ISSN 0016-7029, Geochemistry International, 2022, Vol. 60, No. 8, pp. 792–807. © Pleiades Publishing, Ltd., 2022. Russian Text © The Author(s), 2022, published in Geokhimiya, 2022, Vol. 67, No. 8, pp. 787–804. Radioactivity Assessment of Sapropel Sediments in Small Lakes in the Baraba Lowland and Kulunda Plain, West Siberia V. D. Strakhovenkoa, *, E. A. Ovdinaa, I. N. Malikovaa, and G. I. Malova a Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *e-mail: strahova@igm.nsc.ru Received January 17, 2021; revised January 17, 2021; accepted March 20, 2021 Abstract—Various components of lake systems located in different landscape zones in southern West Siberia were studied for contents of natural and artificial radionuclides, following the requirements of the state standards (GOST). This work aims at assessing the radiation state of lacustrine sapropel sediments in the Baraba lowland and Kulunda plain, taking into account their natural features and the degree of contamination during nuclear tests. It was found out that the values of the total effective specific activity (As) of natural radionuclides in all of the sapropel lacustrine sediments are significantly lower than the standard value (according to the requirements of the GOST). Some sapropel horizons in the lakes contain excess 137Cs reserves that are twice as high as the global background. Such lakes tend to be spatially constrained to areal traces of radioactive fallouts after nuclear tests at the Semipalatinsk test site and undoubtedly belong to lake systems whose waters and soils on catchments were primarily contaminated with radiocesium. Keywords: sapropel, total effective specific activity, natural radionuclides, radiocesium, small lakes, southern West Siberia DOI: 10.1134/S0016702922080080 INTRODUCTION Newly acquired data on the global transfer of sedimentary material have remarkably modified traditional understanding of globally operating laws that control biochemical and mechanical processes of continental sedimentation, and this calls for further studies of lacustrine sediments with the application of modern analytical techniques (Stein, 2008; Wan et al., 2008; Lisitsyn, 2014; and others). Sapropels are formed in anaerobic environments as a result of physicochemical and biological transformations of hydrobionts, at the variable involvement of mineral and organic components, which actively interact with one another (Kemp et al., 1999; Kurzo et al., 2010; Strakhovenko et al., 2016; and others). The composition of sapropels can notably vary depending on their genesis, with the organic components differing primarily in the proportions of the biological contributions of various organisms (Strakhovenko et al., 2014; Serebrennikova et al., 2017; Taran et al., 2018; and others). Terrigenous particles in sapropel are mostly fragments of rocks brought from the catchments. The lakes discussed herein lie mostly in the southeastern West Siberian Platform, which geologically consists of two floors: a folded basement and sedimentary cover. The basement is made up of folded shales, limestones, and sandstones that locally host magmatic rocks. The basement crops out near the city of Novosibirsk and is overlain with the sedimentary cover east of it, with the thickness of the cover reaching 3 km at the western boundary of this territory. The complete vertical section of the sedimentary cover consists of alternating marine and continental Jurassic, Cretaceous, and Paleogene rocks. The surface of the whole territory was uplifted in the latest Paleogene and became free of seawater. A river network was formed there, the rock in adjacent areas in the Altai were eroded, and the eroded material was transported. The Neogene and Quaternary rocks are continental. The surface rocks at the territory are loams and clays, which were immediately involved in the development of the local surface topography, the origin of the soils, and the vegetation. In the Neogene–Quaternary, the local plains (Vasyugan, Ob, Kulunda, and Baraba) subsided at variable rates, and this resulted in that the Baraba lowland became enclosed and undrained, and its absolute elevations reached 100–120 m, i.e., 20 m lower than the elevations of the former three plains. No glaciers covered this territory in the Quaternary, however, the melting of glaciers in nearby Altai areas enhanced the streams. The removal of rock fragments and the products of their weathering thus also increased, as also was enhanced their accumulation on the plains (Explanatory Notes…, 1967). Terrigenous alluvial–proluvial sediments in the West Siberian Lowland were produced by material eroded from Paleozoic formations in the Altai–Sayan and Kazakh- 792 RADIOACTIVITY ASSESSMENT OF SAPROPEL SEDIMENTS IN SMALL LAKES stan, and the radiogeochemical levels of these rocks were close to the average level of the continental crust. The chemical composition of sapropels is controlled not only by the contribution of clastic material but also (and mostly) by the biogeochemical processes in the water and the few uppermost centimeters of the bottom sediments (Newsome et al., 2014; Strakhovenko et al., 2018; and others). Still another important source of material brought to small lakes from nearby watersheds in West Siberia, where soils are formed on loess loams, is the wind erosion of soils, a process that is particularly intense during dust storms, which became more frequent after the tilling of the wild lands (Nalivkin, 1969; Gavshin et al., 1999; Shevchenko et al., 2012; and others). The biological constituent of sapropels of the lakes in question was documented in detail by N.I. Ermolaeva and E.Yu. Zarubina (Institute of Water and Environmental Problems, Siberian Branch, Russian Academy of Sciences). It has been demonstrated in numerous publications that production processes in most of the lakes were more intense than the destruction processes, and the rate of addition of organic compounds to the bottom sediments as a result of decomposition of macrophyte and zooplankton die-off was evaluated. The production of phytoplankton in the lakes broadly varied from 0.01 to 1.96 mg O2/(L × h). The lakes characterized by the microphytogenic type of their sapropel-forming processes and by a high annual production of the phytocenoses (up to 2261 g organic matter/(m2 × year)) are also characterized by the highest concentrations of organic carbon (TOC, total organic carbon). The contributions of various hydrobiont groups to the bottom sediments can significantly differ and variably affect the composition of the sediments (Zarubina, 2013; Ermolaeva et al., 2017; Zarubina et al., 2018; Zarubina and Fetter, 2019; Ermolaeva et al., 2019; and others). Systematic studies of sapropels were launched in 1916 on the initiative of Academicians N.S. Kurnakov and V.I. Vernadsky. The Sapropel Institute, which was established in 1932, conducted extensive academic research and resolved much applied problems. The later development of large oil and gas fields and hydrocarbon chemical processing then hampered studies aimed at the utilization of sapropels and accordingly led to the shrinkage of sapropel studies. Russia possesses uniquely large resources of sapropels, which vary from 38 to 250 billion of cubic meters according to various evaluations (Shtin, 2005), but no more than 2% of them is explored and adequately studied. Modern technological approaches enable the highly efficiently utilization of both sapropels themselves and solid and liquid products of their processing in various fields of economy and various industries, and this revived interest in the utilization of sapropels, including those from deposits in West Siberia. It is also important that the removal of sapropel from water GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 793 bodies improves the quality of the water and conditions for fish farming in these bodies. Sapropels usable in various sectors of national economy must comply with a range of state standards (GOSTs). According to GOST 54000-2010, raw sapropel must meet radiation hygienical standards on the activities of natural and artificial radionuclides. Nuclear tests at the Semipalatinsk and Novaya Zemplya testing sites (starting in 1949) and the nuclear disasters at the Chernobyl Atomic Power Plant in 1986 and at the Fukushima 1 Atomic Power Plant in 2011 resulted in that small areas where regional anthropogenic radiation background was several times higher than the global radiation background emerged practically throughout the whole study area in West Siberia (Izrael’, 2000; Sukhorukov et al., 2001; Izrael’, 2005; Rikhvanov, 2009; and others). Russia and other countries conventionally use 137Cs as an indicator (marker) of nuclear contamination and the level of radiation impact. The 137Cs isotope is a long-lived artificial