1:45 PM - 3:15 PM
[MAG34-P06] Relationships between Cs-137 Concentration and Permeability in Soil under a Japanese Cedar Forest in Fukushima.
Keywords:Fukushima Daiichi Nuclear Power Plant accident, Radiocesium, preferential flow, downward transfer, dye tracer, artificial rainfall equipment
Forestry, one of Fukushima Prefecture's major industries, is important in terms of maintaining the public benefit functions of forests, such as water source recharge and prevention of mountain disasters. However, due to the accident at Fukushima Daiichi Nuclear Power Plant, the area of forests maintained in Fukushima Prefecture was reduced by half, and more than 10 years later, the situation has hardly changed. Therefore, the restoration of forestry is an important issue, but due to the high cost and low effectiveness of forest decontamination, it has been carried out only in the areas adjacent to residential areas. On the other hand, as a result of clear cutting by the Forestry Agency as part of forest decontamination, it was confirmed that there were cases where the air dose rate decreased more than that of the radioactive cesium that was taken out. This is expected to be due to accelerated downward transfer of Cs-137 in the soil, and one of the reasons for this is preferential flow. It is possible that the increased solar radiation from the forest canopy after the logging may have dried the soil and promoted the occurrence of preferential flow. In the future, encouraging downward transfers will be important for forest management and for the restoration of forestry in Fukushima Prefecture, as it will lead to a decrease in Cs-137 absorption by plants. Therefore, this study aimed to investigate the effects of preferential flow and permeability on the spatial distribution of Cs-137 by conducting an artificial rainfall experiment using a dye tracer in a cedar forest before thinning, surveying stained and unstained soil samples at different depths, and analyzing core samples.
The study site was a cedar plantation (initial deposition 720 kBq/m2 ) in Minamisoma, a former planned condemned area where line thinning was conducted in November 2022. Before thinning, a solution of the fluorescent dye Rhodamine B was applied by artificial rainfall equipment at a rainfall rate of about 160 mm and a rainfall intensity of about 90 mm/h. After at least 24 hours, photographs were taken from above at each depth, and soil samples were taken by 100 mL core and visually from stained and unstained areas. The collected soil was dried and sieved through a 2 mm sieve, and the Cs-137 concentration and the Rhodamine B concentration were measured with a germanium semiconductor detector and UV-visible spectrophotometer, respectively. The results of the analysis at each depth (n=3) showed that the concentration of Rhodamine B tended to be significantly higher in the stained soil than in the non-stained soil up to 20 cm. The dispersion of Cs-137 concentrations in unstained and stained soils at each depth (n=3) was also different.
The higher concentration of Rhodamine B in the stained soils at all depths sampled indicates that Rhodamine B, including the surface layer, flows with different ease in different spaces at this rainfall and rainfall intensity, and if the difference in Cs-137 concentration between stained and unstained soils has been caused by the preferential flow from the accident, the path of preferential flow is still the similar after 11 years. The results suggest that preferential flow may have affected the spatial distribution of Cs-137. It was also considered that the preferential flow may be mainly at depths of up to 10 cm. In the future, we plan to analyze the images taken and conduct similar experiments after thinning.
The study site was a cedar plantation (initial deposition 720 kBq/m2 ) in Minamisoma, a former planned condemned area where line thinning was conducted in November 2022. Before thinning, a solution of the fluorescent dye Rhodamine B was applied by artificial rainfall equipment at a rainfall rate of about 160 mm and a rainfall intensity of about 90 mm/h. After at least 24 hours, photographs were taken from above at each depth, and soil samples were taken by 100 mL core and visually from stained and unstained areas. The collected soil was dried and sieved through a 2 mm sieve, and the Cs-137 concentration and the Rhodamine B concentration were measured with a germanium semiconductor detector and UV-visible spectrophotometer, respectively. The results of the analysis at each depth (n=3) showed that the concentration of Rhodamine B tended to be significantly higher in the stained soil than in the non-stained soil up to 20 cm. The dispersion of Cs-137 concentrations in unstained and stained soils at each depth (n=3) was also different.
The higher concentration of Rhodamine B in the stained soils at all depths sampled indicates that Rhodamine B, including the surface layer, flows with different ease in different spaces at this rainfall and rainfall intensity, and if the difference in Cs-137 concentration between stained and unstained soils has been caused by the preferential flow from the accident, the path of preferential flow is still the similar after 11 years. The results suggest that preferential flow may have affected the spatial distribution of Cs-137. It was also considered that the preferential flow may be mainly at depths of up to 10 cm. In the future, we plan to analyze the images taken and conduct similar experiments after thinning.