5. ROOT UPTAKE OF RADIONUCLIDES IN AGRICULTURAL
5.5. Time dependence of radionuclide transfer to plants
equilibrium conditions, that is, at a time long enough after deposition for equilibrium conditions to be approached. However, radionuclides can also be transferred to plants from soil in the year of deposition. Such a situation can be of importance, particularly in the case of acute deposition during the plant growth period. Many models allow the contamination of plants to be calculated for this
TABLE 22. TRANSFER FACTORS (Fv) FROM SOIL TO RICE
Element Soil type N Mean/value GSD/SDa Minimum Maximum
Co All 5 5.1×10–3 1.7 2.2×10–3 1.0×10–2
Cs All 466 8.3×10–3 6.2 1.3×10–4 6.1×10–1
Sand 7 5.9×10–2 3.5 7.1×10–3 1.7×10–1
Loam 24 7.5×10–3 4.1 1.1×10–3 2.8×10–1
Clay 23 2.2×10–2 5.7 1.1×10–3 1.5×10–1
I All 8 3.8×10–3 2.1 1.1×10–3 7.6×10–3
Mn All 5 2.6×10–1 1.7 1.2×10–1 5.2×10–1
Sand 1 2.3×10–1
Loam 4 2.6×10–1 1.9 1.2×10–1 5.2×10–1
Pb All 2 8.4×10–3 4.7×10–3 1.2×10–2
Po All 2 1.3×10–2 9.4×10–3 1.7×10–2
Ra All 40 8.7×10–4 3.1 2.2×10–4 2.8×10–2
Loam 14 7.8×10–4 2.4 2.7×10–4 8.8×10–3
Clay 18 5.7×10–4 1.7 2.5×10–4 2.9×10–3
Sr All 71 2.3×10–2 4.7 2.1×10–3 6.0×100
Sand 6 6.0×10–2 2.6 1.2×10–2 2.2×10–1
Loam 4 9.5×10–2 8.1 5.5×10–3 8.3×10–1
Clay 14 3.2×10–2 3.0 2.1×10–3 1.1×10–1
Tc All 2 <2×10–4
Th All 57 1.6×10–4 3.3 2.2×10–5 3.0×10–2
Loam 22 1.5×10–4 3.1 2.2×10–5 4.0×10–3
Clay 31 1.4×10–4 2.5 2.6×10–5 8.3×10–4
Organic 1 3.0×10–2
U All 65 2.43×10–4 5.98 8.56×10–6 9.00×10–2
Sand 3 5.38×10–3 2.58 1.93×10–3 1.26×10–2
Loam 23 2.07×10–4 6.73 8.56×10–6 2.42×10–2
Clay 29 1.79×10–4 3.57 2.31×10–5 1.80×10–3
Organic 1 9.00×10–2
Zn All 5 1.5×100 1.96 5.80×10–1 2.70×100
Sand 1 2.3×100
Loam 3 1.5×100 2.28 5.80×10–1 2.70×100
a GSD/SD: Geometric standard deviation/standard deviation.
scenario with some uncertainty. Radionuclides acutely deposited during the plant growth period are localized within the soil surface until harvest, because farmland is not ploughed during this period. In such cases the use of Fv values is inappropriate, and it is necessary to use aggregated transfer factors (Tag) specified for the time from deposition until harvest [110–113]. The Tag values for such time dependent scenarios for selected radionuclides (T, Co, Mn, Sr, Cs) are given in the accompanying TECDOC [5].
The rate of decrease of radionuclide uptake by plants is irregular by nature, and several time periods should be considered when applying a half-life approach for data evaluation. In the first years after deposition, the bioavailability of some radionuclides in soil reaches its maximum, resulting in the maximum TABLE 23. TRANSFER FACTORS (Fv) OF STABLE ELEMENT TRANSFER FROM SOIL TO RICE
Element Soil type N Mean/value GSDa Minimum Maximum
Ba All 87 9.4×10–4 2.8 8.5×10–5 7.8×10–3
Ca All 87 6.4×10–3 2.2 1.3×10–3 4.6×10–2
Cd All 87 9.3×10–2 3.2 9.0×10–3 1.2×100
Ce All 60 3.3×10–5 2.7 1.9×10–5 5.7×10–4
Co All 86 6.8×10–4 2.1 1.3×10–4 6.4×10–3
Cr All 87 1.8×10–3 3.5 1.1×10–4 1.9×10–2
Cs All 83 7.3×10–4 2.7 1.1×10–4 1.6×10–2
Loam 26 1.0×10–3 3.7 1.1×10–4 1.6×10–2
Clay 36 6.7×10–4 2.2 1.4×10–4 3.4×10–3
Fe All 87 1.8×10–4 2.2 3.8×10–5 1.3×10–3
I All 40 2.7×10–3 3.2 1.3×10–4 2.0×10–2
K All 87 1.3×10–1 2.3 1.8×10–2 7.8×10–1
La All 79 4.2×10–5 2.2 4.6×10–6 1.4×10–3
Mn All 87 2.9×10–2 2.1 5.4×10–3 1.2×10–1
Na All 87 8.4×10–4 2.0 2.0×10–4 6.9×10–3
Ni All 87 1.4×10–2 2.2 3.0×10–3 8.7×10–2
P All 50 2.4×100 1.8 3.7×10–1 9.4×100
Pb All 63 2.9×10–4 2.6 3.6×10–5 5.9×10–3
Loam 26 2.4×10–4 2.1 7.2×10–5 1.1×10–3
Clay 35 3.0×10–4 2.6 3.6×10–5 3.5×10–3
Rb All 87 8.6×10–2 3.1 7.3×10–3 2.2×100
Se All 67 6.1×10–2 1.9 9.0×10–3 2.9×10–1
Sr All 63 1.9×10–3 2.2 3.8×10–4 8.2×10–3
Loam 26 1.6×10–3 2.1 4.1×10–4 6.4×10–3
Clay 34 2.1×10–3 2.3 3.8×10–4 8.2×10–3
Zn All 87 2.2×10–1 1.6 6.1×10–2 6.8×10–1
a GSD: Geometric standard deviation.