radionuclide, whose half-life is 30.2 years and which is produced at nuclear blasts and in the course of operation of nuclear power plants. It is relatively stable (little migrating) in soils and is relatively easy to identify by currently utilized analytical techniques. The standardized pollution indicators used in Russia is the specific activity (Bq/kg) and fallout density (reserve) of 137Cs in soil in mCi/km2, which characterizes the current state of a given territory and provides a basis for evaluating the accumulated effective radiation dose (cSv) (Myasnikov, 2004). The global activity background of 137Cs in soils in West Siberia is, according to expert estimates based on several archive, calculation, and experimental 137Cs measurements in soils), 50 mCi/km2 (1.85 kBq/m2) as of 1995 (Baranov, 1956; Boltnev et al., 1972; Aleksakhin, 1982; Chernyago et al., 2004; Medvedev et al., 2005; and others). Note that global fallouts in Siberia were only insignificantly contributed (no more than 10%) by radiocesium from the Chernobyl APP, as follows from the activity ratio of 137Cs/134Cs (Gavshin et al., 2000; Sukhorukov et al., 2001). Earlier publications report data on radiocesium reserves in off-system units of radiocesium activity (Ci), including those widely quoted herein, because of which below we report radiocesium reserves in mCi/km2, and which can be recalculated into kBq/m2 as 1 Ci/km2 = 37 kBq/m2 (Pavlotskaya, 1974; Moiseev, 1975; Izrael’, 2000; Mikhailovskaya et al., 2015; and others). Acad. Yu.A. Izrael’ has published, in his paper issued in 2000 and materials of 2005, a map of 137Cs reserves (mCi/km2 ) recalculated to the year 2000, and has demonstrated that the global radiation monitoring carried in the 1990s using aerial gammaspectral survey and practically ubiquitous sampling had revealed a latitudinal zoning in the distribution of 137Cs pollution. As a result of a great number of nuclear tests in the northern hemisphere and, particularly, 2022 794 STRAKHOVENKO et al. atmospheric circulation because of the much lower meridional movement velocity compared to the latitudinal ones, the maximum global radiation background reportedly occurs within two latitudinal belts at 40°– 50° and 50°–60° N (the radiation background caused by 137Cs in the latitudinal belt of 50°–60° varies within the range of mCi/km2 i.e., 0.4–2.0 kBq/m2 in recalculation to the year 2012; Usacheva, 2017), and this value decreases both north- and southward (Izrael’ et al., 2000; Izrael’, 2005). Radiocesium concentrations caused by local and regional fallouts were determined to show a mosaic distribution pattern, which is explained by both the heterogeneity of the primary radioactive fallouts and the uneven secondary redistribution (Sukhorukov et al., 2001; Malikova et al., 2011; and others). Our research was aimed at estimating the radioactive radiation state of lacustrine sapropel sediments in the Baraba lowland and Kulunda plain in West Siberia, with regard to their inherent natural features and their contamination during nuclear tests. MATERIALS This study was carried with sapropel bottom sediment in small lakes in the Baraba lowland and Kulunda plain. The total number of lakes in the Baraba lowland and Kulunda plain exceeds 5000, and the aggregate surface area of the lakes exceeds 8000 km2. Most of the lakes are undrained (basinal) and small: 97.5% are smaller than or equal to 2.5 km2. Our study is based on factual materials collected by the authors, in cooperation with researchers from the Institute of Water and Environmental Problems, Siberian Branch, Russian Academy of Sciences, in the course of fieldwork conducted starting from 2012 thorough 2019. The Baraba lowland and Kulunda plain are characterized by a rolling surface topography, which was produced by suffusion–deflation processes. The local redistribution of moisture and its flow from watersheds to valleys between them (alternating parallel southwest- to northeast-trending elongate topographic highs and depressions) has produced numerous lakes that filled the depressions. The territory is made up of loess rocks ranging from heavy loams and clays in the north to medium and light loams and sandy loams in the south. The rocks contain 1 to 15% carbonates (Syso, 2007). The lakes are hosted in modern Quaternary rocks, which are lacustrine–alluvial loams with sand beds, alluvial sands, and lacustrine– alluvial clays and loams (Explanatory Notes…, 1967). It should be mentioned that the loess loams that are widespread in Baraba and Kulunda and cover the topographic highs as a layer 0.2 to 1.5 m, and more, thick, are of wind-laid (aeolian) genesis, which has significantly homogenized the granulometric and chemical composition of the soil-forming rocks and the soils themselves. For lacustrine sapropel, the soil cover provides organo-mineral material and watersoluble salts, which are brought to lakes with surface and soil-groundwaters and are accumulated at the bottom of the lakes. The composition of the rocks and soils predetermines the wide spread of waters of the soda type over the whole territory. The composition of rocks and soils in the catchments of the lakes was studied in much detail, and the results were summarized in (Syso, 2007; Puzanov et al., 2016; Puzanov et al., 2017; and others). The soils of the catchments and their geochemical features were studied along geochemical– soil profiles across all of the major geomorphological elements (flood plain, bench, and the watershed of the lake basin), and soil profiles were sampled (from selected soil horizons by Yu.V. Ermolov of the Institute of Soil Science and Agrochemistry, Siberian Branch, Russian Academy of Sciences) and A.V. Saltykov (of the Institute of Water and Environmental Problems, Siberian Branch, Russian Academy of Sciences). The lakes studied in the Baraba lowland are constrained mostly within the forest–steppe zone. The structure of the soil cover in the catchments of the lakes is defined by the following alternating soil types: boggy soil → meadow–boggy soil → meadow soil → meadow– chernozem soil → southern and ordinary chernozem → gray wood soil. The quantity and quality of material brought to the lakes depends on concentrations of humic compounds, fine particles, and ions of soluble compounds in the soil cover. The concentrations of humic compounds in the upper parts of the soil profiles broadly vary even within a single catchment, and the degree of their humification ranges from low (0.9%) to high (8.3%). The soils are highly permeable to water and have low water-raising and water-holding capacities. According to their degree of salinization in the catchments, all of the soils are classified into unsalinized (southern and ordinary chernozems, meadow sod–gley soils, meadow–boggy humous soils, and typical forest ones), weakly salinized (solonized meadow–chernozem, boggy humous–gley, meadow humous, and meadow soddy soils), and highly salinized (meadow–boggy humous–gley soils). The pH of the soil cover in the catchments broadly varies, from weakly acidic in the upper parts of the meadow–soddy soils in the vicinities of Tsybovo Lake to strongly alkaline in the lower part of the profile of the meadow– chernozem solonized soils. Carbonates were identified in all of the soil types, except only the gray woody soils typical of the basin of Bol’shie Kaily Lake and occur mostly in lower horizons. Soils in the catchments of lakes in the Kulunda plain are typical of the steppe landscape zone: they are salinized, contain little organic matter, and are highly dense. The following soil types occur in the territory: saline soils (sor-affected, meadow, and meadow–boggy soils), meadow–steppe salinized, meadow salinized soils near lakes, podzols and iron-enriched soddy illuvial podzols in the ribbon forest subzone, and dark brown solonized soils (Puzanov et al., 2016, 2017; Ovdina et al., 2016; and others). The GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 2022 RADIOACTIVITY ASSESSMENT OF SAPROPEL SEDIMENTS IN SMALL LAKES (а) Scale 100 0 200 km 795 (b) Kamen’-na-Obi Kyshtovka da lun Severnoe Ku Slavgorod Kulundinskoe L. Vengerovo Aleisk Barabinsk Tatarsk Chiosoozernoe Ubinskoe Zdvinsk Rubtsovsk Kochki Kupino Zmeinogorsk Krasnozerskoe Karasuk 1 2 3 4 5 7 8 9 10 11 6 Sampling sites of lake components 1 4 7 2 5 8 3 6 9 Fig. 1. (a) Schematic map of the grain-size composition of soils and soil-forming rocks in the Baraba lowland (Soils, 1966) and (b) soil–geographic zoning of the Kulunda plain (Nikol’skaya, 1961) with the sampling sites of components of the lake systems. (a) (1) Loess-like heavy loams and light powdery–silty clays; (2) loess-like heavy powdery and powdery–silty loams; (3) loesslike intermediate and more rare heavy silty–powdery loams; (4) silty–powdery heavy loams and more