Detailed study of post-Chernobyl Cs-137 redistribution in the soils of a small agricultural catchment (Tula region, Russia)

Highlights

• The ¹³⁷Cs inventory in the Plavsk hotspot is 2–6 × higher than the safety standard.

• Contemporary ¹³⁷Cs inventories are very heterogeneous in the study catchment.

• >89% of ¹³⁷Cs reserve is in the top 0–25 cm soil, regardless of land use or location.

Abstract

A detailed study of ¹³⁷Cs redistribution was conducted within a small agricultural catchment in the highly contaminated Plavsk radioactive hotspot in the Tula region of Central Russia, 32 years after the Chernobyl nuclear power plant (NPP) accident, which occurred on April 26, 1986. Although more than three decades have passed since the Chernobyl NPP incident, ¹³⁷Cs contamination is high. The ¹³⁷Cs inventory varies from 67 to 306 kBq·m⁻², which is 2–6 times higher than the radiation safety standard; however, the soils remain suitable for crop cultivation. The initial ¹³⁷Cs fallout within the Plavsk radioactive hotspot was extremely heterogeneous, with a trend of decreasing ¹³⁷Cs inventories from the NW to the SE directions within the studied territory. Contemporary ¹³⁷Cs inventories are also very heterogeneous in the studied catchment. However, the trend of the initial ¹³⁷Cs fallout does not appear in the contemporary ¹³⁷Cs inventories on the slopes. Two methods of interpolation (expert-visual and automatic) were used to calculate the ¹³⁷Cs budget, revealing high similarity in their ¹³⁷Cs loss estimates; however, a large discrepancy was observed in their ¹³⁷Cs gain estimates. A detailed analysis of ¹³⁷Cs redistribution revealed the importance of hollows and “plow ramparts” (positive topographic forms on the boundaries of cultivated fields) in the transport and deposition of sediments. A quarter of the total ¹³⁷Cs gain was deposited within the arable land, whereas a quarter was deposited within the non-plowing sides of the dry valley; the other half was deposited in the valley bottom. About 7–8 × 10⁶ kBq of the ¹³⁷Cs inventory flowed out of the catchment area, which was only about 2% of the ¹³⁷Cs fallout after the Chernobyl NPP accident. About 89% of the total ¹³⁷Cs reserve is concentrated in the top (0–25 cm) layer of soils, regardless of land use or location within the catchment.

Introduction

The Chernobyl accident, which occurred on April 26, 1986, caused radioactive contamination across a large area of the East European Plain (EEP). More than 200 thousand km² of Europe was contaminated with radioactive ¹³⁷Cs (over 0.04 MBq of ¹³⁷Cs per m²), and 71% was deposited in the three most affected countries, Belarus, Russia, and Ukraine (Higley, 2006). Radio-Cs, or ¹³⁷Cs, is a relatively long-lived radionuclide with a half-life of 30.2 years. The spatial distribution of Chernobyl-derived ¹³⁷Cs fallout was controlled by two main factors: the trajectories of air mass transport and the intensity of precipitation 2–3 weeks after the explosion at the Chernobyl nuclear power plant (NPP) (Izrael et al., 1996; Gaydar and Nasvit, 2002). ¹³⁷Cs is rapidly and firmly adsorbed on mineral soil, particularly on fine and clay particles (Cho et al., 1996; Walling et al., 2006; Takahashi et al., 2017). The redistribution of ¹³⁷Cs within the EEP is associated with tillage erosion and the sheet, rill, and ephemeral gully erosion processes that occur during spring snowmelt (March–April) or summer rainstorms (Gusarov et al., 2019).

Some studies were conducted in the Plavsk radioactive hotspot area on the eastern track of the Chernobyl fallout (Golosov et al., 1999a, 2000, 2011; Kvasnikova et al., 2009; Ivanov et al., 2016; Mamikhin et al., 2016; Komissarova and Paramonova, 2019). Golosov et al. (1999a, 2000) revealed considerable spatial heterogeneity in the center of the Plavsk radioactive hotspot: the inventories of ¹³⁷Cs varied from 368 ± 56 to 559 ± 93 kBq·m⁻² in arable chernozems located in different local interfluves. A significant decrease in the ¹³⁷Cs content of arable soils on slopes, and the subsequent ¹³⁷Cs accumulation in the alluvial soils at the bottom of river valleys, were also recorded.

¹³⁷Cs budgets have been estimated during the study of global stratospheric depositions of radionuclides formed after nuclear bomb tests (Longmore et al., 1983; Vanden Berge and Gulinck, 1987; Sutherland and de Jong, 1990; Owens et al., 1997), and of tropospheric fallouts associated with accidents at NPPs (Walling et al., 2000; Panin et al., 2001; Golosov et al., 2018). Detailed estimates of the redistribution of sediments and ¹³⁷Cs were undertaken within small catchments (Ritchie et al., 1974; McHenry and Ritchie, 1975; Golosov et al., 1999b, 2013; Li et al., 2003; Ming-Yi et al., 2006; Porto et al., 2016; Varley et al., 2018). Studies conducted in the first decades after the Chernobyl accident did not indicate significant ¹³⁷Cs redistribution on eroded slopes (Litvin et al., 1996; Golosov and Markelov, 2002). However, ¹³⁷Cs reserves later increased in sediment sinks, particularly in dry valley bottoms, owing to the re-deposition of sediments, delivered by surface runoff from cultivated slopes (Fridman et al., 1997; Panin et al., 2001; Mamikhin et al., 2016).

¹³⁷Cs is one of the most dangerous pollutants; therefore, detailed studies of its migration are relevant and important. Like many other pollutants, ¹³⁷Cs reaches the soil surface with wet fallout and is rapidly and strongly adsorbed by clay and organic colloids within the soil (Ritchie et al., 1974). ¹³⁷Cs has limited mobility through chemical processes. Physical processes associated with the erosion, transport, and deposition of sediment particles, and with cultivation, represent the majority of ¹³⁷Cs redistribution (Sutherland and de Jong, 1990). The lateral migration trends obtained for ¹³⁷Cs can be used to predict the spatial redistribution of other pollutants that also appear from the atmosphere and are transported with solid matter (for example, polycyclic aromatic hydrocarbons, especially benzo[a]pyrene, heavy metals, hexachlorobenzene, DDT, other radionuclides, and many other pollutants). In previous publications (Samonova et al., 2015; Koshovskii et al., 2019) on the studied catchment, other pollutants (heavy metals and polycyclic aromatic hydrocarbons) and sediment transport were also analyzed. This study aimed to evaluate the spatial (lateral) and vertical redistribution features of Chernobyl-derived ¹³⁷Cs within a small agricultural catchment of the first order (Horton system), three decades after the Chernobyl incident.