Soil erosion and radiocesium migration during the snowmelt period in grasslands and forested areas of Miyagi prefecture, Japan

This study aimed to examine the influence of snowmelt on soil erosion processes in mountainous landscapes in the Miyagi prefecture of Japan. The investigated slopes had different expositions and were covered with grasslands and forests. The snowpack thickness, soil frost depth, volume of surface runoff, physicochemical properties of the soil and sediments, cesium composition of the snow and meltwater, and air dose rate were determined. In mid-February, snow cover reached its maximum thickness (100–179 cm). In the forest, the snow depth was always lower by 15–20 cm. The soil did not freeze in winter in any of the plots. Surface runoff was observed only in the grassland plots and depended on the slope aspect. The total volume of surface runoff ranged from 31 to 52 mm and snowmelt soil losses ranged from 2 to 9 kg ha⁻¹ DM. Radiocesium concentrations in runoff samples ranged from 0.1 to 8.4 Bq L⁻¹, below the standard limit for drinking water in Japan (10 Bq L⁻¹). The average organic matter content of the sampled sediments was 0.4%, higher than that in the surface soil. The silt fraction in sediments became dominant for particle size distribution, and the activity concentration of total radiocesium was, on average, 250 Bq kg⁻¹. The air dose rate was always lower than the maximum permissible level (0.2 μSv h⁻¹) and varied from 0.02 to 0.09 μSv h⁻¹ in winter, and from 0.08 to 0.13 μSv h⁻¹ at times of the year without snow.