N. N. Ivanova

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.

CAESIUM-137 BEHAVIOUR IN SMALL AGRICULTURAL CATCHMENTS ON THE AREA OF THE CHERNOBYL CONTAMINATION

The vertical distribution of137Cs from the Chernobyl accident in soils of the basin of the Gusinaya Lapka dry river, situated in the Eastern region, 550 km from the reactor, has been investigated. In this area the137Cs contamination levels range between 250 and 502 kBq m−2. In the accumulation zones, at the bottom of the valley, the sediment deposits over the last ten years represent a layer of not more than 10 cm. On arable lands, the erosion processes are limited and no perceptible redistribution of137Cs could be observed, except at the transition between ploughed lands on the inter-rivers and meadows on the balka slopes. This study demonstrates the potential use of the137Cs from the Chernobyl accident for the quantification of the soil erosion process.

Assessment of the trend of degradation of arable soils on the basis of data on the rate of stratozem development obtained with the use of 137 Cs as a сhronomarker

A new approach for determining the trend of changes in the rate of degradation of arable soils is suggested. It is based on the assessment of volumes of soil material eroded from arable fields and accumulated on the bottoms of first-order valleys during two time intervals: 1954(1963)−1986 and 1986−2015. For dating of this material, 137Cs of global fallout and Chernobyl fallout are used. This approach in combination with a detailed morphometric characterization of the valley bottoms, the pathways of sediment transport from the fields, and the morphology and composition of the sediments accumulated on the bottoms makes it possible to give reliable estimates of the volumes of soil loss from tilled slopes. The benchmarks of 1963 and 1986 are related to maximum 137Cs fallout during nuclear bomb testing and immediately after the Chernobyl accident. As an example, the rates of formation of stratozems (stratified aggraded soils formed due to accumulation of eroded sediments) within the first-order catchment of the Veduga River basin (Voronezh oblast, Russia) are analyzed. The results of the study indicate that the mean annual rate of soil loss from arable fields of the catchment in 1986–2015 was at least two times lower than that in the preceding period from 1954 (the beginning of the global fallout) to 1986 (the Chernobyl accident).

Comparative assessment of the contents of magnetic spherules, 137Cs, and 210Pb in soils as applied for the estimation of soil erosion

The contents of magnetic spherules and 137Cs and 210Pb isotopes were determined in gray forest soils of the Novosil Agroforest Experimental Station in Orel oblast (central part of European Russia). The spatial variability in the contents of these substances was studied, and their distribution in the soil profiles and along soil transects within the afforested and cultivated slopes was analyzed. Factors and processes favoring the accumulation of magnetic spherules and 137Cs and 210Pb isotopes in the soils, as well as their removal from the soils, were revealed. Similarities and distinctions in the patterns of their behavior were interpreted. Prospects for the combined use of these three tracers to estimate the rates of soil erosion and sediment deposition on the soil surface were tested.

Assessing the accumulation of sorbed isotope 137Cs within the upper components of the fluvial network in the zone of Chernobyl contamination

We examine the use of two alternative techniques for assessing the redistribution volumes of sorbed 137Cs within the upper components of the fluvial network, based on the concept of catchment as a lithodynamical system. In terms of one of them, we made a substantive analysis of changes in reserves on accumulative positions where the 137Cs distribution curves showed a significant increase in radionuclide content levels. We carried out a typization o the accumulation surfaces in order to extrapolate data, obtained for a group of soil profiles, to the territory of the entire catchment and to make a direct assessment of the increases in accumulation. An alternative technique was used to assess the volumes of accumulated sediment loads on accumulative positions by analyzing the 137Cs distribution curves, and to correlate with denudation zones in the drainage area under consideration. Values of the wash-out rates for the period 1986–2012 have been obtained, which are necessary for the formation of the accumulative layer observed. We calculated the volume of 137Cs swept away during 26 years after the Chernobyl accident, based on information regarding the volumes of initial reserves in the denudation zones and a correlation between the erosion rate and a specific reduction in reserves. It is established that the technique on the basis of analyzing the wash-out zones featuring high economical efficiency can be used in assessing 137Cs migration for larger territories with relatively low labor-consuming effort.