rare clays; (5) powdery– silty and silty–powdery heavy loams and clays; (6) sandy–silty and silty–sandy heavy and intermediate loams; (7) intermediate sandy–powdery and powdery–sandy loams; (8) intermediate and light silty–sandy and powdery–sandy loams; (9) light silty– sandy loams and more rare sandy clays: (10) sandy clays; (11) cohesive sands. (b) (1) Brown soils; (2) southern chernozems; (3) ordinary chernozems; (4) leached chernozems and gray forest soils; (5) sod–podzol soils of ancient forest terraces; (6) typical chernozems; (7) podzolized chernozems and dark gray forest soils; (8) weakly developed mountainous–meadow soils; (9) chernozems of southern piedmonts. location of the lakes in these areas is shown in schematic soil-cover and soil-forming rock maps published in (Nikol’skaya, 1961; Soils, 1966; Il’in and Syso, 2001) (Fig. 1). It has been demonstrated in our earlier publications that concentrations of natural radionuclides in the soils of different type in the southern West Siberia are similar in various landscape zones (Strakhovenko et al., 2010; Strakhovenko, 2011; Mel’gunov et al., 2011; Malikova and Strakhovenko, 2017). The authors have previously studied in much detail the geochemistry of the sapropels (Strakhovenko et al., 2014; Strakhovenko et al., 2016, Strakhovenko et al., 2019; and others) and have determined that the organic constituent of the sapropels shows a variable GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 contribution of various biological species. X-ray powder diffraction (XRD) data indicate that the mineral component of the soils is composed of a relatively short list of dominant minerals: quartz, plagioclase, potassic feldspar, micas, and calcite. The minor minerals are pyrite, hydromicas, chlorites, dolomite. The accessory minerals are ilmenite, hematite, zircon, monazite, magnetite, apatite, rutile, titanite, etc. Our earlier studies at the same areas have shown that the concentrations of silicon, an element contained in quartz and other rock-forming silicates and in diatom and macrophyte remnants, broadly vary. The Al, K, and Na concentrations vary proportionally, which likely suggests their common source (Strakhovenko et al., 2014). Sedimentation rates in the small sapropel 2022 796 STRAKHOVENKO et al. Density of 137Cs, fallouts, mCi/km2 0 20 40 60 80 100 150 200 250 24.08.56 Trajectory of the air currents carrying nuclear-blast products that caused intense local radioactive fallouts 27 8 5 6 4 12.08.53 Date of the nuclear blast 400 Nuclear yield Kemerovo Novosibirsk Kaf kaim Platovka Istimass Novyi Vostok Fedorovka Makarovka Vasil'chuki Klyuchi Tselinnyi Sampling sites of lacustrine sapropel Kani Petukhi Novovoznesenka Novokormino Barnaul Severka Zapadnyi Ugol Ashigul’ Berezovskii Poluimki Pokrovka Nevodnoe Irkutskii Nikolaevka Rakity Ust’-Kormikha Mikhailovskoe .49 29.08 32 24.08.5 27 Ust’-Volchikha Bor-Forpost Bastan Gorno-Altaisk Valovoi Kordon 20 40 60 mCi/km2 80 100 6 50 13.08.53 3000 Simonovo Malinovoe Kormikha Chernokorovnikovo Borisovka Ozerno-Kuznetsovskii Leskhoz Kuznetsovo L. Bor-Kosobulak Alekseevka Malaya Shelkovka Rakity Bol’shaya Shelkovka Korosteli 2 Mirnyi Pavlovka Uglovskoe Kuibyshevo Lyapunovo Krugloe 10 (a) (b) 0 10 20 30 40 km Korosteli 1 Gor’koe Shelrukha Borisovka 48 8.53 12.0 400 89 Fig. 2. (a) Map of the density of 137Cs fallouts in southern West Siberia (based on original data of the authors) (Rikhvanov, 2009) and (b) a map of the density of 137Cs fallouts in the bottom sediments of the lakes of the Tanatarskaya and Klyuchevskaya systems. lakes in various areas in Siberia (data calculated using the 210Pb and 137Cs isotopes) are 0.15–0.35 cm/year (Strakhovenko et al., 2017). With regard to various approaches to the systematics of sapropels (Korde, 1969; Lopotko, 1978; Shtin, 2005; Kurzo et al., 2010; and others), herein we view sapropels as organic–mineral bottom sediments with ash contents reaching 85%. Based on mineralogical– geochemical and biogenetic data, we classified the organic–mineral bottom sediments according to their composition. The sapropels were subdivided into four types according to their ash content: organogenic (less than 30% ash), organic–mineral (30–50%), mineralorganic (50–70%), and mineralized (70–85%). Bottom sediments with >85% ash were classed with mineral silts (Shtin, 2005; Kurzo et al., 2010; and others). The authors of the paper further classify all sapropel types, except organogenic ones, based on the Si/Ca ratio into three classes: silicon (Si > Ca); calcium (Ca > Si), and mixed (Si ~ Ca). According to the dominant production type, the sapropels were subdivided into planktonic, macrophytic, and planktonic–macrophytic. Radioactive fallouts occurred in the territory of West Siberia during nuclear tests at the Semipalatisk and Novaya Zemlya test areas. The map of nuclear trails across West Siberia indicates that the Baraba and Kulunda areas were affected mostly by the 1953 and 1954 nuclear tests (Selegei, 1997). The total annual beta activity of fallouts in Kulunda in 1961–1963 was 605–1584 mCi/km2 (Robertus, 1993). We have compiled the greatest database of analytical parameters of sapropels in the Baraba lowland and Kulunda plain. The database comprises our own original data and literature materials published before 2018. In cooperation with a large team of researchers, the authors have sampled an extensive territory in southern West Siberia and mapped the density of radiocesium pollution of soils in the Baraba lowland and Kulunda plain, which were published in (Rikhvanov, 2009; Malikova and Strakhovenko, 2011; Mel’gunov et al., 2011; and others). Figure 2 shows a map of the density of 137Cs fallouts in soils in southern West Siberia [the map is based on original data of the authors and was published in the monograph (Rikhvanov, 2009)] and a map of the density of 137Cs fallouts in the bottom sapropel sediments in lakes of the Tanatarskaya and Klychevskaya systems of larger scale, which was con- GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 2022 RADIOACTIVITY ASSESSMENT OF SAPROPEL SEDIMENTS IN SMALL LAKES structed by the authors for each lake system in the Baraba lowland and Kulunda plain separately. METHODS Cores of bottom sediments, soil profiles, soilforming rocks, as well as samples of the waters and dominant plant species, were collected in the course of a fieldwork. Water samples for all analyses were taken in compliance with the state standards GOST 31861, 2012. We have determined the variable physicochemical pars of the waters and bottom sediment. The genesis of sapropel formed in the various lakes was identified in samples of the primary sapropel-forming material (phytoplankton, photosynthetic pigments, zooplankton, phytobenthos and phytoperiphyton, macrophytes, grass cuttings for the biomass, and geobotanical descriptions). The lakes significantly vary in their degree and character of macrophyte development and the production levels of the aquatic phytocenoses. The fluxes of autochthonous organic matter into the lakes vary from 3.2% in hypersaline Malinovoe Lake (in which the sedimentation flux is dominated by allochthonous halite) to 84.2% (in lakes with an autochthonous type of material accumulation, such as Barchin, Kachkul’nya, and other lakes) of the total mass of the sedimentation flux. The maximum production and destruction values were found in lakes with the massive development of blue-green algae. The samples taken for gamma spectrometric analysis included the dominant producers of organic matter in each of the lakes. For example, lakes massively overgrown with macrophytes were sampled for the dominant submerged plants (morass-weed Ceratophyllum, soldier Stratiotes, and charophytes Chorales) or for semisubmerged plants (Phragmites). In lakes of fringing vegetation type, we analyzed a few samples of the dominant biomass, and the further calculations were carried out using the arithmetic means of these samples. Soils were sampled, using a metal ring, throughout the whole depth of the soil profile. Samples were taken from each soil horizon with a sharpened metal ring (82 mm in diameter and 50 mm high). Each individual soil profile was continuously sampled through its upper 30 cm by a ring, and then each of the genetic horizons was sampled. This allowed us to determine radiocesium activity in each horizon, which is particularly important for the identification of a sod horizon and the upper part of the humus–accumulation horizon A, which are able to accumulated the highest radiocesium amounts. The obtained values of the specific radiocesium activity, expressed in Bk/kg, were recalculated into pollution density in mCi/km2 (Malikova and Strakhovenko, 2011). The thickness of the sapropel layers was determined using an acoustic depth finder. Cores of