Soil-plant transfer factors

The ranges of solid-liquid distribution coefficients reported for Co in different soil types are rather similar. In general, Kd values from 0.2 to 20,000 L kg l are representative of sand, loam, clay, and organic soils. Sheppard and Thibault (1990) quote median Kd values of 60, 1300, 540, and 990 L kg for each of these soil types, respectively. Soil—plant transfer factors for numerous crop species have been published by the International Union of Radioecologists (lUR, 1989). These values range from 0.037 for cereal grain to 1.1 for alfalfa fodder. For most of the crop types represented in the lUR (1989) database, a range of soil-plant transfer factors from 0.01 to 0.1 seems most applicable. 

Handl et al. (2000) reported measured transfer factors for silver in pasture grass ranging from 0.009 to 0.065. This range is slightly higher than the rather limited collection of soil-plant transfer factors quoted by lUR (1989) of 0.00027 for lettuce, 0.0008 for tomato, and 0.0013 for radish. Ng et al. (1982) quoted a transfer factor value of 0.15 for Ag in unspecified plant material. The committed effective dose per unit intake following ingestion of " "Ag is 2.8 x 10- Sv Bq for adults older than 17 years of age (IAEA, 1996). 

Experiments on the transfer of i c04 from soil to rice and wheat plants were carried out. The soil/plant transfer factors of " Tc for rice plants were < 0.00.3 on dry weight basis for hulled grains and 1.1 for the lower leaf blade. In contrast, much higher transfer factors were found for wheat plants, i. e. 0.027 for the hulled grains and 2.30 for the lower leaf blade. The level of in the soil solution collected from the flooded soil used for rice plants was found to decrease rapidly with time. For wheat plants grown in non-flooded soil, the decrease of the "Tc level in the soil solution was rather slow. Obviously, y.smTcO" was readily transformed under the reducing conditions in the flooded soil to in.soluble forms of Tc, which could explain the low transfer factor for rice plants [64]. 

Iodine Soil—Plant Transfer Factors The most convenient way to assess plant uptake of iodine is to consider the transfer factor (TF), since it inherently takes into account the relationship between soil and plant elemental concentrations. This is determined according to the formula... 

Traditionally the overall process of incorporation of radionuclides into plant tissues from contaminated soils via the root uptake process has been quantified using the soil-to-plant transfer factor, or TF. This is defined simply as the ratio of the specific radionuclide activities in the plant tissue of concern (usually some edible organ) and the soil, with the dry masses of each material usually being taken into account ... 

While the ratios defined in Eqs. (6) and (7) conveniently yield dimensionless factors, the last of these equations yields rather cumbersome units of m2 kg-1. These three transfer factor equations give some idea of the range of ways in which individual radioecologists have tackled the problem of expressing the gross uptake of radionuclides by plants in some easily quantifiable form, but it is Eq. (6) which provides us with the most commonly used definition which has been adopted by the International Union of Radioecologists (IUR) as the working definition for their soil-to-plant transfer factor data base. 

Table 7-13. Supplementary information appended to values of soil-to-plant transfer factors for radionuclides within the IUR data base.

While the actinides Pu, Am, Np and Cm are critically important to assessments of the exposure of humans to environmental sources of radioactivity due to their highly hazardous a emissions, the role which the plant root absorption pathway plays in this exposure is relatively small. A conservative estimate of the degree to which plants will incorporate Pu from soil, for instance, is 10% (ie, a soil-to-plant transfer factor of... 

Fig. 7-12. Concentration ratios (ie, soil-to-plant transfer factors) for plutonium and americium at various sites associated with the US nuclear programme. Key to sites NTS, Nevada test site SR, Savannah River, South Carolina OR, Oak Ridge, Tennessee RF, Rocky Flats, Colorado NTS, Enewetak Atoll, South Pacific (from Dahlman et al., 1976).

IUR (1982). Report on a Workshop on the Measurement of Soil to Plant Transfer Factors for Radionuclides. Part /, Wageningen International Union of Radioecologists, EURATOM-ITAL. 

