirmed by their close correla tion in the recent sediments of the lakes Chetyrekhverstnoe, Airanne and Raivattalan lampi, where high concentrations of Th were registered (Figure 8).However, in genera in all the studied lake sediments, significant correlations between Th and U were not de tected.As noted earlier, for recent geological formations (peatlands and bottom sediment of water bodies), positive geochemical U anomalies are not uncommon in the areas with developed ore mineralization of this metal [23].Studies of peatlands and lacustrine sedi ments enriched in U are reviewed in [13,21,22].Notably, the peat enrichment in U can occur directly from the parental rock (for example, in mountainous areas) or through soi occurrences of Th, associated, as a rule, with the clastic rocks (quartz-pebble conglomer ates) of the early Proteorozoic age having a U-thorium nature, are known from the terri tory of Karelia [43].The connection between Th and U is confirmed by their close correla tion in the recent sediments of the lakes Chetyrekhverstnoe, Airanne and Raivattalan lampi, where high concentrations of Th were registered (Figure 8).However, in genera in all the studied lake sediments, significant correlations between Th and U were not de tected.As noted earlier, for recent geological formations (peatlands and bottom sediment of water bodies), positive geochemical U anomalies are not uncommon in the areas wit developed ore mineralization of this metal [23].Studies of peatlands and lacustrine sedi ments enriched in U are reviewed in [13,21,22].Notably, the peat enrichment in U ca occur directly from the parental rock (for example, in mountainous areas) or through so As noted earlier, for recent geological formations (peatlands and bottom sediments of water bodies), positive geochemical U anomalies are not uncommon in the areas with developed ore mineralization of this metal [23].Studies of peatlands and lacustrine sediments enriched in U are reviewed in [13,21,22].Notably, the peat enrichment in U can occur directly from the parental rock (for example, in mountainous areas) or through soil and groundwater [22].It is believed that U is leached from rocks and migrates in ionic form with water flows [21].In water and sediments, U forms insoluble complexes with clay and silty particles or/and organics.An analysis of the accumulation of U in the sediments of lakes in the Canadian province of Saskatchewan suggests that the transport pathways via run-off, surface flow or groundwater flow is an important stage in the migration of U from the source of pollution (mines) toward water bodies [22].Studies of the accumulation of U in the sediments of Lake Baikal have shown that uranium enters the water and then is absorbed by organic particles and solid particles of iron and manganese oxides [44].The described processes probably occurred during the U-enrichment of the sediments of the studied lakes in Karelia.For example, there are no exits of indigenous rocks in the area of Lake Gryaznoe, and the nearest explored deposits are tens of kilometers from the water body.In this case, the received U could have accumulated via groundwater or surface flow.Lake Airanne, on the contrary, is at the foot of a mountain consisting of magmatic rocks.In this case, U could have accumulated directly from the source, if we assume that the U mineralization is localized in the mountain rocks.
Accumulation of U in lake sediments resembles U accumulation in the peats of the humid zone.In the core sediments of Lake Syrytkul (South Ural), more than 11,500 years old, the maximum U content (88.2 mg/kg) was registered for carbonate sediments with peat layers accumulated in humid conditions of climate warming [41].Minimum U concentration in the sediments of Lake Syrytkul (3.7 mg/kg) were characteristic of the lake clay.In the core of the Holocene sediments of Lake Plesne (Czech Republic), aged ~15 kyr, the U content in organic-poor layers varied between 16 and 51 mg/kg, while in the organic sediment layers, the U content was the maximum for the entire sediment sequence (1127 mg/kg) [23].The same was observed in Lake Gryaznoe (Karelia): in the clayey silt underlying the organic sediments with an elevated U content, the concentration of this metal was 4.4 mg/kg; however, organic sediment layers accumulated the largest amount of uranium for all the lakes of Karelia studied in this work (Figure 2).Thus, these cases confirmed that U better accumulates in the sediments rich in organic matter.This is not always the case, given that the sediments of Lake Raivattalanlampi, with elevated concentration of U and Th, have a low content of organic matter [45].However, in this case, the proximity of the geological source of U is probably more important, since Lake Raivattalanlampi is surrounded by the rocks that may have ore mineralization.