bottom sediments were taken from a twin-hull boat, using a cylindrical corer with a vacuum lock (designed and manufactured at the Taifun Research and Production GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 797 Company, Russia) 82 mm in diameter and 120 mm long. Samples were then collected from each layer of the bottom sediment cores, with the sampling sites spaced 5 to 25 cm apart, depending on the composition, with the mineral silt sampled from the basins of the lakes. All of the samples were analyzed for moisture and ash contents and for the composition of the organic and inorganic constituents of the sapropel. The morphology and phase composition of the bottom sediment samples of various classes were studied under a TESCAN MIRA 3 (Tescan, Czech Republic) scanning electron microscope equipped with an OXFORD XMAX 450+ (Oxford Instruments, United Kingdom) energy-dispersive spectrometer. Natural radionuclides and radiocesium were determined by gamma spectroscopy, using gamma spectrometers with NaI(Tl) well-type scintillation crystals 200 × 200 and 150 × 150 mm. The masses of the analyzed samples were varied between 100 and 450 g. The detection limits at mass analyses was evaluated at 1–3 Bq/kg. The accuracy and reproducibility of the analyses were determined by replicate analysis of the SA-1 and SI-1 standards and the BSILT (7126-94) reference standard sample of Baikal silt. The accuracy of analyses for natural radionuclides was estimated by comparing with analyses of IAEA standard reference samples, and the analytical laboratory has previously successively participated in the certification of these samples. Replicate analyses of soil and bottom sediment samples were used to estimate the reproducibility: replicate analysis of every tenth sample was conducted in strict compliance with the analytical procedures. According to the results of statistical processing of data on 70 bottom sediment samples that contained 75 to 25% organic matter and were analyzed by two or more techniques, the deviations between the 226Ra activity values were no greater than 15%, and those for Th and K were 10%. Detailed descriptions of the analytical techniques can be found in (Gavshin et al., 2004; Mel’gunov et al., 2011; Malikova and Strakhovenko, 2017). To assay how much the sapropels comply with the radiation−hygienical standards, we calculated the total effective specific activity (As) caused by natural radionuclides by the formula (with regard to the coefficients) As = АRa + 1.31 ATh+ 0.085 AK1 , where АRa, ATh, and AK1 are the specific activities of the corresponding radionuclides. The standardized values of the As values of natural radionuclides are no higher than 300 Bq/kg, and those for artificial radionuclides are no greater than 1 relative unit, which is assumed to be equal to the global background value (GOST R 54519, 2011). The analyses were conducted at the Analytical Center for Multielemental and Isotope Research, Siberian Branch, Russian Academy of Sciences. The data on the specific activity of radiocesium were recalculated with regard to the radioactive decay as of the 2022 798 STRAKHOVENKO et al. Table 1. Statistical parameters of geochemical data on the specific activity (Bq/kg) of natural radionuclides and 137Cs in the various components of lake systems in the Baraba lowland (52 lakes) and Kulunda plain (41 lakes) Variables Specific activity of Ra Specific activity of Th Specific activity of K Total effective specific activity Specific activity of 137Cs Specific activity of Ra Specific activity of Th Specific activity of K Total effective specific activity of Specific activity of 137Cs Specific activity of Ra Specific activity of Th Specific activity of K Total effective specific activity of Specific activity of 137Cs Average Minimum value Maximum value Standard deviation Asymmetry Excess 29 19 438 89 20 Soil (2137 samples) 1 1 10 3 0 112 59 751 234 198 11 22 146 37 42 0.7 2.4 0.6 1.2 2.9 1.0 5.9 1.6 3.3 8.4 32 12 470 90 3 Biota (112 samples) 1 156 1 152 10 1720 2 264 0 24 28 18 432 62 6 2.0 5.0 1.0 1.2 2.1 5.2 35.4 0.1 0.8 4.0 26 15 248 66 12 Sapropel (3956 samples) 1 143 1 58 10 870 3 187 0 342 16 10 176 33 31 1.8 0.7 0.4 0.5 4.8 7.0 0.2 0.4 0.5 30.4 year 2010 and put into the database. The analytical data were statistically processed, including the estimation of radionuclide distribution in the soils, sapropel deposits, and biota, tests of hypotheses of the types of the distributions, and the evaluation of the correlations, were done using the Statistica 8 software and MS Excel application. Cluster analysis was applied to graphically represent the grouping of the analytical data on the whole set of the analyzed elements in the bottom sediments and soils (Mikhail’chuk et al., 2006). The calculations were conducted with a т × п matrix (where п is samples of bottom sediments, biota, and/or soils, and т is the number of factors or variables (Ca, Mg, Na, Al, Fe, Si, U(Ra), Th, and K). The number of variables in the solutions was varied to identify stable relationships between variables and to obtain stable groups of lakes. The calculations were conducted for R analysis−factors (elements). The metrics of R analysis is the correlation coefficient. The solutions were presented in the form of dendrograms of correlations between chemical elements within a specified set of objects. The QGIS and ArcGIS program packages were applied to build models for the layer by layer and areal distributions of the 137Cs reserves (mCi/km2) and the total effective activity (As) of natural radionuclides in the sapropels and soils in the catchments for the lakes studied in the various lake systems in the Baraba lowland and Kulunda plain. An example is presented in Fig. 2. RESULTS AND DISCUSSION Considered together with preexisting dataset on the soil profiles, the newly obtained materials on the sapropels, biomass, and waters in lakes in the Baraba lowland and Kulunda plain show that, in spite of the significant variations in the concentrations of U(Ra), Th, and K in the datasets, the average values for the soil profiles and bottom sediment columns vary relatively insignificantly (Table 1). This fact obviously indicates that the concentrations are unevenly distributed in the soil-forming rocks, from which the elements are transferred first into the soils and then into the sapropels. During the very first evolutionary stages of the lakes, their bottom sediments were formed on a mineral source material. The topographic features had then not been completely shaped, and the surface topography of the catchments was actively formed by erosion. The corresponding rocks make up the lowermost portions of bottom-sediment columns in small lakes in West Siberia, and the thickness of this layer is commonly 2−6 cm (at the total thickness of the sediments of 3 to 29 m). The system was equilibrated with time, and the bulk of the sediment-forming material was brought from the catchments into lakes by streaming waters and air flows (allochthonous material), with the authigenic organic and mineral material formed by hydrobionts (autochthonous material). The concentrations of natural radionuclides in the soil profiles correspond to those in the loess loams. We statistically GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 2022 Number of observations Number of observations RADIOACTIVITY ASSESSMENT OF SAPROPEL SEDIMENTS IN SMALL LAKES 100 90 80 70 60 50 40 30 20 10 0 700 600 500 400 300 200 100 0 799 (а) (b) (c) Histogram of: Ас К-С d = 0.13385, p < 0.05, Lilliefors p < 0.01 Normal expectation Histogram of: Ас К-С d = 0.03047, p > 0.20, Lilliefors p < 0.05 Normal expectation Histogram of: Ас К-С d = 0.14512, p < 0.05, Lilliefors p < 0.01 Normal expectation Ас Soil –50 0 50 100 150 200 250 Upper boundaries, x ≤ boundary (d) Histogram of: 137Сs К-С d = 0.3462, p < 0.01, Lilliefors p < 0.01 Normal expectation 137 Сs Soil –50 0 50 100 150 200 250 300 350 Upper boundaries, x ≤ boundary 600 500 400 300 200 100 0 70 60 50 40 30 20 10 0 Ас Sapropel –50 0 50 100 150 200 Upper boundaries, x ≤ boundary (e) Histogram of: 137Сs К-С d = 0.31472, p < 0.01, Lilliefors p < 0.01 Normal expectation 137 Сs Sapropel –50 0 50 100 150 200 Upper boundaries, x ≤ boundary 45 40 35 30 25 20 15 10 5 0 70 60 50 40 30 20 10 0 Ас Biota –50 0 50 100 150 200 250 300 Upper boundaries, x ≤ boundary (f) Histogram of: 137Сs К-С d = 0.34108, p < 0.01, Lilliefors p < 0.01 Normal expectation 137 Сs Biota –5 0 5 10 15 20 25 Upper boundaries, x ≤ boundary Fig. 3. Histograms of the total effective specific activity (As) (Bq/kg) of (a, b, c) natural radionuclides and (d, e, f) 137Cs in the soils, sapropel, and biota. processed geochemical data on concentrations of natural radionuclides and 137Сs in the soils, sapropel bodies, and biota. The results show that