Grass, U mining area
With Equation 29.1 and Equation 29.2, the soil-to-plant transfer factor (TF), the percentage yearly reduction in soil activity can be calculated as follows:
TF x Yield
Wsoil where Wsoil is the weight of the contaminated soil layer (kg ha-1). As made clear by Equation 29.3, the annual removal percentage increases with yield and TF. However, TF and yield values are not independent: high yield is often associated with lower TFs because of growth dilution effects.
Phytoextraction requires several years and the future trend in radionuclide concentration in the soil can be calculated according to:
I Wsoil ti/2 J
The second term in the exponent of Equation 29.4 was included to account for radioactive decay (tV is the half-life of the radionuclide). Given the half life of 238U (4.5 109 years), this component will not affect the phytoextraction potential. For 137Cs and 90Sr, with half-lives of 30 years, the phytoextraction potential will be affected (2.33% yearly loss in activity merely through radioactive decay). Equation 29.4 assumes a constant bioavailability of the contaminant, i.e., a constant TF.
For a soil depth of 10 cm and a soil density of 1.5 kg dm-3, soil weight is 1,500,000 kg. Table 29.2 shows the percent of annual removal for different crop yields and TFs. Yields of more than 20 ton ha-1 and TFs higher than 0.1 (Table 29.1) may be regarded as average values or upper limits, except for Sr. This would result in an annual reduction percentage of 0.1% (excluding decay). In the case of a TF of 1, annual reduction is about 1%.
Rearranging Equation 29.4 allows calculation of the number of years needed to attain the required reduction factor as a function of annual removal percentage. Table 29.3 tabulates the years required to attain a reduction of the contaminant concentration up to a factor of 100, given an
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