Soil-to-plant transfer

When implementing measures for mitigating the accident consequences and for rehabilitating the contaminated territories, it is important to take into account the availability of radionuclides to plants and their interaction with soil constituents. These factors determine the transfer rate of radionuclides into food chains [15].

The available fraction of radionuclides in the soil for root uptake is determined by two parameters: the radionuclide content in exchangeable form and in mobile form on the one hand, and the value of the soil-to-plant transfer coefficient of ¹³⁷Cs on the other. These parameters make it possible to correlate the radionuclide concentration in plants with the soil contamination per unit area. As a result of alteration in its non-exchangeable state, the quantity of ¹³⁷Cs desorbed from samples of the soil into 1M KCl solution decreased by a factor of three between 1987 and 1992. The availability of ¹³⁷Cs for plants is not high: in 1994, it was responsible for 5–29% of the total ¹³⁷Cs activity in the soil [23]. The availability of ⁹⁰Sr for plants is characterized by the prevalence of easily mobile forms (exchangeable and water-soluble substances) which account for 50–90% of the total ⁹⁰Sr content. From 1996, the plant uptake of ⁹⁰Sr continued to increase [20, 23, 24].

On the territory of the Russian Federation, the “condensation-generated” type of fallout predominated. In 1986, the content of ¹³⁷Cs exchangeable forms in the soil was higher than in

1986, the current content of ¹³⁷Cs exchangeable forms is 3–5 times lower than immediately after the accident, and the content of its mobile forms (i.e. that displaced by 1N HCl) 1–4 times lower. The dynamics of ¹³⁷Cs forms depends on soil properties. The sorption of this radionuclide has a higher rate in automorphic soils normally moisturized by atmospheric condensation: the content of ¹³⁷Cs exchangeable forms in automorphic soils was 1–10 times lower than in constantly overmoisturized hydromorphic soils [14]. Generally, the content of

137Cs acid-soluble forms is higher in hydromorphic soils [16].

During the first post-accident year, the ¹³⁷Cs content in plants was determined by aerial contamination and reached its maximum value. During the second post-accident year (1987), the ¹³⁷Cs content in the plants dropped by a factor of 3–6 as a result of the prevalence of root uptake over other routes of ¹³⁷Cs uptake by plants. Since 1987, the transfer rate of ¹³⁷Cs to plants has continued to decrease, although the rate of decrease has slowed: within the past eight years, the transfer coefficients of ¹³⁷Cs decreased on average by a factor of 1.5–7.0.

Beginning in 1989, the soil-to-plant transfer coefficients of ¹³⁷Cs showed a tendency to stabilize. The differences in radionuclide accumulation in the plants were mainly determined by different soil properties. The accumulation in herbiage of ¹³⁷Cs from soddy-podzolic and sod soils was 1.2–3.2 times lower than from the soddy-gley and soddy-gley peat ones (Gley is water-logged grey clay soil). The maximum values of the transfer coefficient for ¹³⁷Cs were found in peat-boggy soils. These values were 1.5–6.0 times higher than in automorphic soils [24, 25].

In order to predict future radioecological conditions, it is necessary to determine the duration of the period after which a relative stabilization may be expected in the soil-to-plant transfer of ¹³⁷Cs. Both for automorphic and hydromorphic soils, two periods may be distinguished, 1986–1989 and 1989–1994, when the change in the concentration of ¹³⁷Cs in exchangeable and mobile forms proceeded with different intensities. Despite the significant variability of experimental data, the integral parameters characteristic for the ¹³⁷Cs sorption in the soil during these periods could be approximately evaluated [19].