Study of Different U Fractions
Studying small lakes in Siberia also showed that U accumulates in carbonate lake sediments with a high level of mineralization and high pH, where U exists in the form of carbonate uranyl-ion complexes.There is a high degree of correlation between the concentrations of U and Ca, as well as between those of U and Sr [46].It is known that U can have a correlation with Ca and carbonates in general in the pore groundwater, although such species are usually dissolved [16].In the territory of Southeast Fennoscandia, carbonate sediments are rare [24,25].Additionally, there is not a high Ca concentration in the surface water of Karelia [30].Therefore, the key factor in the U accumulation in the studied lake sediments is organic matter and terrigenous (possibly clay) fractions, as shown in the studies in different regions of the world mentioned above.
Indeed, in some lakes of Karelia, the close correlation between the concentrations of U and Ca (for example, in Lake Gryaznoe) was notable; however, in the other studied lakes this correlation was not observed (Lake Chetyrekhverstnoe).In the water and sediments of lakes in Karelia, Ca was found in silicates and aluminosilicates, so the apparent U selectivity in this case is associated with a greater or lower connection of this metal with the mineral phase of the sediments.
In the Murmansk region (also Fennoscandian Shield), where carbonate sediments are uncommon, the U anomalies in the freshwater sediments are confined to direct sources of this metal and are found in the lakes, regardless of their organic matter content [18,36,47].For example, high U concentrations were detected in the sediments of Lake Imandra (the largest lake of Murmansk region), where the content of this metal reached 17 mg/kg, which is 10 times higher than the Earth's crust [48].In the sediments of Lake Umbozero, located near Imandra, the U concentrations varied between 4 and 46 mg/kg, which in 2.4-27.1 times higher than the Earth's crust [36].Since both water bodies are located near the alkaline massif, where U mineralization was detected [49,50], it is obvious that U accumulated in the sediments of lakes Imandra and Umbozero as a result of a natural process.However, it is impossible to exclude that this process could have been accelerated by anthropogenic intervention, given that apatite ore has been mined in the area since the 1930s.Canadian studies by limnologists have shown that anthropogenic aerial transport of U from mining sites of this metal is a key factor in migration towards lakes located far from the source of pollution [22].
In the north of the Murmansk region, one deposit and more than 45 ore occurrences of U were explored [51].This territory is called the Litsa geological area.The U mineralization of this area is associated with pegmatite granites, metasomatites, including oligoclasites and albitites, and various terrigenous rocks.This resulted in the elevated level of U accumulation in the sediments of some small lakes in the north of the Murmansk region [18].For example, in the sediments of the urban Lake Severnoe, U concentrations varied between 77 and 204 mg/kg, which is 45-120 times higher than the Earth's crust.Notably, in the sediments of this lake, U was mainly associated with the mineral phase (40-70%); however, 30 to 50% of the total concentrations was associated with organic matter [18].Studying the main U fractions in the sediments of some lakes in Karelia showed that the stable (mineral) phase also plays a key role in the accumulation of this metal (Figure 9).In the sediments of Lake Chetyrekhverstnoe, the mineral fraction accounted for 51-64% of the total U concentrations.At the same time, only 7-8% of U was connected with the organic matter of the studied sediments of the city lake.On the contrary, the mobile and potentially bioavailable forms of U in the sediments of Lake Chetyrekhverstnoe accounted for 14-26% of the total concentrations of the metal, which creates a potential hazard for the biota and humans [52].
largest lake of Murmansk region), where the content of this metal reached 17 mg/kg, which is 10 times higher than the Earth's crust [48].In the sediments of Lake Umbozero, located near Imandra, the U concentrations varied between 4 and 46 mg/kg, which in 2.4-27.1 times higher than the Earth's crust [36].Since both water bodies are located near the alkaline massif, where U mineralization was detected [49,50], it is obvious that U accumulated in the sediments of lakes Imandra and Umbozero as a result of a natural process.However, it is impossible to exclude that this process could have been accelerated by anthropogenic intervention, given that apatite ore has been mined in the area since the 1930s.Canadian studies by limnologists have shown that anthropogenic aerial transport of U from mining sites of this metal is a key factor in migration towards lakes located far from the source of pollution [22].
In the north of the Murmansk region, one deposit and more than 45 ore occurrences of U were explored [51].This territory is called the Litsa geological area.The U mineralization of this area is associated with pegmatite granites, metasomatites, including oligoclasites and albitites, and various terrigenous rocks.This resulted in the elevated level of U accumulation in the sediments of some small lakes in the north of the Murmansk region [18].For example, in the sediments of the urban Lake Severnoe, U concentrations varied between 77 and 204 mg/kg, which is 45-120 times higher than the Earth's crust.Notably, in the sediments of this lake, U was mainly associated with the mineral phase (40-70%); however, 30 to 50% of the total concentrations was associated with organic matter [18].Studying the main U fractions in the sediments of some lakes in Karelia showed that the stable (mineral) phase also plays a key role in the accumulation of this metal (Figure 9).In the sediments of Lake Chetyrekhverstnoe, the mineral fraction accounted for 51-64% of the total U concentrations.At the same time, only 7-8% of U was connected with the organic matter of the studied sediments of the city lake.On the contrary, the