all of the radionuclides are characterized by a normal or lognormal distribution (Fig. 3). A lognormal distribution indicates that the contribution of samples with an elevated radiation background was small. were found. Inasmuch as waters in the territories are mostly alkaline, this is favorable for high U mobility in the form of uranyl−carbonate Na compounds (Evseeva and Perel’man, 1962; and others). In the reducing environments of lakes with a stagnant hydrodynamic regime, uranium that has been previously adsorbed on colloid particles is reduced (Titaeva, 2005). Concentrations of Th and K in the sapropels of the lakes generally correspond to those in soils in the catchments and are generally lower than in the soils. Uranium concentrations in the bottom sediment of all of the lakes are lower than or equal to those in soils in the catchments of these lakes, with rare exceptions of lakes in the steppe and taiga zones. The exceptions are some soda lakes whose waters have high pH (>9), in which U concentrations in the sapropels are higher. The depletion of the sapropels in potassium is most likely explained by its leaching from minerals and organic matter in the sediments. Analysis of components of the lakes for U, Th, and K enabled us to compare the contributions of the specific activity of natural radionuclides in various landscape zones of southern West Siberia and the contribution of their total specific activity (Fig. 5). The values of As of the biota soils broadly vary but never exceed 300 Bq/kg. The values obtained for the soils are consistent with literature data on the natural radioactivity of rocks (Titaeva, 2000; Rikhvanov, 2009). The total effective specific activity of natural radionuclides (As) in the bottom sediments of all of the small lakes is lower than in soils in the catchments and never exceeds the standardized maximum value of 300 Bq/kg (GOST R 54519, 2011). It is worth mentioning that the As values of the sapropels generally only insignificantly inherit features of the catchments, i.e., the soil-forming rocks. This is explained by the significant contribution of the biota to the As values, with this contribution broadly varying even for a single lake (dependence on the species), from one lake to another within a single landscape zone, and from one landscape zone to Cluster analysis of the concentrations of trace elements and radionuclides in the soil and bottom-sediment samples has demonstrated the strongest positive correlations of Al with Si, Th, K, and Na, which are strongly correlated with many trace elements that are mostly contained in clastic minerals of the bottom sediments, such as quartz, feldspars, micas, mafic silicates, and aluminosilicates (Fig. 4). No correlations between U(Ra) and these elements (including Sr) GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 2022 800 STRAKHOVENKO et al. (а) 1.0 0.8 0.6 0.4 0.2 Fe V Al K Ba Na Si Ni Cu Cr Th Mn Co Zn Cd Pb Hg U Mg Sr Ca P (b) 0 –0.2 –0.4 –0.6 –0.8 –1.0 1.0 0.8 0.6 0.4 Na Th Hg K Al Si Cr Cu Ni Fe P Cd Pb Co Zn U Mg Sr Ba Ca Mn Cluster R Silicon class 0.2 0 –0.2 –0.4 –0.6 –0.8 –1.0 Cluster R Calcium class (c) 1.0 Na Mg Sr Ba U Ca P Al Si Fe Ni Co Cu Th Cr Zn Cd Pb Hg K Mn 0.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –1.0 Cluster R Mixed class Fig. 4. Dendrograms of correlations between chemical elements (R-cluster analysis) based on analytical data on the concentrations of major and trace elements (Si, Ca, Na, K, Al, Mg, Fe, Ti, P, Mn, Sr, Ba, Pb, Cd, V, Cu, Zn, Co, Ni, Cr, Hg, U, and Th) in the distinct classes of the sapropels: (a) silicon, (b) calcium, and (c) mixed. another. This is explained by the significant contribution of potassium, whose concentrations in the biomass are high, to the total specific activity. Relations between the U, Th, and K concentrations and their specific activity are reported in (Rikhvanov, 2009). To comprehensively estimate the radiation state of the sapropel deposits and the outlooks of their industrial use, we systematized our results on relations between the ash contents of the sediments and the total specific activity As of natural radionuclides in various water bodies of the lake systems in various landscapes (Fig. 6). No dependence of the As values on the ash contents of the sediments were detected, i.e., further evidence was obtained that biogeochemical processes largely control the chemical composition of the sapropels. Detailed data on the dependence of As values on the mineralogical composition of the samples of sapropel of various classes led us to determine the following. As expected, the minimum As values were detected in the carbonate sapropels, which is explained by that both calcite and dolomite practically cannot adsorb trace elements and do not contain them as admixtures, except only Sr, Mn, and Ba. Conversely, the occurrence of much micas and/or feldspars is the sapropels leads to an increase in the As values, because much potassium is contained in minerals of the terrigenous fraction. A decrease in the Th, U, and K concentrations in the sapropels of the lakes in all landscape zones, and hence, a decrease in the As values, is explained by the dilution of the sediments GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 2022 200 Taiga landscape 150 (Kyshtovskaya lake system) 50 0 0 ilo vo U rm an no Sh e ch uc h’ eP Le ne Sh vo ch uc h’ eL eK D an ch ’ ab Sh ch u Ka r 200 (Klyuchevskaya lake system) 150 (central Baraba) Ch ul Ba ym r Ka chin m ba l Ka a Be ily Ka rgul za ’ to Ya vo rg B ol’ Pe . Ka sc ily ha n Ch oe ist S oe B. uet Ku ok r Sa gan rb Bu aly gr k Ve isto rk e h N nee izh ne e 50 200 Steppe landscape 150 50 0 0 0 Soil Biota Ko ro s ali kh elt y Zh Pe tu M e ch ’ L. Ku re kh L. tel ev sk oe B. Ta sso Ly r ap un ik ha Sh ub a Ba lan so r 50 no vo Io e dn o Ka e ra Ta tan na tar Ta -6 na ta Ta r 4 na tar D em 2 ki n Ru o ble v Pr o es no e 50 ov oS 100 r’ 100 ov oP 100 Pe tu (Uglovskaya lake system) (Tanatarskaya lake system) 150 801 Forest–steppe landscape 150 100 200 Bq/kg 200 100 aly k Bq/kg RADIOACTIVITY ASSESSMENT OF SAPROPEL SEDIMENTS IN SMALL LAKES Bottom sediments Fig. 5. Total effective specific activity of natural radionuclides (As, Bq/kg, per dry weight) in the soils, biota, and bottom sediments of lakes occurring in various landscape zones of the Baraba lowland and Kulunda plain. The names of the lakes are listed along the X axis. Lakes in the Baraba lowland Lakes in the Kulunda plain Kachkul’nya Yargol’ Kusgan160 Peschanoe Kr. Lyaga 140 Kaily Khoroshee 120 B. Kaily Kankul’ 100 Bol. Chicha Itkul’ Zhiloe 80 60 40 20 0 Presnoe Balansor Zalivnoe M. Minzelinskoe Chistoe Tsybovo Bil’gen’ Bugristoe Suetok Kazatovo Zhiloe K. Bol. Kurgan Karagan Chulym Mostovoe B. Tassor Kurech’e Sarbalyk Kuklei Barchin Bergul’ Kambala 160 140 120 100 80 60 40 20 0 Rublevo Petukhovo S. Korostelevskoe Zheltyr’ Petukhovo P Malinovoe Tanatar 6 Ash content Demkino Tanatar 4 Lyapunikha Iodnoe Shuba Gor’koe As Fig. 6. Total effective specific activity of natural radionuclides (As, Bq/kg, per dry weight) and ash content of sapropels in lakes in the Baraba lowland and Kulunda plain. Circled lake names indicate to lakes with high Ca concentrations in their sapropels. with silica (quartz sand of aeolian genesis). Note that the presence of fringing-type vegetation of macrophytes at the lakes also hampers the influx of minerals of aeolian genesis into the sapropel deposits. We have previously determined that the trace-element compoGEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 sition of the lacustrine silts and soils in the steppe landscapes of the small lakes are identical, and this led us to hypothesize that the dominant sources of material for the bottom sediments of the lakes were particles of soil that rests on loess loams, and the main transport 2022 802 STRAKHOVENKO et al. (а) Bq/kg 20 40 60 80 0 100 10 10 20 20 30 30 Depth, cm Depth, cm 0 (b) Bq/kg 40 50 60 70 80 137 Cs Barchin Bol. Kaily Bugristoe Zhiloe-K Lenevo 20 40 60 80 100 40 50 Gor’koe 60 70 137 Presnoe Cs Rakity 80 Fig. 7. Vertical distribution of the specific activity of 137Cs (Bq/kg) in the profiles of sapropel bodies in lakes in the (a) Baraba lowland and (b) Kulunda plain. The lakes differ in composition and have an uneven distribution of radiocesium. The type of the symbols denotes the class of the sapropel body: squares correspond to the calcium class (Ca > Si), circles are the silicon class (Si > Ca), and triangles mark the mixed class (Si ~ Ca). agent was dust storms, which are widespread in southern West Siberia (Gavshin et al., 1999; Strakhovenko, 2011; and others). In lakes with open shores (such as Zheltyr’, Shuba, and Zhiloe K), high As values are caused by that the sediments are rich in muscovite, although loams on the shores contain only trace amounts