Técnicas e instrumentos de recolección de datos

The uptake of radionuclides, as well as of other trace elements, by plant roots is a competitive process [3.50]. For radiocaesium and radiostrontium the main competing elements are potassium and calcium, respectively. The major processes influencing radionuclide transport processes within the rooting zone are schematically represented in Fig. 3.21, although the relative importance of each component varies with the radionuclide and soil type.

The fraction of deposited radionuclides taken up by plant roots differs by orders of magnitude, depending primarily on soil type. For radiocaesium and radiostrontium, the radioecological sensitivity of soils can be broadly divided into the categories listed in Table 3.5. For all soils and plant species, the root uptake of plutonium is negligible compared with the direct contamination of leaves via rain splash or resuspension.

Transfer from soil to plants is commonly quantified using either the transfer factor (TF,

0 20 40 60 80 100 0–2 2–4 4–6 6–8 8–10 10–14 14–18 18–22 22–26 26–30 30–34 34–38 38–42 42–46 46–50 50–54 54–60 60–65 65–100

Part of radionuclide activity (%)

Americium-241

Europium-154

Strontium-90

Caesium-137

Depth (cm)

FIG. 3.20. Depth distributions of radionuclides in low humified sandy soil (in per cent of total activity) measured in 1996 [3.47].

Soil organic matrix PLANTS Soil Solution _{[RN] p} [RN] d ROOTS, Mycorhiza [RN] min [RN] mic

Soil mineral matrix

Additional non -radioactive pollutants

[RN] org Plants Soil solution Roots, mycorrhiza Soil microorganisms

Soil mineral matrix

Fertilizers

Soil organic matrix

FIG. 3.21. Radionuclide pathways from soil to plants with consideration of biotic and abiotic processes [3.43].

dimensionless, equal to plant activity concentration, Bq/kg, divided by soil activity concentration, Bq/kg) or the aggregated transfer coefficient (T_ag, m2_/kg,

equal to plant activity concentration, Bq/kg, divided by activity deposition on soil, Bq/m2_).

The highest 137_{Cs uptake by roots from soil to}

plants occurs in peaty, boggy soils, and is one to two orders of magnitude higher than in sandy soils; this uptake often exceeds that of plants grown on fertile agricultural soils by more than three orders of magnitude. The high radiocaesium uptake from peaty soils became important after the Chernobyl accident because in many European countries such soils are vegetated by natural unmanaged grassland used for the grazing of ruminants and the production of hay.

The amount of radiocaesium in agricultural products in the medium to long term depends not only on the density of contamination but also on the soil type, moisture regime, texture, agrochemical properties and plant species. Agricultural activity often reduces the transfer of radionuclides from soil to plants by physical dilution (e.g. ploughing) or by adding competitive elements (e.g. fertilizing). There are also differences in radionuclide uptake between plant species. Although among species variations in uptake may exceed one or more orders of

magnitude for radiocaesium, the impact of differing radioecological sensitivities of soils is often more important in explaining the spatial variation in transfer in agricultural systems.

The influence of other factors that have been reported to influence plant root uptake of radionu- clides (e.g. soil moisture) is less clear or may be explained by the basic mechanisms discussed above; for example, the accumulation of radiocaesium in crops and pastures is related to soil texture. In sandy soils the uptake of radiocaesium by plants is approx- imately twice as high as in loam soils, but this effect is mainly due to the lower concentrations of its main competing element, potassium, in sand.

The main process controlling the root uptake of radiocaesium into plants is the interaction between the soil matrix and solution, which depends primarily on the cation exchange capacity of the soil. For mineral soils this is influenced by the concentrations and types of clay minerals and the concentrations of competitive major cations, especially potassium and ammonium. Examples of these relationships are shown in Fig. 3.22 for both radiocaesium and radiostrontium. The modelling of soil solution physicochemistry, which takes account of these major factors, enables prediction of the root uptake of both radionuclides [3.51, 3.52].

TABLE 3.5. CLASSIFICATION OF RADIOECOLOGICAL SENSITIVITY FOR SOIL–PLANT TRANSFER OF RADIOCAESIUM AND RADIOSTRONTIUM

Sensitivity Characteristic Mechanism Example

Radiocaesium

High Low nutrient content Absence of clay minerals High organic content

Little competition with potassium and ammonium in root uptake

Peat soils

Medium Poor nutrient status, consisting of minerals, including some clays

Limited competition with potassium and ammonium in root uptake

Podzol, other sandy soils

Low High nutrient status

Considerable fraction of clay minerals

Radiocaesium strongly held to soil matrix (clay minerals), strong competition with potassium and ammonium in root uptake

Chernozem, clay and loam soils (used for intensive agriculture)

Radiostrontium

High Low nutrient status Low organic matter content

Limited competition with calcium in root uptake

Podzol sandy soils

Low High nutrient status

Medium to high organic matter content

Strong competition with calcium in root uptake

Umbric gley soils, peaty soils

Thus differences in radioecological sensitiv- ities of soils explain why in some areas of low deposition high concentrations of radiocaesium are found in plants and mushrooms harvested from seminatural ecosystems and, conversely, why areas of high deposition can show only low to moderate concentrations of radiocaesium in plants. This is illustrated in Fig. 3.23, in which the variability in activity concentrations of radiocaesium and radio- strontium in plants is shown for a normalized concentration in soil.