Spatial variations in radiocesium deposition and litter-soil distribution in a mountainous forest catchment affected by the Fukushima nuclear accident

The Fukushima Dai-ichi Nuclear Power Plant accident caused serious Cs-137 contamination in mountainous forest areas. To understand the spatial variation in soil Cs-137 inventory in complex mountainous topography and the influencing factors, a whole-area investigation of Cs-137 deposition in a broad-leaved forest catchment of a mountain stream was conducted using grid sampling. Across the catchment, organic and surface mineral soil layers were collected at 42 locations in 2013 and 6 locations in 2015. Cesium-137 deposition on the forest floor exhibited high spatial heterogeneity and altitude-dependent distribution over the catchment. The Cs-137 retention ratio in the organic layer, determined as the inventory in the organic layer divided by the soil (organic and mineral soil layers) inventory, ranged from 6% to 82% in 2013, and the coefficient of variation was 0.6. The Cs-137 retention ratios had positive correlations with the material inventory in the organic layer and the elevation. The Cs-137 retention ratios in the organic layer were less than 20% in 2015, even at the locations where the retention ratio was higher than 55% in 2013. Although there was spatial variation in the migration speed, Cs-137 migration from the organic layer to mineral soil was almost completed within 4 y of the deposition, suggesting a decrease in Cs-137 circulation within the forest ecosystem. This study also examined a relationship between the Cs-137 inventory and the air dose rate to assess the potential of using the air dose rate to estimate soil Cs-137 inventory. Soil Cs-137 inventories and air dose rates were highly positively correlated, indicating that measurement of air dose rate can provide an easier and quicker alternative to measurement of soil Cs-137 inventory in forest ecosystems.