Tome, F. V., Rodriguez, M. P. B., and Lozano, J. K. (2003). Soil-to-plant transfer factors for natural radionuclides and stable elements in a Mediterranean area. J. Environ. Radioact. 65, 161-175. 

Element uptake from soil and transfer into the edible parts of plants have been addressed in several other studies. Soil-to-plant transfer factors in fruit and vegetables grown in various agricultural conditions have been determined for, for example, Pt [100], T1 [101], and various other metal contaminants [102], In a study on stable isotopes of fission product elements (Ce, Cs, Sr), an in vitro enzy-molysis method has been applied to investigate the solubilization of the analytes from fodder in a simulated ruminant digestion [103], The effect of inhibitors of fission product solubility was also considered and essential elements were determined simultaneously to evaluate potential nutrition problems for the animals from the use of such inhibitors. Selective leaching of individual classes of metal complexes with different ligands and sequential enzymolysis have been recently applied to estimate the potential bioavailability to humans of Cd and Pb in cocoa powder and related products [104]. 

In ecosystems, Pu is present mainly in the form of sparingly soluble Pu(IV) dioxide or hydroxide and is therefore rather immobile. It stays mainly in the upper layers of the soil and its uptake by roots is very small (soil-plant transfer coefficients <0.001 d/kg). However, plants may be contaminated with Pu by deposition from the air. Resorption factors in the gastrointestinal tract of animals are also very small (/r < 10 " ). On the other hand, up to 5% of inhaled Pu is found in blood and up to 15% in the lymph glands. About 80% of resorbed Pu is deposited in bones, the rest in kidneys and liver. Biological half-lives reported in the literature vary between 500 and 1000 d for the lymph glands and between 1 and lOO y for the skeleton. 

Nisbet, A. F., and Woodman, R. F. M. (2000). Soil-to-plant transfer factors for radiocesium and radiostrontium in agricultural systems. Health Phys. 78, 279-288. 

Shaw, G., and Bell, J. N. B. (1989). The kinetics of cesium absorption by roots of winter wheat and the possible consequences for the derivation of soil-to-plant transfer factors for radiocesiutn../. Environ. Radioact. 10, 213-232. 

Twining, J. R., Payne, T. E., and Itakura, T. (2004). Soil-water distribution coefficients and plant transfer factors for Sr and Zn under field conditions in tropical... 

The variability of radiocesium concentration in undisturbed environments such as alpine pastures is very high and a high minimum of soil and plant samples would be required to estimate the radiocesium distribution median with a 95% confidence interval and a tolerable error of at least 20%. Low cost sampling with only a few samples would result in a biased estimate of the real radionuclide contamination. There is some evidence that the high variability is caused more by microscale runoff phenomenon directly after the Chernobyl accident rather than by long-term erosion processes. The high spatial variability of the Cs, the random distribution and the concentration in the first centimeter of the soil possibly causes the missing correlation of the radiocesium activity concentrations in soils and plants and therefore does not allow a soil-to-plant transfer factor to be calculated. 

Another relevant general review summarizes the knowledge on the behavior of series radionuclides in soils and plants and is intended to provide a comprehensive source of information for environmental impact studies (Mitchell et al. 2013). The summary of the data on plant to soil concentration ratios that depends on the specific soil and type of plant and the distribution of uranium within the parts of the plant is especially important. The dependence of the sorption of dissolved uranium compounds on the type of soil (like the clay content) and the parameters mentioned earlier (pH, complex forming agents, anions, presence of iron, organic matter, etc.), based mainly on studies of the (distribution factor) of spiked soil samples, is discussed. It is noted that in general the uranium concentration in plants is several orders of magnitude lower than in soil, but some plants may efficiently absorb uranium and translocation within the plant is quite common (Mitchell et al. 2013). These features, and especially the soil-to-plant transfer factors, will be discussed in Section 3.4 that deals with the uranium content in plants and soil and the relation between them. 

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