ISSN 1064-2293, Eurasian Soil Science, 2021, Vol. 54, No. 2, pp. 291–303. © Pleiades Publishing, Ltd., 2021. Russian Text © The Author(s), 2021, published in Pochvovedenie, 2021, No. 2, pp. 247–260. SOIL ЕROSION Erosion as a Factor of Transformation of Soil Radioactive Contamination in the Basin of the Shchekino Reservoir (Tula Region) V. N. Golosova, b, c, M. M. Ivanova, b, A. S. Tsyplenkova, M. A. Ivanovc, Y. Wakiyamad, A. V. Konoplevd, E. A. Konstantinovb, and N. N. Ivanovaa, * aLomonosov Moscow State University, Moscow, 119991 Russia bInstitute of Geography, Russian Academy of Sciences, Moscow, 119017 Russia cKazan Federal University, Kazan, 420008 Russia dInstitute of Environment Radioactivity, Fukushima University, Kanayagava, Fukushima, 1960–1296 Japan *e-mail: [email protected] Received April 30, 2020; revised June 16, 2020; accepted July 13, 2020 Abstract—Redistribution of sediments and Chernobyl-derived 137Сs transported with them were estimated using a set of field methods and erosion model calculations for the Shchekino reservoir (Tula region) catch- ment, and changes in the contents of 137Сs in soils of various types that occurred over 1986–2018 were deter- mined. The rate of snowmelt soil erosion on arable land during this period has decreased by about a half in comparison with that in 1960–1985 due to a reduction in soil freezing depth in winter. The rainfall erosion rate increased by about a third between 1986–2003 due to an increase in the rainfall erosivity index; after that, it tended to decrease synchronously with a decrease in the rainfall erosivity. The total average annual soil loss related to water erosion varies in the range of 1.3–1.6 t ha–1 depending on the soil type. The erosional loss of 137Сs from arable land averaged 1.5–2% of its total inventory, which decreased by more than a half in com- parison with the initial inventory in May 1986 due to natural decay. On 0.4% of the arable land with maximum rates of erosion, the decrease in the 137Сs inventory reached 12–40% from the initial inventory. More than 90% of 137Сs washed away with sediments from arable lands were redeposited along the transportation path- way from arable fields to permanent streams. The total soil 137Сs inventory exceeded its initial inventory at the time of fallout from the atmosphere in May 1986 in the bottoms of hollows in areas from the lower edge of the arable land to the upper reaches of dry first-order valleys due to high sedimentation rates. The 137Сs inventory exceeded the lower threshold of permissible radioactive contamination of soils (37 kBq m–2) also in some other sediment sinks (bottoms of dry valleys, foot of plowed slopes, and a low floodplain of rivers) due to the accumulation of contaminated sediments. Keywords: radioactive contamination, snowmelt erosion, rainfall erosion, 137Cs inventory, sediments redis- tribution, Luvic Chernic Phaeozems, Luvic Greyzemic Chernic Phaeozems, Luvic Retic Greyzemic Phae- ozems, Stagnic Phaeozems (Colluvic, Pachic) DOI: 10.1134/S106422932102006X INTRODUCTION as the Plavsk cesium spot (Fig. 1). It is found in the Soils of a part of Europe were radioactively con- agricultural region with a high proportion of arable 137 taminated (mainly, with 137Cs) after the Chernobyl lands. The content of Cs, which is quickly and disaster at the end of April 1986 [28]. On a larger part firmly fixed on soil particles, determines the level of of polluted territory, the levels of radioactive contam- radioactive contamination of soils [3, 32]. In regions ination of soils did not exceed the permissible values with a high proportion of arable lands, erosion-accu- and were comparable in magnitude with the pollution mulative processes are the main mechanism for the from 137Cs global fallout as a result of nuclear explo- subsequent redistribution of radionuclides within river sions in the atmosphere since the beginning of the basins [31]. In the north of the forest-steppe zone of 1950s and until 1963. However, locally, areas with the European part of Russia, wind erosion is not contamination levels exceeding the maximum permis- developed [12]. Snowmelt erosion in spring and rain- sible level were formed, including those in the Euro- fall erosion in summer and fall play the major role in pean part of Russia. For example, in the southern half the lateral migration of 137Cs. Over more than 30 years of Tula region, the maximum fallout of 137Сs and other that have passed since the Chernobyl disaster, the con- radionuclides was observed in the area extending from tent of 137Cs in soils has already been halved due to nat- Chernskii to Bogoroditskii districts; this area is known ural decay (the half-life period of 137Cs is 30.2 years). 