mobile and potentially bioavailable forms of U in the sediments of Lake Chetyrekhverstnoe accounted for 14-26% of the total concentrations of the metal, which creates a potential hazard for the biota and humans [52].In another studied water body of Karelia (Lake Raivattalanlampi), the proportion of mobile U forms in the sediments was lower than that in the urban Lake Chetyrekhverstnoe (Figure 10).Although the proportion of U associated with organic matter in the sediments of Lake Raivattalanlampi was higher than in Lake Chetyrekhverstnoe, the mineral phase of U prevailed in the sediments of both lakes.In lake Raivattalanlampi, up to 68% of total U concentrations were associated with insoluble compounds.In recent sediments and soil, residual fractions are associated more with the terrigenous component.Studying river sediments and soils in the territory of Karelia [42] showed that in the rivers of southern In another studied water body of Karelia (Lake Raivattalanlampi), the proportion of mobile U forms in the sediments was lower than that in the urban Lake Chetyrekhverstnoe (Figure 10).Although the proportion of U associated with organic matter in the sediments of Lake Raivattalanlampi was higher than in Lake Chetyrekhverstnoe, the mineral phase of U prevailed in the sediments of both lakes.In lake Raivattalanlampi, up to 68% of total U concentrations were associated with insoluble compounds.In recent sediments and soil, residual fractions are associated more with the terrigenous component.Studying river sediments and soils in the territory of Karelia [42] showed that in the rivers of southern Karelia, the U content is 1.34 mg/kg, which is higher than the median U content in the sediments of the lakes.In the soils of Karelia, the U concentrations range from 0.07 to 0.96 mg/kg, reaching the maximum values in the mineral soil horizons.Thus, similarly to lake sediments, in river sediments and soils, U is mainly associated with the terrigenous (stable) component and not the organic matter.Karelia, the U content is 1.34 mg/kg, which is higher than the median U content in the sediments of the lakes.In the soils of Karelia, the U concentrations range from 0.07 to 0.96 mg/kg, reaching the maximum values in the mineral soil horizons.Thus, similarly to lake sediments, in river sediments and soils, U is mainly associated with the terrigenous (stable) component and not the organic matter.The difference in the U mobility in the sediments of lakes Chetyrekhverstnoe and Raivattalanlampi is likely due to the anthropogenic factor, since Lake Chetyrekhversnoe is located in the urbanized area, near in the Ptitsefabrika mineral deposit of U. It is known that more intensive use of city landscapes in recent times compared with the landscapes of the background areas leads to greater mobility of chemical elements in the process of their migration from rocks, quaternary deposits and soils to water bodies.Chemical elements migrate through dust [53,54] and then form what is known as urban surface deposited sediment on land [55] and metal-rich silts in water bodies [5].This is natural both for potentially toxic elements and for elements with a natural status (for example, U).To a certain extent, this hypothesis is reflected in the nature of the vertical distribution of U in the column of the recent sediments of Lake Chetyrekhverstnoe (Figure 6), where the content of the studied metal increased from the lower layers (26-28 cm) to the upper ones (2-4 cm).The same was observed in the recent sediments of Lake Lamba, also located in the city of Petrozavodsk (Figure 11), which additionally supports the conclusions about the technical and natural nature of U migration in the city landscapes of Karelia.A similar accumulation of U has been noted in modern sediments of Lake Bolshoy Vudjavr (Murmansk region, Russia).An increase in the concentration of this metal is noted in the second half of the twentieth century, which can be associated both with nuclear explosions and with the intense influence of urban and industrial areas near the lake [4].At the same time, for both urban lakes in Karelia, the proximity of the Ptitsefabrika mineral deposit of U to the studied waterbodies is a potential factor in the anthropogenic accumulation of U in recent sediment layers.The difference in the U mobility in the sediments of lakes Chetyrekhverstnoe and Raivattalanlampi is likely due to the anthropogenic factor, since Lake Chetyrekhversnoe is located in the urbanized area, near in the Ptitsefabrika mineral deposit of U. It is known that more intensive use of city landscapes in recent times compared with the landscapes of the background areas leads to greater mobility of chemical elements in the process of their migration from rocks, quaternary deposits and soils to water bodies.Chemical elements migrate through dust [53,54] and then form what is known as urban surface deposited sediment on land [55] and metal-rich silts in water bodies [5].This is natural both for potentially toxic elements and for elements with a natural status (for example, U).To a certain extent, this hypothesis is reflected in the nature of the vertical distribution of U in the column of the recent sediments of Lake Chetyrekhverstnoe (Figure 6), where the content of the studied metal increased from the lower layers (26-28 cm) to the upper ones (2-4 cm).The same was observed in the recent sediments of Lake Lamba, also located in the city of Petrozavodsk (Figure 11), which additionally supports the conclusions about the technical and natural nature of U migration in the city landscapes of Karelia.A similar accumulation of U has been noted in modern sediments of Lake Bolshoy Vudjavr (Murmansk region, Russia).An increase in the concentration of this metal is noted in the second half of the twentieth century, which can be associated both with nuclear explosions and with the intense influence of urban and industrial areas near the lake [4].At the same time, for both urban lakes in Karelia, the proximity of the Ptitsefabrika mineral deposit of U to the studied waterbodies is a potential factor in the anthropogenic accumulation of U in recent sediment layers.

