Using reservoir sediment deposits to determine the longer-term fate of chernobyl-derived 137Cs fallout in the fluvial system.

Vast areas of Europe were contaminated by the fallout of 137Cs and other radionuclides, as a result of the Chernobyl accident in 1986. The post-fallout redistribution of Chernobyl-derived 137Cs was associated with erosion and sediment transport processes within the fluvial system. Bottom sediments from lakes and reservoirs can provide a valuable source of information regarding the post-fallout redistribution and fate of 137Cs released by the Chernobyl accident. A detailed investigation of sediment-associated 137Cs in the bottom sediments of a reservoir in a Chernobyl-affected area in Central Russia has been undertaken. A new approach, based on the vertical distribution of 137Cs activity concentrations in the reservoir bottom sediment makes it possible to separate the initially deposited bottom sediment, where the 137Cs activity reflects the direct fallout of Chernobyl-derived 137Cs to the reservoir surface and its subsequent incorporation into sediment deposited immediately after the accident, from the sediment mobilized from the catchment deposited subsequently. The deposits representing direct fallout from the atmosphere was termed the "Chernobyl peak". Its shape can be described by a diffusion equation and it can be distinguished from the remaining catchment-derived 137Cs associated with sediment accumulated with sediments during the post-Chernobyl period. The 137Cs depth distribution above the "Chernobyl peak" was used to provide a record of changes in the concentration of sediment-associated 137Cs transported from the upstream catchment during the post-Chernobyl period. It was found that the 137Cs activity concentration in the sediment deposited in the reservoir progressively decreased during the 30-year period after the accident due to a reduction in the contribution of sediment eroded from the arable land in the catchment. This reflects a reduction in both the area of cultivated land area and the reduced incidence of surface runoff from the slopes during spring snowmelt due to climate warming.

Natural attenuation of Fukushima-derived radiocesium in soils due to its vertical and lateral migration.

Processes of vertical and lateral migration lead to gradual reduction in contamination of catchment soil, particularly its top layer. The reduction can be considered as natural attenuation. This, in turn, results in a gradual decrease of radiocesium activity concentrations in the surface runoff and river water, in both dissolved and particulate forms. The purpose of this research is to study the dynamics of Fukushima-derived radiocesium in undisturbed soils and floodplain deposits exposed to erosion and sedimentation during floods. Combined observations of radiocesium vertical distribution in soil and sediment deposition on artificial lawn-grass mats on the Niida River floodplain allowed us to estimate both annual mean sediment accumulation rates and maximum sedimentation rates corresponding to an extreme flood event during Tropical Storm Etau, 6-11 September 2015. Dose rates were reduced considerably for floodplain sections with high sedimentation because the top soil layer with high radionuclide contamination was eroded and/or buried under cleaner fresh sediments produced mostly due to bank erosion and sediments movements. Rate constants of natural attenuation on the sites of the Takase River and floodplain of Niida River was found to be in range 0.2-0.4 year-1. For the site in the lower reach of the Niida River, collimated shield dose readings from soil surfaces slightly increased during the period of observation from February to July 2016. Generally, due to more precipitation, steeper slopes, higher temperatures and increased biological activities in soils, self-purification of radioactive contamination in Fukushima associated with vertical and lateral radionuclide migration is faster than in Chernobyl. In many cases, monitored natural attenuation along with appropriate restrictions seems to be optimal option for water remediation in Fukushima contaminated areas.

Behavior of accidentally released radiocesium in soil-water environment: Looking at Fukushima from a Chernobyl perspective.

Quantitative characteristics of dissolved and particulate radiocesium wash-off from contaminated watersheds after the FDNPP accident are calculated based on published monitoring data. Comparative analysis is provided for radiocesium wash-off parameters and distribution coefficients, Kd, between suspended matter and water in rivers and surface runoff on Fukushima and Chernobyl contaminated areas for the first years after the accidents. It was found that radiocesium distribution coefficient in Fukushima rivers is essentially higher (1-2 orders of magnitude) than corresponding values for rivers and surface runoff within the Chernobyl zone. This can be associated with two factors: first, the high fraction of clays in the predominant soils and sediments of the Fukushima area and accordingly a higher value of the radiocesium Interception Potential, RIP, in general, and secondly the presence of water insoluble glassy particles containing radiocesium in the accidental fallout at Fukushima. It was found also that normalized dissolved wash-off coefficients for Fukushima catchments are 1-2 orders of magnitude lower than corresponding values for the Chernobyl zone. Normalized particulate wash-off coefficients are comparable for Fukushima and Chernobyl. Results of the investigation of radiocesium's ((134)Cs and (137)Cs) vertical distribution in soils of the close-in area of the Fukushima Dai-ichi NPP - Okuma town and floodplain of the Niida river are presented. The radiocesium migration in undisturbed forest and grassland soils at Fukushima contaminated area has been shown to be faster as compared to the Chernobyl 30-km zone during the first three years after the accidents. This may be associated with higher annual precipitation (by about 2.5 times) in Fukushima as compared to the Chernobyl zone, as well as the differences in the soil characteristics and temperature regime throughout a year. Investigation and analysis of Fukushima's radiocesium distribution in soils of Niida river catchment revealed accumulation zones of contaminated sediments on its floodplain. Average sediment deposition rates varied from 0.3 to 3.3 cm/year.

