165
( ess U) should be found in agricultural fields due to the phosphate fertilizer application (Rothbaum et al. 1979; Jones 1992; Tagami and Uchida 2006; Takeda et al.
2006; Yamaguchi et al. 2009). When Yamaguchi et al. (2009) applied chemical
extraction methods to identify constituents that contributed to U fixation in the studied soil samples, they observed that the physicochemical forms of U initially
included and added were different. According to their results, most of the U from
the phosphate fertilizer was incorporated into the soil organic matter and amorphous
Fe/Al oxy-hydroxides in the soil, which meant the ess U was not readily available to
crops; but it was not clear whether the ess U in soil correlated with the U concentrations in crops or not.
If the concentrations of ess U in agricultural soils were estimated and then the
concentrations between ess U in soil and U in crops were compared, it might be possible to determine whether or not the soil-to-crop transfer of added U, that is ess U,
would affect the U concentrations in crops. If ess U in soils and U in crops were well
correlated, ess U might have different soil-to-crop transfer factors compared to those
that initially included U have. Such results would make it difficult to apply TF values obtained by measuring total U in soil and crops to the mathematical models
needed for dose assessment from radioactive waste disposal sites. In this chapter,
therefore, we try to estimate ess U in agricultural field soil, that is, paddy field and
upland field, using the initial U/Th ratio in soil because the Th concentration in
phosphate fertilizer is generally low (Komura et al. 1985; Tsumura and Yamasaki
1993; Pantelica et al. 1997). Thus, Th in agricultural fields would not increase much
even if phosphate fertilizer was applied in those fields, which would allow us to
calculate the initial U from the Th. After examining the effect of estimated ess U to
the U contents in crops, we provided the TF-U values for ten crop groups collected
from studies done in Japan.
2 Estimation of Excess Amount of U in Agricultural Fields
According to Th and U concentration data obtained for about 3000 Japanese river
sediment samples by Imai et al. (2004), a good correlation factor between Th and U
concentrations was found (Fig. 1), i.e., R = 0.859 (p < 0.001), between logarithms
of the concentrations of Th and U (log-normal curves fit the data better than normal
curves, so that logarithms of the data were used). However, for agricultural fields,
we previously reported no correlation between Th and U concentrations for 37
paddy field soil samples and a correlation factor of R = 0.69 (p < 0.001) for 45
upland field soil samples which is lower than that of the river sediment samples
(Tagami and Uchida 2006). Although we increased the sample numbers in this
study, i.e., N = 98 for paddy soil samples and N = 139 for upland soil samples collected throughout Japan, the correlation factor for each soil use was still low, i.e.,
R = 0.472 (p < 0.001) for paddy field soil samples and R = 0.659 (p < 0.001) for
upland field soil samples (Fig. 2). Apparently, application of phosphate fertilizers to
agricultural fields affected these observation results.
Soil-to-Crop Transfer Factor: Consideration on Excess Uranium…
( ess U) should be found in agricultural fields due to the phosphate fertilizer application (Rothbaum et al. 1979; Jones 1992; Tagami and Uchida 2006; Takeda et al.
2006; Yamaguchi et al. 2009). When Yamaguchi et al. (2009) applied chemical
extraction methods to identify constituents that contributed to U fixation in the studied soil samples, they observed that the physicochemical forms of U initially
included and added were different. According to their results, most of the U from
the phosphate fertilizer was incorporated into the soil organic matter and amorphous
Fe/Al oxy-hydroxides in the soil, which meant the ess U was not readily available to
crops; but it was not clear whether the ess U in soil correlated with the U concentrations in crops or not.
If the concentrations of ess U in agricultural soils were estimated and then the
concentrations between ess U in soil and U in crops were compared, it might be possible to determine whether or not the soil-to-crop transfer of added U, that is ess U,
would affect the U concentrations in crops. If ess U in soils and U in crops were well
correlated, ess U might have different soil-to-crop transfer factors compared to those
that initially included U have. Such results would make it difficult to apply TF values obtained by measuring total U in soil and crops to the mathematical models
needed for dose assessment from radioactive waste disposal sites. In this chapter,
therefore, we try to estimate ess U in agricultural field soil, that is, paddy field and
upland field, using the initial U/Th ratio in soil because the Th concentration in
phosphate fertilizer is generally low (Komura et al. 1985; Tsumura and Yamasaki
1993; Pantelica et al. 1997). Thus, Th in agricultural fields would not increase much
even if phosphate fertilizer was applied in those fields, which would allow us to
calculate the initial U from the Th. After examining the effect of estimated ess U to
the U contents in crops, we provided the TF-U values for ten crop groups collected
from studies done in Japan.
2 Estimation of Excess Amount of U in Agricultural Fields
According to Th and U concentration data obtained for about 3000 Japanese river
sediment samples by Imai et al. (2004), a good correlation factor between Th and U
concentrations was found (Fig. 1), i.e., R = 0.859 (p < 0.001), between logarithms
of the concentrations of Th and U (log-normal curves fit the data better than normal
curves, so that logarithms of the data were used). However, for agricultural fields,
we previously reported no correlation between Th and U concentrations for 37
paddy field soil samples and a correlation factor of R = 0.69 (p < 0.001) for 45
upland field soil samples which is lower than that of the river sediment samples
(Tagami and Uchida 2006). Although we increased the sample numbers in this
study, i.e., N = 98 for paddy soil samples and N = 139 for upland soil samples collected throughout Japan, the correlation factor for each soil use was still low, i.e.,
R = 0.472 (p < 0.001) for paddy field soil samples and R = 0.659 (p < 0.001) for
upland field soil samples (Fig. 2). Apparently, application of phosphate fertilizers to
agricultural fields affected these observation results.
Soil-to-Crop Transfer Factor: Consideration on Excess Uranium…