17:15 〜 18:30
[MAG38-P01] 福島県の森林土壌におけるCs-137の空間不均一性および化学形態に対する選択流の影響
キーワード：選択流、染料トレーサー、下方浸透
1. Introduction
Rainfall infiltration is one of the most important mechanisms of downward migration of Cs-137 in forest soils. It has been pointed out that rainwater infiltration in soil can be divided into two types, which are relatively uniform matrix flow and non-uniform preferential flow through coarse pores and root surfaces. This preferential flow would contribute the downward migration of various substances, therefore it is possible that this preferential flow is a factor in the spatial heterogeneity of the vertical distribution of Cs-137 in soil. However, there have been few studies that have quantitatively evaluated the effects of preferential flow on the vertical distribution of Cs-137. In the presnt study, we conducted an artificial rainfall experiment using dyes in a forest and collected soil samples from dyed and un-dyed areas at different depths in order to quantify the spatial heterogeneity and chemical form of Cs-137.
2. Material and methods
The study site was a cedar forest located in Akougi district, Namie-town, Fukushima Prefecture. The estimated deposition amount by airborne monitoring was about 4700 kBq/m2. One plot with 1 m x 1 m was set up in the forest, and 10 L of distilled water containing 2 g/L of Rhodamine B was sprayed by a simple sprinkler system as a dye, corresponding to a rainfall amount of 10 mm and the rainfall intensity of about 120 mm/h. Two days after sprinkling, a scraper plate was placed under the litter layer, and the visually dyed soil were collected little by little using a spoon, and then the un-dyed soil were collected. The sampling depths were 0-2 cm, 2-4 cm, 4-6 cm, and 6-10 cm. Although the soil was classified as a non-allophanic Andosol, these surface soil samples did not have andic properties. After air-drying and sieving, the soil samples were sequentially extracted (F1: water-soluble form, F2: exchangeable form, F3: Fe-Mn oxides form, F4: organic matter-bound form, F5: acid-soluble form, F6: residue). The concentrations of Cs-137 and several main elements in each solution were measured using a Ge semiconductor detector and atomic absorption spectrometer. In addition, several chemical properties of soil samples were determined.
3. Results
The dyed soil samples in all layers had higher root and fresh organic matter contents per unit soil weight, higher soil organic matter content, higher CEC, and higher water-soluble and exchangeable cation contents than those in the un-dyed soil samples. The ratios of Cs-137 concentrations in the dyed and un-dyed samples at the same depth were 0.98, 1.48, 1.51, and 2.64 for 0-2, 2-4, 4-6, and 6-10 cm, respectively. This suggests that the preferential flow played an important factor in the spatial heterogeneity of Cs-137 vertical distribution and the pathway of the preferential flow was relatively stable even after nearly 10 years from the accident. As a result of sequential extraction, the average percentages of Cs-137 at each depth in dyed and un-dyed samples were 0.031% and 0.022% in the F1 fraction, 1.10% and 0.97% in the F2 fraction, 0.57% and 0.50% in the F3 fraction, 3.64% and 3.12% in the F4 fraction, and 43.2% and 40.7% in the F5 fraction, showing that the dyed samples tended to be slightly higher in all fractions except the residue. This result suggested that plant-available Cs-137, especially water soluble and exchangeable form, may be accumulated in areas where preferential flow frequently permeate, such as around the roots. Therefore, it is considered that the effects of preferential flow are important not only for the downward migration but also for the evaluation of root absorption.
Rainfall infiltration is one of the most important mechanisms of downward migration of Cs-137 in forest soils. It has been pointed out that rainwater infiltration in soil can be divided into two types, which are relatively uniform matrix flow and non-uniform preferential flow through coarse pores and root surfaces. This preferential flow would contribute the downward migration of various substances, therefore it is possible that this preferential flow is a factor in the spatial heterogeneity of the vertical distribution of Cs-137 in soil. However, there have been few studies that have quantitatively evaluated the effects of preferential flow on the vertical distribution of Cs-137. In the presnt study, we conducted an artificial rainfall experiment using dyes in a forest and collected soil samples from dyed and un-dyed areas at different depths in order to quantify the spatial heterogeneity and chemical form of Cs-137.
2. Material and methods
The study site was a cedar forest located in Akougi district, Namie-town, Fukushima Prefecture. The estimated deposition amount by airborne monitoring was about 4700 kBq/m2. One plot with 1 m x 1 m was set up in the forest, and 10 L of distilled water containing 2 g/L of Rhodamine B was sprayed by a simple sprinkler system as a dye, corresponding to a rainfall amount of 10 mm and the rainfall intensity of about 120 mm/h. Two days after sprinkling, a scraper plate was placed under the litter layer, and the visually dyed soil were collected little by little using a spoon, and then the un-dyed soil were collected. The sampling depths were 0-2 cm, 2-4 cm, 4-6 cm, and 6-10 cm. Although the soil was classified as a non-allophanic Andosol, these surface soil samples did not have andic properties. After air-drying and sieving, the soil samples were sequentially extracted (F1: water-soluble form, F2: exchangeable form, F3: Fe-Mn oxides form, F4: organic matter-bound form, F5: acid-soluble form, F6: residue). The concentrations of Cs-137 and several main elements in each solution were measured using a Ge semiconductor detector and atomic absorption spectrometer. In addition, several chemical properties of soil samples were determined.
3. Results
The dyed soil samples in all layers had higher root and fresh organic matter contents per unit soil weight, higher soil organic matter content, higher CEC, and higher water-soluble and exchangeable cation contents than those in the un-dyed soil samples. The ratios of Cs-137 concentrations in the dyed and un-dyed samples at the same depth were 0.98, 1.48, 1.51, and 2.64 for 0-2, 2-4, 4-6, and 6-10 cm, respectively. This suggests that the preferential flow played an important factor in the spatial heterogeneity of Cs-137 vertical distribution and the pathway of the preferential flow was relatively stable even after nearly 10 years from the accident. As a result of sequential extraction, the average percentages of Cs-137 at each depth in dyed and un-dyed samples were 0.031% and 0.022% in the F1 fraction, 1.10% and 0.97% in the F2 fraction, 0.57% and 0.50% in the F3 fraction, 3.64% and 3.12% in the F4 fraction, and 43.2% and 40.7% in the F5 fraction, showing that the dyed samples tended to be slightly higher in all fractions except the residue. This result suggested that plant-available Cs-137, especially water soluble and exchangeable form, may be accumulated in areas where preferential flow frequently permeate, such as around the roots. Therefore, it is considered that the effects of preferential flow are important not only for the downward migration but also for the evaluation of root absorption.