Spatial and Temporal Reconstruction of Chernobyl (137)Cs Initial Fallout Field on Soil Within upper Lokna River Basin.

The study area is located within the upper Lokna River basin with a catchment area of about 35 km(2). The schematic map of (137)Cs initial fallout after the Chernobyl disaster in 1986 on soil was drawn. The method of selecting reference sites and soil sampling scheme are given in detail-for statistically correct description of radionuclide initial fallout field. 12 soil samples were selected from each of the six reference sites to characterize the average amount of radionuclide in the upper 30 cm of the soil profile. Additionally, some single sampling points were used on erosion-stable areas adjacent or located within the catchment, as well as the "truncated" samples of the radionuclide inventory from the catchment bottom. The cesium soil contamination in 1986 immediately after the Chernobyl accident was restored taking into account the half-life of (137)Cs. The schematic map adequately correlated with the aerial photography data performed by Hydromet in 1986.

Fallout radionuclide-based techniques for assessing the impact of soil conservation measures on erosion control and soil quality: an overview of the main lessons learnt under an FAO/IAEA Coordinated Research Project.

This paper summarizes key findings and identifies the main lessons learnt from a 5-year (2002-2008) coordinated research project (CRP) on "Assessing the effectiveness of soil conservation measures for sustainable watershed management and crop production using fallout radionuclides" (D1.50.08), organized and funded by the International Atomic Energy Agency through the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture. The project brought together nineteen participants, from Australia, Austria, Brazil, Canada, Chile, China, Japan, Morocco, Pakistan, Poland, Romania, Russian Federation, Turkey, United Kingdom, United States of America and Vietnam, involved in the use of nuclear techniques and, more particularly, fallout radionuclides (FRN) to assess the relative impacts of different soil conservation measures on soil erosion and land productivity. The overall objective of the CRP was to develop improved land use and management strategies for sustainable watershed management through effective soil erosion control practices, by the use of ¹³7Cs (half-life of 30.2 years), ²¹°Pb(ex) (half-life of 22.3 years) and 7Be (half-life of 53.4 days) for measuring soil erosion over several spatial and temporal scales. The environmental conditions under which the different research teams applied the tools based on the use of fallout radionuclides varied considerably--a variety of climates, soils, topographies and land uses. Nevertheless, the achievements of the CRP, as reflected in this overview paper, demonstrate that fallout radionuclide-based techniques are powerful tools to assess soil erosion/deposition at several spatial and temporal scales in a wide range of environments, and offer potential to monitor soil quality. The success of the CRP has stimulated an interest in many IAEA Member States in the use of these methodologies to identify factors and practices that can enhance sustainable agriculture and minimize land degradation.

The spatial variability of Chernobyl-derived 137Cs inventories in a small agricultural drainage basin in central Russia.

Little information currently exists regarding the small-scale spatial variability of Chernobyl radiocaesium fallout and associated inventories. This contribution reports the results of a study of the variability of 137Cs inventories within the 2.18-km2 Lapki balka catchment located near Tula in central Russia. The local area was characterized by 137Cs inventories in excess of 200 kBq m-2 immediately after the Chernobyl accident and pre-existing bomb-derived inventories can be ignored in view of their very low magnitude. Field sampling and measurements included both collection of soil cores for subsequent laboratory analysis and in situ field measurements using a CORAD portable detector. The results obtained show evidence of a systematic south-north increase in the reference inventory across the basin, which must be taken into account when interpreting subsequent radiocaesium redistribution within the basin. Random spatial variability of 137Cs inventories of a similar magnitude to that reported for bomb-derived fallout was also documented. The extent of random spatial variability varied between different geomorphological units. Maximum variability, with coefficients of variation up to 20%, was associated with areas of sediment accumulation within the balka bottoms. Substantial variability (cv. typically ca. 15%) was found within flat cultivated areas and undisturbed areas both on the interfluves and on the balka sides, all of which could serve as reference sites. Minimum variability (cv. typically ca. 12%) was associated with the cultivated slopes with no evidence of sediment accumulation.