of muscovite. Similar to any mica mineral, muscovite forms platy crystals (thin platelets and flakes), which can be readily transported by wind and can be brought from the catchment areas. It is worth mentioning that radiocesium activity in the soils and sapropels is high and is sometimes much higher than the background activity. Radioactive fallouts occurred in some areas in Baraba and Kulunda, as well as the territory of West Siberia as a whole, and soils and bottom sediments in these territories are still contaminated with radiocesium. Our database on 137Cs activity, recalculated to the year 2010 with the QGIS and ArcView program packages, were used to develop detailed models for the areal distribution of 137Cs reserves (mCi/km2) of the lake systems in various parts of the Baraba lowland and Kulunda plain and soils at the catchments of these systems. This material is generally consistent with our earlier maps, for example, the map of radiocesium reserves in the upper humus−accumulation horizon of soils in the Novosibirsk and Altai territories (Ad) (Malikova et al., 2005; Malikova and Strakhovenko, 2011; and others). According to the character of the vertical distribution of 137Cs in the sapropel deposits of the lake sys- tems, these systems can be classified into two major types. One of these types of 137Cs distribution in sapropel deposits pertains to lakes with two or more peaks of 137Cs activity in the bottom sediments, with this activity decreasing both up and down the vertical sections of the sapropel deposits (Fig. 7). Anomalously high 137Cs concentrations at deep levels of the sediments provide evidence that the sediments were originally contaminated by fallouts after nuclear tests starting from 1949. They can be explained by the passage of radioactive clouds over the lake systems and radioactive fallouts from these clouds. These lakes tend to be spatially constrained to radioactive fallout trails and are undoubtedly systems whose lakes were originally contaminated (Selegei, 1997; Rikhvanov, 2009). The sapropel deposits of these lakes are noted for an elevated radiocesium activity, which is twice or more higher than the global background (34 mCi/km2 as of 2010). The density of global 137Cs fallouts in the midlatitudes of Siberia, recalculated to 2010, is according to various evaluations (including those of the authors of this paper) 34 to 53 mCi/km2 (0.9–1.5 kBq/m2) (Chernyaga et al., 2012; and others). A feature of the other type is the fact that radiocesium is enriched in the upper horizons, and its activity gradually diminishes toward lower levels to a depth of 40−50 cm, where it approaches zero (Fig. 8). No 137Сs was found in the silts at depths greater than 40−50 cm. Our earlier studies have demonstrated that the depth of 40−50 cm corresponds (according to the plot of 210Pb distribution) to the beginning of nuclear tests at GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 2022 RADIOACTIVITY ASSESSMENT OF SAPROPEL SEDIMENTS IN SMALL LAKES 20 0 (а) Bq/kg 40 60 80 0 10 80 Tanatar-4 Kambala 20 Yargol’ 30 Suetok Bol. Kurgan Tsybovo 50 Depth, cm Kazatovo 40 Rublevo Bol. Tassor 30 Lyapunikha 40 Zalivnoe Kurich’e 50 Peschanoe 60 70 60 10 20 Depth, cm 20 (b) Bq/kg 40 803 Zheltyr’ 60 137 137 Cs 70 80 Cs 80 Fig. 8. Vertical distribution of the specific activity of 137Cs (Bq/kg) in the profiles of sapropel deposits in lakes in the (a) Baraba lowland and (b) Kulunda plain. The lakes differ in composition and show a gradual radiocesium enrichment with decreasing depth, starting at a depth of 40–50 cm. The type of the symbols denotes the class of the sapropel deposits: squares correspond to the calcium class (Ca > Si), circles are the silicon class (Si > Ca), and triangles mark the mixed class (Si ~ Ca). the Semipalatinsk test area (Strakhovenko et al., 2010; Strakhovenko et al., 2017; and others). Such distributions were found in many of the lakes. This distribution is likely explained by that radionuclides were continuously redistributed at the bottom–water interface, and this is associated with 137Cs influx from the catchment to lake with soil particles: the bulk of 137Cs is concentrated in the upper sod soil horizon even nowadays, and the destruction of this layer leads to the release of the radionuclides and their removal (Table 2). Only the past two decades were marked by the equalization and even a decrease in 137Cs addition to the bottom sediment compared to earlier decades. The decrease in the 137Cs activity in the uppermost horizon is controlled by the decrease in the concentration of the radioisotope in the sod horizon of the soil because of decay. This distribution type is thus related to the secondary redistribution of the “retained” reserves of soil 137Cs between the sinking remnants of the dead biomass and the new accumulation of radionuclides by the rooted aquatic vegetation and benthos. If the soils gradually release their artificial radionuclides as a result of chemical and physical processes, these radionuclides are accumulated in the lakes, i.e., the primary radioactive contamination of the lakes is overprinted by effects of secondary processes. For most sapropel deposits in the lakes, regardless of their chemical composition, the contamination level with 137Cs corresponds to the global background. GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 Evidently, it is the heterogeneous distribution of radiocesium in the soils and bottom sediments (because of the uneven fallouts of atmospheric precipitation during the nuclear tests) that is the main reason for that the 137Cs activity in the sapropels of the lakes is not correlated with the location of the lakes in various landscape zones. The local landscape environments affect the erosion–accumulation processes and lithochemical migration. Known facts indicate that radiocesium can migrate from upper soil horizons downward, but this has never been detected in any of the studied soil profiles (Table 2) (Izrael’, 2005; Rikhvanov, 2009; and others). The distribution of natural radionuclides over the whole lengths of cores from the sapropel deposits is practically homogeneous throughout the whole time period in question and depends, unlike what is typical of the soil profiles, mostly not only the composition of the soil-forming rocks (Malikova and Strakhovenko, 2017; Strakhovenko et al., 2017) but also on the composition of the biomass of the organic constituents of the sediment. At some of the lakes, their fringing reed tussock were found out to serve as reactive barriers, which seem to be able to fix soil particles brought from the shores (Ovdina et al., 2019). The reed tussock partly adsorb dissolved uranium, potassium, and radiocesium species and accumulate them in the root parts of the tussocks (the root systems of reedmace and reed). 2022 804 STRAKHOVENKO et al. Table 2. Morphological structure and vertical distribution of 137Cs specific activity (Bq/kg) in genetically different soil horizons in the catchments of the lakes 137Сs, Bq/kg 137Сs, Bq/kg Horizon Depth Horizon Depth Baraba lowland, Zhiloe Lake Kulunda plain, Demkino Lake Ordinary chernozem Iron-rich illuvial sod-podzol A 0–5 cm 54 О 0–2 cm 43 2–7 cm 27 A 5–10 cm 31 A1 7–12cm 12 A 10–16 9 A1 12–17 cm 2 AB 16–31 cm 0 A1 17–27 cm 0 B 31–54 cm 0 A 1A 2 27–35 cm 0 BC >54 cm 0 A2 B 1 35–75 cm 0 Meadow-chernozem solonized soil B1 0–6 cm 36 B2 75–104 cm 0 Ad >104 cm 0 A 6–18 cm 12 B3 B 18–40 cm 0 Meadow solonchak soil 0–5 cm 54 BC >40 cm 0 S1 5–8 cm 32 Boggy humous-gley soil S1 0–5 cm 89 Abound 8–13 11 Ap 5–10 cm 36 Abound 13–18 0 Ap 10–15 cm 12 Abound 18–20 cm 0 Ap 15–20cm 0 AS2 20–35 cm 0 Ap 35–50 cm 0 C 20–24 cm 0 S2 >50 cm 0 C >24 cm 0 S3 Tsybovo Lake Malinovoe Lake Meadow sod soil Sor solonchak 0–5 cm 49 S1 0–5 24 Ad 5–10 cm 13 A 5–10 cm 15 S2 10–15 cm 4 A 10–15 cm 4 S2 15–20 cm 0 A 15–22 cm 0 S2 20–36 0 AC 22–37 cm 0 S3 36–47 0 C >37 cm 0 S4 47–59 0 Mostovoe Lake S4S5 >59 cm 0 Typical gray forest soil S5 A 0–5 cm 22 Iodnoe Lake A 5–10 cm 11 Iron–rich illuvial sod–podzol 0–5 17 A 10–15 cm 2 A1 5–10 7 A 15–22 cm 0 A1 10–15 2 AB 22–44 cm 0 A1 15–21 0 B 44–74 cm 0 A1 21–60 0 BC >74 cm 0 A1A2 60–82 0 Bol’shie Kaily Lake A2B1 >82 cm 0 Typical gray forest soil B1 A 0–5 cm 38 Krasnovishnevoe Lake 5–10 21 Meadow–steppe solonized soil 0–5 cm 21 AB 10–15 3 A/ AB 15–20 cm 0 5–10 cm 4 A/ AB 20–31 0 10–15 cm 0 A/ B >31 cm 0 15–20 cm 0 A/ Meadow sod soil 20–25 0 A// Ad 0–17 cm 31 AB 25–30 0 30–35 0 A 17–40 cm 0 B1 35–40 0 C >40 cm 0 B1 40–58 0 Peschanoe Lake ABr Meadow sod soil BC 58–76 0 0–5 cm 42 С >76 cm 0 Ad 0–10 cm 23 Sor solonchak Ad 0–5 cm 24 A 10–15 cm 11 S1 5–10 cm 15 A 15–20 cm 3 S1 10–15 cm 4 AB 20–30 cm 0 S1 15–20 cm 0 AB 30–59 cm 0 S2 21–26 0 C 59–78 cm 0 S3 26–50 0 C >78 cm 0 S4 GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 2022 