radionuclide transfer rate to plants. From the data it can be concluded that the ecological half-live of 137Cs in plants is in the range of 1–2 years in the first years after deposition, increasing to 12–20 years in the long term. The ecological half-live of 90Sr tends to be slightly longer, at an estimated 20–30 years.
Unfortunately, data are rather scarce in the literature, even for 90Sr and 137Cs, and any such estimates should be interpreted with caution.
Evaluation of radionuclide transfer in the environment implies consideration of the decrease of radionuclide activity concentrations in plants over time after a single release of radionuclides to the environment. This decrease occurs because radionuclides transferred to the environment are gradually fixed by natural sorbents (soils, bottom sediments in water ecosystems, etc.) and are lixiviated to lower soil layers, becoming less biologically available for inclusion in food chains.
Time dependent behaviour of radionuclides is often quantified by reference to the ecological half-life, which is an integral parameter relating to the reduction of activity or activity concentration in a specific medium. According to the definition given in Section 2, the ecological half-life is equal to the period of time over which the concentration of a radionuclide, in a given component of a trophic chain, is decreased by a factor of two, excluding the effects of radioactive decay.
Although field data on variations in radionuclide transfer factors over time after clearly defined depositions are rather scarce, there are three prime sources of information on radionuclide half-lives in plants: global fallout, and the Kyshtym and Chernobyl accidents [87, 89, 91, 94, 95, 114–116].
5.6. APPLICATION OF DATA
Assessment of Fv values based on sources in the literature is always associated with various constraints, and often considerable judgement must be exercised in evaluating the available data. First, some data are based on studies that were not originally intended for transfer factor assessments. Second, the experimental design of the research may deviate from the design required by the transfer factor definition. For example, vertical distribution of radionuclides in soil profiles can depart from the uniform distribution assumed by the transfer factor definition. Radionuclide transfer to plants depends on numerous factors including the physical and chemical forms of the radionuclide in the soil, soil properties, plant species, plant compartments and farming practices. Such factors result in high variability, and the individual Fv values themselves can vary by over five orders of magnitude [5].
To decrease the uncertainty associated with soil and/or plant factors, several classifications were developed, and soil to plant transfer factors were reviewed
and grouped according to the selected plant and soil categories. These data, providing information for specific plant and soil groups, allow more precise radiological assessments in different areas around the world. However, even for temperate environments, clear gaps in transfer factor values remain for a substantial number of radionuclides, plants and soil groups.
Far fewer data are available for tropical and subtropical ecosystems than for temperate environments. Additional uncertainty in the application of the tropical and subtropical data provided in this handbook can be assigned with the use of different climate classification schemes. Fv values are highly variable, which constitutes a strong limitation for their application. The concentration of a radionuclide in a soil is not the only factor influencing its uptake by plants. Mean values reported in this section should be used only as an indication of the tendency of the radionuclide transfer from soil to plants.
Soil and plant classifications facilitate application of the recommended Fv values for radiological assessments and increase the robustness of such estimates.
However, site specific information on soils, plants and climatic conditions should be considered when using the Fv values from the tables given in this section.
Transfer factors are not appropriate for natural and semi-natural ecosystems because of the layered structure of those soils and the highly inhomogeneous distribution of root systems. Therefore, as an alternative, aggregated transfer factors (Tag) are used to quantify radionuclide availability to various types of natural or semi-natural vegetation and animal products. Tag is defined as the ratio ofthe radionuclide activity concentration in the plant (Bq kg–1 fresh weight or Bq kg–1 dry weight) or any other food product, divided by the total deposition on the soil (Bq m–2). The concept of Tag is adopted as a reasonable empirical measure to normalize radionuclide accumulation in semi-natural products, regardless of variations in the vertical radionuclide distribution and availability in the soil profile, which greatly depend on the site.