Conclusions
Studying the level of U accumulation in the sediments of the water bodies in the Re public of Karelia showed that sediments in the region had low U concentrations relativ to the mean content of this element in the upper part of the Earth's crust.However, in certain areas of Karelia, associated with the deposits or ore occurrences of U, the elevated content of this metal was revealed in the lake sediments.The highest level of U accumu lation was detected in the sediments of Lake Gryaznoe, influenced by the North Oneg ore geochemical district, where vanadium deposits and ore occurrences with an elevated content of U, Fe, Mo and Cu are widespread.The close connection of U and Cu (R 2 = 0.85 as well as that of U and Mo (R 2 = 0.98) in the studied sediments of Lake Gryaznoe confirm the relationship of U anomalies with the natural (geological) influence on the geochemis try of sedimentation in the water body.In some lake sediments, the anomalies of Th which often accompanies U in ore geological formations, were found.Analysis of the U fractions in the sediments of some lakes in Karelia revealed the key role of the minera (insoluble) phase in U accumulation.For example, in the sediments of lake Chetyrekhverstnoe and Raivattalanlampi, up to 64% and up to 68% of the total U concen trations, respectively, were associated with the mineral phase.The proportion of the or ganic fraction in the U accumulation for both lakes was not large and varied between and 15%, relative to the total concentrations of the metal.The example of the lakes in Pet rozavodsk shows that U accumulation in urban water bodies may be connected with th anthropogenic influence on the migration of lithophile elements from technogenically changed landscapes (rocks, quartery deposits, soils).U can likely enter into the water bod ies as part of urban dust.

