17:15 〜 19:15
[MAG34-P05] 選択流による土壌中Cs-137存在量のばらつきと土壌理化学性の関係
★招待講演
キーワード：人工降雨実験、染料トレーサー、下方移行メカニズム、福島第一原子力発電所事故、森林土壌
Preferential flow is one of the still poorly studied mechanisms of downward migration of Cs-137. Preferential flow accounts for the permeation of a significant portion of the percolating water, although the preferential flow pathways occupy only a small proportion of the soil matrix. Therefore, preferential flow is expected to have a strong effect on Cs-137 because it has a very high adsorption ability on soil particles, but research on this remains limited. In this study, an artificial rainfall experiment using a dye tracer was conducted in a cedar plantation forest in the former evacuation zone of Fukushima Prefecture. Soil samples from stained and unstained areas were collected at 2.5 cm intervals and rainfall infiltration water samples under litter layer and in the soil at different depths were collected using a zero-tension lysimeter to measure Cs-137 concentrations to determine the influence of preferential flow on the spatial distribution and downward migration. In addition, 25 core samples were taken at three depths (0-5 cm, 10-15 cm and 20-25 cm) and their Cs-137 concentrations and physico-chemical properties (three-phase distribution, porosity, saturated hydraulic conductivity, bulk density, total carbon, total nitrogen and radiocesium interception potential) that may contribute to preferential flow and partition coefficient were measured to discuss their interrelationships.
The water-extracted rhodamine B concentration in the soil of the stained area is more than 100 times higher than that of the unstained area, indicating the occurrence of preferential flow from the surface layer although even with a rainfall intensity of approximately 90 mm and a rainfall rate of 160 mm. The Cs-137 concentration also tended to be higher in the stained area, and in particular, the dispersion of the Cs-137 concentration in stained area was significantly higher at especially 10-20cm. The Cs-137 concentration in the rainfall infiltration water was almost the same at 10 cm and 20 cm depth. Although the amount of infiltrated water decreased the deeper the depth, it was confirmed that Cs-137 was infiltrating downward without being much adsorbed. These results suggest the influence of preferential flow.
Core sampling results showed an increased variance of the Cs-137 inventory at 10-15 cm compared to the surface layer, as well as significant correlations between selective flow and associated soil physical properties such as bulk density, saturated hydraulic conductivity and coarse porosity. The fact that the relationships were found despite the study being conducted 11 years after the accident suggests that the pathways of selective flow may not have changed significantly after the accident. On the other hand, no such relationship was observed at depths of 0-5 cm and 20-25 cm. This was thought to be because at 0-5 cm, the pathway of the selective flow was easily changed due to plant root growth and death, and the penetration was uniform, so the variation was small. At 20-25 cm, it was thought to be because the amount of Cs-137 that reached the surface was small compared to the selective flow. In addition, a high positive correlation was observed between Cs-137 concentration and total carbon content, and exchangeable cations. Many studies have reported that total carbon and exchangeable cations increase within the range of the influence of preferential flow. These results suggest that Cs-137, carbon and cations have a similar spatial distribution pattern in the soil due to preferential flow. Therefore, 137Cs has the potential to be used as a tracer for tracking these dynamics and preferential flow, and further research is expected to develop.
The water-extracted rhodamine B concentration in the soil of the stained area is more than 100 times higher than that of the unstained area, indicating the occurrence of preferential flow from the surface layer although even with a rainfall intensity of approximately 90 mm and a rainfall rate of 160 mm. The Cs-137 concentration also tended to be higher in the stained area, and in particular, the dispersion of the Cs-137 concentration in stained area was significantly higher at especially 10-20cm. The Cs-137 concentration in the rainfall infiltration water was almost the same at 10 cm and 20 cm depth. Although the amount of infiltrated water decreased the deeper the depth, it was confirmed that Cs-137 was infiltrating downward without being much adsorbed. These results suggest the influence of preferential flow.
Core sampling results showed an increased variance of the Cs-137 inventory at 10-15 cm compared to the surface layer, as well as significant correlations between selective flow and associated soil physical properties such as bulk density, saturated hydraulic conductivity and coarse porosity. The fact that the relationships were found despite the study being conducted 11 years after the accident suggests that the pathways of selective flow may not have changed significantly after the accident. On the other hand, no such relationship was observed at depths of 0-5 cm and 20-25 cm. This was thought to be because at 0-5 cm, the pathway of the selective flow was easily changed due to plant root growth and death, and the penetration was uniform, so the variation was small. At 20-25 cm, it was thought to be because the amount of Cs-137 that reached the surface was small compared to the selective flow. In addition, a high positive correlation was observed between Cs-137 concentration and total carbon content, and exchangeable cations. Many studies have reported that total carbon and exchangeable cations increase within the range of the influence of preferential flow. These results suggest that Cs-137, carbon and cations have a similar spatial distribution pattern in the soil due to preferential flow. Therefore, 137Cs has the potential to be used as a tracer for tracking these dynamics and preferential flow, and further research is expected to develop.