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a TF: Bq kg-1 dry mass plant per Bq kg-1 dry mass soil.

b 95% confidence intervals of the mean (i.e., geometric mean ± tn standard errors) (tn ~2 if n > 10).

a TF: Bq kg-1 dry mass plant per Bq kg-1 dry mass soil.

b 95% confidence intervals of the mean (i.e., geometric mean ± tn standard errors) (tn ~2 if n > 10).

Source: IUR, Sixth report of the IUR working group soil-to-plant transfer factors: report of the working group meeting in Guttannen, Switzerland, 198924-26, Bilthoven, RIVM: 240, 1989.

[32] that mobility of Th in soil may be less affected by soil pH than by soil organic matter. Tetravalent thorium may be strongly complexed with soil organic matter, thus increasing the mobility of Th in soil.

29.2.2.3.2 Phytoextraction Potential of Th

Information on Th-TF is rather scant. Shtangeeva et al. [33] report TFs for Couch grass ranging from ~0.01 to 0.05 and for plantain from 0.008 to 0.03 g g-1 — the higher values for the shoots and the lower values for the roots. For garden crops cultivated on high background areas, Linsalata et al. [34] found values ranging from 10-5 to 10-3 g g-1. Whicker et al. [35] measured values ranging from 2 x 10-5 to 2 x 10-3 g g-1 for crops grown on a contaminated lake bed. Only Vera Tome et al. [36] reported rather high Th-TF, ranging from 0.013 to 0.270 g g-1for grass pasture samples collected in granitic and alluvial soils around a disused uranium mine.

As was said for radium, no information was found by the author on studies dedicated to the phytoextraction of thorium or methods to promote radium availability in the soil-plant environment. Because the highest TF for thorium observed is around 0.1 g g-1 (yet most TFs observed are a factor of 100 to 10,000 lower), the maximal amount annually extracted by plants is of the order of 0.1%.

29.2.2.4 Caesium

29.2.2.4.1 Behavior of Cs in Soil and Soil Factors Affecting Cs Availability After the Chernobyl accident, the study of the fate of radiocaesium in the environment was of particular importance, given its relatively long half-life, its widespread contamination, and its similarity with K favoring its uptake by plants [37-40].

The soil K status affects Cs availability: in general, the higher the soil K is, the lower is the TF. This K effect is explained by the increased solution concentration of a cation, which competes with radiocaesium for uptake. The consensus now is that the solid/liquid partitioning of radiocae-sium in the soils, and thus its soil bioavailability, is governed by the presence of micaceous, potassium-bearing clay minerals [41-43]. The process relates to the action of a small number of

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