137Cs behavior in the system soil – plant in the stationary sampling sites located within the 30-kilometer zone of the Chernobyl NPP in the period 1987-1992: II. Vertical migration in soils and accumulation of 137Cs in natural meadow grasses
Authors: Vyacheslav S. Anisimov, Natalia I. Sanzharova, Rudolph M. Alexakhin
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It was shown that on the territory of 30-km area around the Chernobyl nuclear power plant highlighted 2 zones, which differ in the form of depositions of radionuclides. The first, which was called the "near" zone, dominated by the fuel component; the second, named "remote" zone, dominated by the condensation component. In the "near" to the reactor zone properties of 137Cs were largely determined by the occurrence of the radionuclide within transformed fuel particles and composite reactor’s materials (primarily, graphite) besides of SAC (soil adsorbtion complex). The radiocesium incorpoprated in composition of graphite particles, during the entire period of study (1986-1992) have been gradually leaching out of them, while remaining readily available for plant uptake. At the same time, most of 137Cs located in SAC, due to irreversible fixation by clay minerals of the soil became inaccessible for plants. As a result, during the period of research the mobility of radiocaesium in the system "soil-plant" was higher in "near" to the reactor zone, than in "remote" with a predominance of condensation forms of the radioactive fallout: the transfer factors (TF) of 137Cs in "near" zone, in average, turned out in 1988 to 1.6 in 1989 to 2.4 in 1990 to 3.1, and in 1992 to 4.0 times higher than in "remote" zone.
In lysimetric waters collected at sites with a predominance of the fuel components of the radioactive fallout, there was marked increased 137Cs volume activity. Nevertheless, in the soil of the area located in the "near" zone, the content of the exchange form of the radionuclide was 2.5-7 times less in comparison with other plots, located in the "remote" zone. This may be due to the transition of 137Cs in the lysimetric water by leaching from graphite particles and transformed nuclear fuel. However, in the whole, the concentrations of 137Cs in the lysimetric waters have been negligible, which have lead to the very slight removal of radionuclide from the upper layers of the soils with the help of gravitational current of moisture. So, outside the layer 0-2 cm of soils for both "near" and "remote" zones with the help of gravitational moisture current during the growing seasons in 1988 and in the 1990's, there were removed, respectively, from 1.1×10-2 to 6.0×10-2 % and from 6.0×10-3 to 2.6×10-2 % of the total 137Cs content in the layer.
The main factor influencing the uptake of 137Cs by plants within a 30-km zone around ChNPP, as revealed by the research results, was a deposition form of the radionuclide. Next, in order of decreasing influence are following: type of soil water regime, then follow the agro-chemical and physical properties of soil (in order of decreasing the influence on radiocaesium uptake by plants: pH, the content of mobile forms of phosphorus, manganese, potassium, CEC).
Uptake of 137Cs in natural grass herbage decreases over time exponentially. Periods of half-decreasing of 137Cs accumulation by plants, from 1987 to 1992, ranging between 2.1-7.3 years for automorphous and hydromorphous soils of "near" and "remote" zones.
Experimental data on vertical distribution of 137Cs in the soils of permanent sample sites were processed using two-component quasi-diffusion model of the radionuclide migration in the soil. As a result, the calculated effective half-reducing periods of the radionuclide from the layer 0-5 cm ranged from 18.8 to 27.7 years, from the layer 0-10 cm from 27.3 years to 30.1 years. Environmental half-reducing periods of radiocesium (not taking into account the process of radioactive decay), differed for hydromorphous and associated automorphous soils. For hydromorphous soils, a period for the layer 0-5 cm 172-461, 0-10 cm – 282-758 years. For automorphous soils the relevant periods is 393-763 and 646-1253 years respectively.