291 292 GOLOSOV et al. (a)(a) (c)(c) ((b)b) TulaTula 7 Depth, m 0−1 ShR-9/1,ShR-9/1, 9/29/2 SShR-8/1,hR-8/1, 88/2/2 UpaUpa 6 1−2 ShR-6/1,ShR-6/1, 6/26/2 ShR-5/1,ShR-5/1, 5/25/2 1 2−3 PlavaP l a − v 3 4 a 2 5 4−5 PlavskPlavsk 8 UpaUpa 5−6 ShR-7/1,ShR-7/1, 77/2/2 4 − 3 6 7 ShR-3,ShR-3, 4 Density of contamination 137Cs, kBq/m2 0 0.25 0.50 1.0 1.5 2.0 km −9 0 3.7 10 20 37 185 555 Fig. 1. Research area, location of gauging stations, weather stations, and sampling points for soil and bottom sediments: (a) loca- tion of Tula region; (b) map of radioactive contamination of Tula oblast in 1986 [4] ((1) boundary of the Upa River basin, (2) catchment of the Shchekino reservoir; soil sampling sites: (3) grassed bottom of the hollow and plowed slope, (4 and 5) flood- plains of the Lokna and Upa rivers, respectively; gauging stations on the Upa River: (6) Orlovo, (7) Tula; (8) weather station in Plavsk); and (c) bathymetric map of the Shchekino reservoir ((9) sampling points of bottom sediment cores). The water erosion contributes to the redistribution of catchment of the Shchekino reservoir under the impact washed off soil particles with 137Сs fixed on them of erosion-accumulative processes. within river catchments. As a result, secondary pollu- tion areas are formed, where the total 137Сs inventory exceeds the levels of its initial deposition [22, 45]. OBJECTS AND METHODS These areas include unplowed lower parts of slopes, Study area. The Shchekino water reservoir was cre- bottoms of dry valleys, floodplains, and water bodies, ated in 1950 in the upper reaches of the Upa River as a in which the accumulation of 137Cs transported with cooling pond of the Shchekinskaya electric power sta- soil particles takes place [36, 37, 39, 41, 47]. tion. Its catchment (1350 km2) is found southeast of Available estimates of 137Cs redistribution are the center of Tula oblast (Fig. 1), in the agricultural mainly based on the use of erosion models, or their region with a predominance of arable land and the combined use with actual measurements of the total absence of large settlements. 137 Cs inventory in the areas of erosion and accumula- According to the Köppen–Geiger climate classifi- tion [31]. In some cases, for the catchments of small cation, the investigated part of the Upa River basin is 137 ponds, the Cs budget was assessed taking into located in the humid continental (Dfb) climate zone account its accumulation in bottom sediments [35]. At without dry season and with hot summer. The mean the same time, no quantitative assessments of 137Cs annual precipitation slightly decreases from the north- redistribution for catchments of large reservoirs have west to the southeast from 630 to 592 mm with about been carried out so far. 460 mm in the warm season. The upper reaches of the This study aims at a quantitative assessment of the Upa River lie near the main divide between the Volga transformation of the field of radioactive soil contam- and Don River basins. The studies territory is a part of ination in 32 years after the Chernobyl disaster for the the eastern slope of the Central Russian Upland and it EURASIAN SOIL SCIENCE Vol. 54 No. 2 2021 EROSION AS A FACTOR OF TRANSFORMATION OF SOIL RADIOACTIVE 293 is characterized by a relatively slightly dissected topog- interpretation and subsequent manual digitization. raphy with extensive nearly level watersheds. The For other land uses, we used the automatic classifica- maximum relative elevation of the relief is observed in tion by the Random Forest method with the number the western part of the Upa River basin, where the of trees equal to 100. Thus, three vector layers of land river sharply turns to the northeast towards the use maps for different years were obtained. Their over- Shchekino reservoir. This part of the basin was most lay showed the areas of reduction and increase of ara- heavily contaminated after the Chernobyl disaster ble land and other land categories for the estimated (Fig. 1b). The network of permanent streams includes time intervals. The calculation of the areas of used and the Upa River, its large right-bank tributary—the abandoned arable land was carried out in the MapInfo Uperta River—and their confluents. The soil cover of program. A more detailed description of the methods the basin consists of leached chernozems (Luvic Cher- of image interpretation is given in [10]. nic Phaeozems), podzolized chernozems (Luvic To calculate rainfall erosion, the revised universal Greyzemic Chernic Phaeozems), and dark gray forest soil loss equation (RUSLE) was used [45]: soils (Luvic Retic Greyzemic Phaeozems) on the interfluves covered with loesslike calcareous loam as YRKLSCP= , (1) the parent material is [16].