Figure 1 .
Figure 1.Study area showing sampling locations of lake sediments.

Figure 1 .
Figure 1.Study area showing sampling locations of lake sediments.

Water 2023 , 16 Figure 2 .
Figure 2. Variation of U concentration in the sediments of lakes in the Republic of Karelia with the main statistical parameters; from bottom to top: the minimum value of the sample, the lower quartile (25th percentile), the median, the arithmetic mean (indicated by a cross), the upper quartile (75th percentile), the maximum value of the sample, the extremely high values that stand out from the total sample (indicated by circles).

Figure 3 .
Figure 3. Vertical distribution of U in the sediments of Lake Gryaznoe (Karelia).

Figure 2 .
Figure 2. Variation of U concentration in the sediments of lakes in the Republic of Karelia with the main statistical parameters; from bottom to top: the minimum value of the sample, the lower quartile (25th percentile), the median, the arithmetic mean (indicated by a cross), the upper quartile (75th percentile), the maximum value of the sample, the extremely high values that stand out from the total sample (indicated by circles).

Figure 2 .
Figure2.Variation of U concentration in the sediments of lakes in the Republic of Karelia with the main statistical parameters; from bottom to top: the minimum value of the sample, the lower quartile (25th percentile), the median, the arithmetic mean (indicated by a cross), the upper quartile (75th percentile), the maximum value of the sample, the extremely high values that stand out from the total sample (indicated by circles).

Figure 3 .
Figure 3. Vertical distribution of U in the sediments of Lake Gryaznoe (Karelia).

Figure 3 .
Figure 3. Vertical distribution of U in the sediments of Lake Gryaznoe (Karelia).

Figure 4 .
Figure 4. Study area with indication of places of lake sediments with U anomalies (the range of U concentration is indicated in parentheses) and some deposits and ore occurrences of U.

Figure 5 .
Figure 5. Correlation between the concentration of U and Mo, and U and Cu in the sediments o Lake Gryaznoe.

Figure 4 .
Figure 4. Study area with indication of places of lake sediments with U anomalies (the range of U concentration is indicated in parentheses) and some deposits and ore occurrences of U.

Figure 4 .
Figure 4. Study area with indication of places of lake sediments with U anomalies (the range of U concentration is indicated in parentheses) and some deposits and ore occurrences of U.

Figure 5 .
Figure 5. Correlation between the concentration of U and Mo, and U and Cu in the sediments o Lake Gryaznoe.

Figure 5 .
Figure 5. Correlation between the concentration of U and Mo, and U and Cu in the sediments of Lake Gryaznoe.

Figure 6 .
Figure 6.Vertical distribution of U in the sediments of lakes Chetyrekhverstnoe, Airanne and Raivattalanlampi.

Figure 6 .
Figure 6.Vertical distribution of U in the sediments of lakes Chetyrekhverstnoe, Airanne and Raivattalanlampi.

Figure 7 .
Figure 7. Variation of Th concentration in the sediments of lakes in the Republic of Karelia with th main statistical parameters; from bottom to top: the minimum value of the sample, the lower quartil (25th percentile), the median, the arithmetic mean (indicated by a cross), the upper quartile (75t percentile), the maximum value of the sample, the extremely high values that stand out from th total sample (indicated by circles).

Figure 8 .
Figure 8. Correlation between the concentration of U and Th in the sediments of some Karelian lake with high Th content.

Figure 7 .
Figure 7. Variation of Th concentration in the sediments of lakes in the Republic of Karelia with the main statistical parameters; from bottom to top: the minimum value of the sample, the lower quartile (25th percentile), the median, the arithmetic mean (indicated by a cross), the upper quartile (75th percentile), the maximum value of the sample, the extremely high values that stand out from the total sample (indicated by circles).

Figure 7 .
Figure 7. Variation of Th concentration in the sediments of lakes in the Republic of Karelia with th main statistical parameters; from bottom to top: the minimum value of the sample, the lower quartil (25th percentile), the median, the arithmetic mean (indicated by a cross), the upper quartile (75t percentile), the maximum value of the sample, the extremely high values that stand out from th total sample (indicated by circles).

Figure 8 .
Figure 8. Correlation between the concentration of U and Th in the sediments of some Karelian lake with high Th content.

Figure 8 .
Figure 8. Correlation between the concentration of U and Th in the sediments of some Karelian lakes with high Th content.

Figure 11 .
Figure 11.Vertical distribution of U in the sediments of Lake Lamba (Petrozavodsk).