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 137Cs contamination in mountainous forest areas. To understand the spatial variation in soil 137Cs inventory in complex mountainous topography and the influencing factors, a whole-area investigation of 137Cs 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 137Cs 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 137Cs retention ratios had positive correlations with the material inventory in the organic layer and the elevation. The 137Cs 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, 137Cs migration from the organic layer to mineral soil was almost completed within 4 y of the deposition, suggesting a decrease in 137Cs circulation within the forest ecosystem. This study also examined a relationship between the 137Cs inventory and the air dose rate to assess the potential of using the air dose rate to estimate soil 137Cs inventory. Soil 137Cs 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 137Cs inventory in forest ecosystems.