RADIOACTIVITY ASSESSMENT OF SAPROPEL SEDIMENTS IN SMALL LAKES 805 Our data do not reveal any significant differences in the accumulation and redistribution of radiocesium in the cores of the sapropels of any type and class. extraction from the bottoms of the lakes is necessary for the rational management of the natural resources and should maintain the balance of the lake systems. CONCLUSIONS The bottom sediments of the small lakes show a total effective activity (As) no higher that the standardized limiting values of 300 Bq/kg (GOST, 2011). The value of As of sapropel body of any one lake is controlled primarily by the mineralogical composition of the sediment (which depends on biogeochemical processes in the water and in the uppermost layer of the sediment), radiogeochemical characteristics of soils in the catchment, and hence, also the underlying rocks, and the presence of fringing macrophyte vegetation around the lakes, which hampers the introduction of the aeolian components of the terrigenous fraction of dust storms. The sapropel deposits of some of the lakes were found out to include horizons whose 137Cs reserves are twice or more higher than the global background (32 mCi/km2 as of 2010). These lakes tend to be spatially constrained within the areal trails of radioactive fallouts after nuclear tests in the Semipalatinsk Test Area and undoubtedly belong to lake systems with primary contamination of both the water bodies and soils in the catchments. If the soils gradually get rid of artificial radionuclides as a result of their decay, these lakes accumulate the radionuclides, because their primary radioactive contamination is overprinted by secondary processes of transport from the catchments. Anomalously high (peak) concentrations of 137Cs in some depth intervals provide evidence of the primary (starting in 1949) contamination of the sediments by products of nuclear tests. According to Article 19 of the Federal Law of Environmental Protection 7-FZ of January 10, 2002, which postulates standards not only for the quality of the environment but also for the permissible influence on the environment at economic and other activities to guarantee environmental safety, some horizons in the sapropel deposits cannot be directly utilized because of their contamination by artificial radiocesium. It is necessary to decide whether the material of these horizons should be first diluted with material with a low radiocesium activity (sand, sandy clay, etc.), or these horizons with a high radiocesium activity, which are normally no thicker than 10 cm, i.e., make up no more than 1% of the total thickness of the sapropel deposits, can be utilized directly. If the sapropels are brought from the bottom of the lakes, ecological problems of these lakes are simultaneously solved: silt deposition in the lake is terminated, and this drastically reduces the internal eutrophyzing load, ensures the sustainable functioning of the natural ecological system, and prevents the degradation of the lakes. Scientifically grounded sapropel FUNDING GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 This work is done on state assignment of IGM SB RAS with the financial support of the Ministry of Science and Higher Education of the Russian Federation. CONFLICT OF INTEREST The authors declare that they have no conflicts of interest. REFERENCES R. M. Aleksakhin, Nuclear Energy and Biosphere (Energoizdat, Moscow, 1982) [in Russian]. V. I. Baranov, Radiometry (AN SSSR, Moscow, 1956) [in Russian]. M. V. Bocharov, V. M. Loborev, I. P. Matveichuk, and V. V. Sudakov, “Global radioactive environmental contamination of the Northern Hemisphere and contribution of Soviet nuclear test to it,” Atomnaya Energiya 78 (1), 50–53 (1993). L. I. Boltneva, Yu. A. Izrael, V. A. Ionov, and I. M. Nazarov, “Global 137Cs and 90Sr contamination and doses of the external radiation on the USSR territory,” Atomnaya energiya. 42 (5), 355–360 (1977). B. P. Chernyago, V. G. Bychinskii, and G. I. Kalinovskii, “Global cesium-137: from Baikal to Arctic Ocean,” Proc. 2nd International Conference “radioactivity and Radioactive Elements in the Human Habitat (ID "Tandem-Art”, Tomsk, 2004), pp. 647–648 [in Russian]. B. P. Chernyago, A. I. Nepomnyashchikh, and V. I. Medvedev, “Current radiation environment in the central ecological zone of the Baikal natural territory,” Russ. Geol. Geophys. 53 (9), 1206–1218 (2012). N. I. Ermolaeva, E. Yu. Zarubina, V. V. Kirillov, D. M. Bezmaternykh, E. Yu. Mitrofanova, O. N. Vdovina, G. V. Vinokurova, L. A. Dolmatova, and M. I. Sokolova, “Facctor characteristics of hydrobiocenosis of lakes the dry steppe subzone of the Ob—Irtysh interfluve, V.Ya. Levanidova Readings (2019), No. 8, pp. 46–55. N. I. Ermolaeva, E. Yu. Zarubina, V. D. Strakhovenko, R. E. Romanov, A. V. Puzanov, and E. A. Ovdina, “Assesment of influence of abiotic factors on the production of ecosystems of minor lakes of West Siberia,” Organic Matter and Biogenic Elements in the Inner Basins and Seawaters. Proc. 6th All-Russian Symposium with International Participance (Barnaul, 2017), pp. 78–83 [in Russian]. L. S. Evseeva and A. I. Perelman, “Uranium Geochemistry in the Supergene Zone (Gosatomizdat, Moscow, 1962) [in Russian]. V. M Gavshin, F. V Sukhorukov, V. A Bobrov, M. S Melgunov, L. V Miroshnichenko, J Klerkx, S. I Kovalev, and P. A. Romashkin, “Chemical composition of the uranium tail storages at Kadji-Sai (southern shore of Issyk-Kul Lake, Kyrgyzstan), Water Air Soil Pollut. 154, 71–83 (2004). 2022 806 STRAKHOVENKO et al. V. M. Gavshin, B. L. Shcherbov, V. D. Strakhovenko, M. S. Melgunov, V. A. Bobrov, and V. M. Tsibulchik, “137Cs and 210Pb in the lacustrine deposits of steppe Altai as indicator of dynamics of anthropogenic changes of geochemical background during 20th century,” Geol. Geofiz. 40 (9), 1331–1341 (1999). V. M. Gavshin, F. V. Sukhorukov, and I. N. Malikova, “Distribution of radionuclides over Altai krai,” Nuclear tests, environment, and wealth of inhabitants of the Altai krai. Proc. Researchers (Barnaul, 1993), pp. 34–72 [in Russian]. V. M. Gavshin, F. V. Sukhorukov, V. S. Parkhomenko, I. N. Malikova, and M. S. Melgunov, “Traces of the Chernobyl accident in West Siberia,” Radioactivity during Nulcear explosions and Accidents. Proc. International Conference, Moscow, 2000 (Gidrometeoizdat, St-Petersburg, 2000), Vol. 1, pp. 178–182 [in Russian]. GOST 31861, Water. General Requirements to Sampling. International Standard (2012) [in Russian]. GOST R 54519, Organic Fertilizers. Sampling Methods. RF National Standard (2011) [in Russian]. V. B. Il’in and A. I. Syso, Trace Elements and Heavy Metals in soils of the Novosibirsk Oblast (SO RAN, Novosibirsk, 2001) [in Russian]. Yu. A. Izrael, “Anthropogenic radioactive pollution of the Earth’s planet,” Radioactivity After Nuclear Explosions and Accident: Proc. International Conference (Gidromet, Moscow, 2005), pp. 13–24 [in Russian]. Yu. A. Izrael, E. V. Kvasnikova, I. M. Nazarov, and E. D. Stukin, “Cesium-137 radioactive contamination of Russia at the turn of centuries,” Meteorol. Gidrol., no. 4, 20–31 (2000). A. E. S. Kemp, R. B. Pearce, I. Koizumi, J. Pike, and S. J. Rance, “The role of mat-forming diatoms in the formation of Mediterranean sapropels,” Nature 398 (6722), 57–61 (1999). Classification of Russia’s Soils, Ed. by L. L. Shishov V. D. Tonkonogov, and I. I. Lebedev (Pochv. Inst. Im. V.V. Dokuchaev, Moscow, 1997) [in Russian]. N. V. Korde, Biostratigraphy and Typology of Russian Sapropels (AN SSSR, Moscow, 1969) [in Russian]. B. V. Kurzo, O. M. Gaidukevich, and M. V. Kuzmitskii, “Improvement of methodology of prospecting of sapropel deposits, technology of mining and processes of sapropel for increasing efficiency of its use,” Novosti Nauki Tekhnol. 16 (3), 16–26 (2010). A. P. Lisitsyn, World Ocean. Volume 2. Physics, Chemistry, and Biology of Ocean. Sedimentation in Ocean and Interaction of the Earth’s Geosphere, Ed. by L.I. Lobkovsky and R. I. Nigmatulin (Nauch. mir, Moscow), pp. 331– 571 (2014) [in Russian]. M. Z. Lopotko, Lakes and Sapropel (Minsk, 1978) [in Russian]. M. Z. Lopotko, G. A. Evdokimova, and O. M. Bukach, “Methodical Indications on Searching and Prospecting of Lacustrine Sapropel Deposits of BSSR (Nauka i tekhnika, Minsk, 1986). I. N. Malikova and V. D. Strakhovenko, “Uranium, thorium, and Th/U ratio in soils of the southern West Siberia,” Probl. Biogeokhim. Geokhim. Ekol. 15 (1), 26– 39 (2011). I. N. Malikova and V. D. Strakhovenko, “The effect of landscape factors on natural radioactivity of soils in Siberia,” Int. J. Environ. Res. 11 (5–6), 653–665 (2017). I. N. Malikova, V. D. Strakhovenko, F. V. Sukhorukov, and A. Yu. Devyatova, “Ekological state of soils of the Altai krai: contamination by radiocesium,” Sibirsk. Ekol. Zh. 12 (6), 985–998 (2005). V. I. Medvedev, L. G. Korshunov, and B. P. Chernyago, “Radiation influence of the Semipalatinsk nuclear test at South Siberia (experience of long-term studies on Eastern and Middle Siberia and comparison of results with data on West Siberia),” Sibirsk. Ekol. Zh. 6 (12), 1055–1071 (2005). M. S. Melgunov, V. M. Gavshin, F. V. Sukhorukov, I. A. Kalugin, V. A. Bobrov, and J. Klerkx, “Anomalies of radioactivity on the southern coast of Lake Issyk-Kul (Kyrgyzstan),” Khimiya v Interesakh Ustoich. Razvitiya, No. 6, 869–880 (2011). L. N. Mikhailovskaya, I. V. Molchanova, and M. G. Nifontova, “Radionuclides of global Fallouts in plants of terrestrial ecosystems of the Ural region,” Ekologiya. 