The first effective clearance half-life (1986–1989) related to exchangeable ¹³⁷Cs was approximately three years; the second one (1989–1994) approximately 12 years. These estimates are based upon the following observations [26]:

(a) For natural herbage of dry meadows in areas of soluble aerosol fallout, the first clearance half-life (1986–1989) of exchangeable ¹³⁷Cs was determined to be approximately two years, and the second one (1989–1994) approximately 4–12 years; and

(b) The effective clearance half-life also depends on the type of meadow and the soil properties. For dry meadows on soils with heavy physical composition and for lowland meadows on peat soils, the first clearance half-life (1986–1989) of the exchangeable ¹³⁷Cs was shorter and the second one (1989–1994) two to three times longer than for dry meadows on soils with lighter physical composition.

The higher caesium uptake from peaty soil is important because such soils underlie natural unmanaged grassland used for cattle grazing and hay production. The radical improvement of such lands has been a major factor in the reduction of radiocaesium concentration in milk [25, 26].

The level of radioactive contamination of agricultural products also depends on soil properties. The transfer coefficients of radionuclides into plants growing on soddy-podzolic loamy soils are one to three times lower than the same coefficients for soddy-podzolic sandstone soils. The agrochemical properties of soils, which determine their level of fertility, also have an important influence on radionuclide accumulation in agricultural crops. For instance, an increase in the content of humus in the soddy-podzolic soils of Belarus from 1% to 3% has resulted in a decrease in the level of ¹³⁷Cs contamination of perennial grasses by a factor of 1.7, and of ⁹⁰Sr contamination by a factor of 1.9 (Fig. 9). The use of organic fertilizers has increased the agricultural crop harvest and slightly reduced the soil-to-plant transfer of radionuclides (by 15–20%) [20, 25].

Fig. 9. Influence of soil characteristics on perennial grass radionuclide uptake from soddy-podzolic loamy sand soil related to a deposition of 370 kBq/m^{2 137}Cs and of 37 kBq/m^{2 90}Sr.

By increasing the content of exchangeable potassium in the soil to optimal values, (200–

300 mg of K₂O per kg for sandy and loamy soddy-podzolic soils), a drop in the ¹³⁷Cs transfer into perennial grasses by a factor of three can be achieved, and in the ⁹⁰Sr transfer, by a factor of up to 1.5–1.8 (Fig. 9). The increased use of potassium fertilizers on soils with low and medium content of exchangeable potassium leads to a drop in the ¹³⁷Cs transfer from the soil into agricultural crops by a factor of 1.3–2 [27]. The influence of phosphorous fertilizers is lower, but they also reduce radionuclide contamination of plants. The traditional approach of soil liming makes it possible to reduce the acidity of soddy-podzolic soils from pH5.0 to pH6.0–6.5, and to reduce the ¹³⁷Cs and ⁹⁰Sr contamination in crops by a factor of two (Fig. 9).

In the case of highly-acidic soils (pH4.0–4.5), liming may reduce radionuclide transfer by a factor of up to three. The soil humidity has an even more important influence on these processes. The ¹³⁷Cs contamination of perennial grasses on soddy-podzolic and sod automorphic soils with a normal degree of humidity is some 10–27 times lower than on gley soils with a permanently increased degree of humidity. The transfer coefficients for ⁹⁰Sr are also up to two times lower in normally humid soils than in excessively humid ones, although the difference is less significant [20, 27].

Therefore, the improvement of soddy-podzolic soils through the complex use of organic fertilizers, liming and high doses of potassium and phosphorous fertilizers makes it possible to reduce the ¹³⁷Cs contamination of agricultural crops by a factor of 4–6 and, when accompanied by irrigation control, by a factor of up to 10.

The results obtained make it possible to conclude that, five years after the Chernobyl accident, the ¹³⁷Cs soil-to-plant transfer had become stable, and marked the completion of the period of quasi-non-equilibrium increase in the stability of ¹³⁷Cs sorption in the soil solid phase.

The process of radionuclide trapping in the soil slows down with time. The main mechanism of durable radionuclide trapping in the soil is its inclusion into the crystal lattice of clay minerals.

The subsequent decrease in the ¹³⁷Cs transfer into plants will occur at a slower rate [22].