11:30 〜 11:45
[AGE28-09] Radiocesium (Cs) horizontal and vertical distribution at a decontaminated backyard hillslope and a residential grove
★Invited Papers
キーワード：福島、放射性セシウム、除染、森林土壌
Among the major radioactive materials released from the Fukushima Daiichi NPP in 2011, 72~75% of the main extent of 137Cs, having a relatively long half-life of 30 years, is deposited in forests. Cs has been widely used in soil erosion studies to trace moving soils, where Cs concentration decreases with soil loss; meanwhile, Cs concentration on the soil surface increases where soil is deposited. Since Cs transport from the forest ecosystem to the outside of the system is limited, Cs may remain in the forest ecosystem for an extended period. Removing Cs deposited with undergrowth and fallen leaves was carried out in residential areas; however, the forests outside a 20-meter radius of houses were untouched. When the pristine forest with Cs resided in a higher part of a hillslope in the backyard of the houses, Cs moving downward to the decontaminated areas became a big concern. Therefore, it is essential to understand the Cs's behavior in the forest environment. We studied concentration distribution at the soil surface of a decontaminated slope at Iitate Village, Fukushima Prefecture, from April 2015 to February 2019. We found that Cs-remained and upper Cs-removed sampling sites experienced increasing Cs concentrations, suggesting that Cs was transported downward from the contaminated area in the upper slope than Cs-remained sampling sites. We also found that Cs concentration decreased faster than the natural decay at the surface of the bottom and lower Cs-removed sites, meaning that Cs moved downward into the soil. We collected soil samples in 5 cm increments to 15 cm near all sampling sites in 2020 to investigate the Cs concentration profile. The lower and bottom Cs-removed sites had similar Cs concentrations from 0 cm to 15 cm. Cs concentrations tend to be similarly distributed in the profile when soil is deposited. Our result suggests that Cs was transported with soil particles on the slope surface from the upper slope to near the foot of the hill. Soil particles associated with Cs moved down on the slope surface and reached the foot of the hill, as expected. The degree of Cs-recontamination was insufficient for the decontamination procedure because Cs concentration on the slope surface decreased at the lower and bottom Cs-removed sites. The vertical distribution of Cs concentration at the surveyed sites changed due to soil erosion. In December 2021, soil samples for measuring Cs concentration were collected to a depth of 155 cm in a residential grove in Iitate Village, unaffected by soil erosion. The soil was weathered granite containing vermiculite, which fixes Cs in its structure. 57% of Cs was present between the soil surface and 3 cm deep, and 95% of Cs was presented to 10 cm deep; very little Cs was found below 50 cm deep. The Beck equation can approximate the vertical distribution of radioactive materials in soil. The smaller fitting parameter, α(cm-1), indicates that the radionuclides are more likely to move vertically. The α value for the study site was 0.13, smaller than the value previously reported for agricultural soils in Fukushima Prefecture. Faster vertical transport may be attributed to the transport of Cs by ion exchange. We will also discuss how Cs were distributed in the soil profile at the study site.