1, 9–15 (2015). A. A. Mikhalchuk, E. G. Yazikov, and V. V. Ershov, Statistical Analysis of Ecological—Geochemical Information: a Textbook (TPU, Tomsk, 2006) [in Russian]. A. A. Moiseev, Cesium-137 in Biosphere, Ed. by A. A. Moiseev and P. V. Ramzaev (Atomizdat, Moscow, 1975) [in Russian]. D. V. Nalivkin, Windstorm, Rainstorm, and Tornado. Geographical Features and Geological Activity (Nauka, Leningrad, 1969) [in Russian]. L. Newsome, K. Morris, and J. R. Lloyd, “The biogeochemistry and bioremediation of uranium and other priority radionuclides,” Chem. Geol. 363, 164–184 (2014). Yu. P. Nikolskaya, Salination in Lakes and Waters of the Kulunda Steppe (SO AN SSSR, Novosibirsk, 1961) [in Russian]. Explanatory Note to the Map of the Quaternary Deposits. Scale 1 : 200 000. Kulundin–Barabinskaya Series. Sheet N-44–I (1967) [in Russian]. E. A. Ovdina, V. D. Strakhovenko, N. I. Ermolaeva, E. Yu. Zarubina, and A. V. Saltykov, “Modern Mineral Formation in lakes Petukhovo of the Kulunda steppe,” Water Resources: Study and Operation (Limnological School–Practice). Proc. 5th International Conference of Youth Scientists, Petrozavosk, Russia, 2016 (Karel’sk. Nauchn. Ts. RAN, Petrozavodsk, 2016), pp. 210–217 [in Russian]. E. A. Ovdina, V. D. Strakhovenko, N. I. Ermolaeva, E. Yu. Zarubina, A. I. Syso, and Yu. V. Ermolov, “Distribution of radionuclides in Lake Sarbalyk (Baraba Plain),” Radioactivity and Radioactive Elements in the Human Inhabitancy. Proc. 5th International Conference (2016), pp. 475–477 [in Russian]. E. A. Ovdina, V. D. Strakhovenko, N. I. Yermolaeva, E. Yu. Zarubina, and Yu. V. Yermolov, “Radionuclide distribution in components of the Sarbalyk limnetic system (Baraba lowland, Western Siberia),” Russ. J. Earth Sci. 19 (6), Art.ES6013 (2019). Doi: https://doi.org/10.2205/2019ES000681 F. I. Pavlotskaya, Migration of Radioactive Products of Global Fallouts in Soils (Atomizdat, Moscow, 1974) [in Russian]. GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 2022 RADIOACTIVITY ASSESSMENT OF SAPROPEL SEDIMENTS IN SMALL LAKES Soils of the Novosibirsk Oblast (Nauka, Novosibirsk, 1966) [in Russian]. A. V. Puzanov, T. A. Rozhdestvenskaya, S. V. Baboshkina, O. A. Elchininova, D. N. Balykin, S. N. Balykin, A. V. Saltykov, I. V. Gorbachev, and I. A. Troshkova, “Content and distribution of heavy natural radionuclides (238U, 232Th, 40K) in soils of the Verkhnii Alei basin (southwestern Altai krai),” Radioactive and Radioactive Elements in the Human Inhabitance. Proc. 5th International Conference (2016), pp. 534–537 [in Russian]. A. V. Puzanov, S. N. Balykin, A. V. Saltykov, and I. A. Troshkova, “Soils of the Catchment Area of the Minzelinskoe, Itkul, Kankul, and Kachkulnya Minor Sapropel Lakes (Novosibirsk oblast),” Izv. Altaisk. Otdel. Russk. Geograf. O-va 1 (44), 80–84 (2017). L. P. Rikhvanov, Radioactive Elements in the Environment and Radioecological Problems: a Textbook (STT, Tomsk, 2009) [in Russian]. Yu V. Robertus, V. I. Fatin, O. B. Rylov, and S. L. Shamov, “Anomalous increase of radioactive background at the territory of the Altai krai,” Nuclear Tests, Environment, and Wealth of the Inhabitants of the Altai Krai. Proc. of Studies 1 (1), 112–116 (1993) [in Russian]. V. V. Selegei, Radioactive Contamination of Novosibirsk: Past and Present (Novosibirsk fil. seti fondov Sorosa, Novosibirsk, 1997) [in Russian]. O. V. Serebrennikova, E. B. Strelnikova, P. B. Kadychachov, I. V. Russkikh, and E. D. Elchaninova, “Vertical distribution of organic compounds in the bottom sediments of two steppe lakes in southern Siberia,” Water Res. 44 (5), 774–784 (2017). S. M. Shtin, Lake Sapropels and Principles of their Complex Exploration (Mosk. Gos. Univ., Moscow, 2005) [in Russian]. R. Stein, Arctic Ocean Sediments. Processes, Proxies, and Paleoenvironment (Elsevier, Amsterdam, 2008). N. M. Strakhov, Selected Works. Sedimentation in Modern Basins (Nauka, Moscow, 1993) [in Russian]. N. M. Strakhov, N. G. Brodskaya, L. M. Knyazeva, A. N. Razzhivina, M. A. Rateev, D. G. Sapozhnikov, and E. S. Shitova, Formation of Sediments in Modern Basins (AN SSSR, Moscow, 1954) [in Russian]. V. D. Strakhovenko, Extended Abstract of Candidate’s Dissertation in Geology and Mineralogy (IGM SO RAS, Novosibirsk, 2011) [in Russian]. V. D. Strakhovenko, B. L. Shcherbov, I. N. Malikova, and Yu. S. Vosel, “The regularities of distribution of radionuclides and rare-earth elements in bottom sediments of Siberian lakes,” Russ. Geol. Geophys. 51 (11), 1501– 1514 (2010). V. D. Strakhovenko, O. P. Taran, and N. I. Ermolaeva, “Geochemical characteristics of the sapropel sediments of small lakes in the Ob’–Irtysh interfluve,” Russ. Geol. Geophys. 55 (10), 1466–147 (2014). V. D. Strakhovenko, N. A. Roslyakov, A. I. Syso, N. I. Ermolaeva, E. Yu. Zarubina, O. P. Taran, and A. V. Puzanov, “Hydrochemical characteristic of sapropels in Novosibirsk oblast,” Water Res. 43 (3), 539–545 (2016). V. D. Strakhovenko, I. N. Malikova, E. A. Ovdina, and A. A. Denisenko, “Distribution of natural radionuclides in the bottom sediments of lakes in different landscape areas of Western Siberia,” International Mul- GEOCHEMISTRY INTERNATIONAL Vol. 60 No. 8 807 tidisciplinary Scientific GeoConference SGEM, STEF92 Technology Ltd. 17 (11), 703–710 (2017). V. D. Strakhovenko, E. A. Ovdina, N. I. Ermolaeva, E. Yu. Zarubina, O. P. Taran, V. V. Boltenkov, and T. I. Mishchenko, “Genesis of lacustrine sapropel deposits in the central Baraba plain,” Sedimentary Geology of the Urals and Adjacent Regions: Present and Future. Proc. 12th Uralian Lithological Conference (IGG UrO RAN, Yekaterinburg, 2018), pp. 334–337 [in Russian]. V. D. Strakhovenko, G. I. Malov, E. A. Ovdina, N. I. Ermolaeva, and E. Yu. Zarubina, “Actual problems of preservation and use of sapropel lodes of minor lakes of the Baraba lowland and Kulunda plain,” Lakes of Eurasia: Problems and Ways of their Solution. Proc. 2nd International Conference, Kazan, Russia, 2019 (Akad. Nauk RT, Kazan, 2019), Vol. 2, 184–189 (2019). F. V. Sukhorukov, I. N. Malikova, M. A. Malgin, V. M. Gavshin, B. L. Shcherbov, A. V. Puzanov, V. D. Strakhovenko, and S. I. Kovalev, “Radiocesium in soils of Siberia (experience of long-term studies),” Sibirsk. Ekol. Zh., No. 2, 131–142 (2001). A. I. Syso, Regularities of Distribution of Chemical Elements in Soil-Forming Rocks and Soils of West Siberia (SO RAS, Novosibirsk, 2007) [in Russian]. O. P. Taran, V. V. Boltenkov, N. I. Ermolaeva, E. Yu. Zarubina, I. V. Delii, R. E. Romanov, and V. D. Strakhovenko, “Relations between the chemical composition of organic matter in lacustrine ecosystems and the genesis of their sapropel,” Geochem. Int. 56 (3), 256–265 (2018). N. A. Titaeva, Nuclear Geochemistry (MGU, Moscow, 2000) [in Russian]. N. A. Titaeva, Geochemistry of Natural Radioactive Decay Series (GEOS, Moscow, 2005) [in Russian]. D. Wan, Zh. Jin, and Y. Wang, “Geochemistry of eolian dust and its elemental contribution to Lake Qinghai sediments,” Appl. Geochem. 27 (8), 1546–1555 (2008). E. Yu. Zarubina, “Primary production of macrophytes of three sapropel lakes of different types of southern West Siberia (within the Novosibirsk oblast) in 2012,” Mir Nauki, Kultury, i Obrazovaniya 5 (42), 441–444 (2013). E. Yu. Zarubina and G. V. Fetter, “Production and destruction of organic matter in the mountain lakes of Russian Altai,” XII Conference of Hydrobiological Community at RAS. Proc. Reports, Petrozavodsk, Russia, 2019 (KarNTs, Petrozavodsk, 2019), pp. 165–166 [in Russian]. E. Yu. Zarubina and M. I. Sokolova, “Role of zonal factors in the formation of productivity of minor lakes of the southern Ob’–Irtysh interfluve,” Lakes of Eurasia: Problems and Ways of their Solution. Proc. International Conference (AN RT, Kazan, 2019), Vol. 1, pp. 80–84 [in Russian]. E. Yu. Zarubina, N. I. Ermolaeva, V. D. Strakhovenko, E. A. Ovdina, R. E. Romanov, and O. P. Taran, “Relation of chemical composition of sapropels with productivity of plankton and macrophytes in lakes of southern West Siberia,” Proc. All-Russian Scientific–Practical Conference, Sochi, Russia, 2018 (Lik, Novocherkassk, 2018), Vol. 1, 333–339 [in Russian]. Translated by E. Kurdyukov 2022