Soil-to-plant transfer factors

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]. 

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. 

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. 

The difference in the uranium concentration in some plant species between the control site and the mining area is evident. For example, in clover, rape seed, and chives, the uranium content was three to seven times higher in the mining area, while in onions, tomatoes, apples, and peeled potatoes, the uranium concentration was quite similar in both areas. These results reflect the differences in uranium content in soil as well as the different uptake, or soil-to-plant transfer, factors (Anke etal.2009). 

Figure 2 Soil to plant transfer factors for Swiss chard grown in loam and peat based...

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