The anthropogenic fallout radionuclides in soils of Mount Khuko (the Western Caucasus) and their application for determination of sediment redistribution.

The purposes of this study are to determine the content and origin of anthropogenic fallout radionuclides (FRN) in soils of Mount Khuko, located in the western sector of the Caucasus Mountains and to assess the possibility to use them for evaluation of sediment redistribution for the alpine grasslands,. The field study was carried out in August 2019 near the top of Mount Khuko, located in the western part of the main Caucasus Mountain Ridge. Integral and incremental soil samples were collected from the different morphological units of the studied area. The content of 137Cs and 241Am in soil samples was evaluated using laboratory gamma-spectrometry. A part of samples was selected for Pu isotopes extraction and then alpha-spectrometric analysis. It was established that the 137Cs contamination of soils in the studied area has at least two sources of origin. The first source is the 137Cs bomb-derived fallout after the bomb tests in 1950-60th, which is widespread across the globe. The second source is 137Cs Chernobyl-derived fallout High random variability (Cv = 25-42%) was found within reference sites, located at the undisturbed areas on the local flat interfluves due to high variability of soil characteristics (grain size, density, organic matter content etc.). However minimum spatial variability (range 12,2-14,3 kBq/m2) was identified for the mean value of 137Cs inventories for all 5 reference sites located in the different parts of the studied area. It is difficult to separate individual peaks of the bomb-derived and Chernobyl-derived 137Cs falloutin sediment sinks with low sedimentation rates. Application 239,240Pu as an additional chronological marker allows to identify the origin of above mention peaks in the soils of alpine grasslands and of dry lake bottom.

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

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.

Use of natural and artificial radionuclides to determine the sedimentation rates in two North Caucasus lakes.

The specific activities of natural (210Pb, 226Ra, and 232Th) and artificial (137Cs, 239,240Pu, and 241Am) radionuclides in the sediments of two North Caucasus lakes were determined. The two lakes, Lake Khuko and Lake Donguz-Orun, differ in their sedimentation conditions. Based on the use of unsupported 210Pbex and both Chernobyl-derived and bomb-derived 137Cs as chronological markers, it was established that the sedimentation rates in Lake Khuko over the past 55-60 y did not exceed 0.017 cm y-1. Sedimentation rates in Lake Donguz-Orun were found to be more than an order of magnitude higher. In the latter case, the sedimentation rates for the period from 1986 to the present were over 1.5 times higher than they were for the period 1963-1986. The differences in sedimentation rates were due to differences in the rates of denudation of their respective catchment areas. The specific activities of artificial radionuclides (137Cs, 2600 Bq kg-1; 239,240Pu, 162 Bq kg-1; and 241Am, 36 Bq kg-1) and their ratios in the sediments of Lake Khuko show that their deposition was mainly due to global stratospheric fallout of technogenic radionuclides associated with nuclear bomb testing during 1954-1963-rather than fallout from the Chernobyl accident. Several factors, including the mode of precipitation, features of the surface runoff, and location of Lake Khuko, were responsible for the accumulation of artificial radionuclides.

Contribution of climate and land cover changes to reduction in soil erosion rates within small cultivated catchments in the eastern part of the Russian Plain during the last 60 years.

The eastern part of the Russian Plain is an important agricultural region of European Russia with high proportion of cultivated lands in the steppe, forest-steppe and forest (southern part) landscape zones. Soil erosion is the main process of land degradation and surface water contamination there. Climate and land cover changes have been observed in this region during the last 30 years. However, field quantitative assessments of soil erosion rates are not available for the eastern part of European Russia due to the lack of monitoring data as well as the evaluation of erosion/deposition processes in cultivated catchments using other field methods. Three representative small cultivated catchments with high (> 80%) proportion of cultivated lands were selected in the forest (southern part), forest-steppe and steppe zones of the study region to evaluate sedimentation rates in dry valley bottoms of the catchments for two-time intervals (1963-1986 and 1987-2016) based on the application of the bomb-derived and Chernobyl-derived 137Cs isotope for sediment dating. The 3-4 depth 137Cs profiles were used to assess the sedimentation rates within the each investigated catchment. It was established that the sedimentation rates have considerably decreased (at least 2-3 times) over the last 30 years compared to 1963-1986 in all the investigated catchments. This is in agreement with results of erosion rate calculations using erosion models for the forest zone, however not consistent with erosion rates assessments for the forest-steppe and steppe zones. According to the model calculations, erosion rates show a slight decrease in the forest-steppe zone and increase in the steppe zone. The reduction in surface runoff during snowmelt period is one of the reasons for decrease in erosion rates within cultivated slopes for all the investigated catchments. The increase in proportion of perennial grasses in the regional crop rotation is another important reason for the decrease in erosion rates for the catchment located in the south of the forest zone. The importance of land cover changes in a major decrease of soil losses from the cultivated fields of the investigated catchments located in the forest-steppe and steppe zones cannot be identified due to the lack detailed information about crop rotation for those particular sites. However, available regional information about crop rotation changes for the two-time intervals (1960-1980 and 1996-2012) do not explain very high reduction in sedimentation rates in the dry valley bottoms after 1986.