Siltation and radiocesium pollution of small lakes in different catchment types far from the Fukushima Daiichi nuclear power plant accident site

摘要

The Fukushima Daiichi nuclear power plant accident caused radioactive pollution in northeastern Honshu Island, Japan. This study examined the influence of snowmelt and rainfall on soil erosion pro-cesses and siltation of small lakes in Miyagi Prefecture (150 km northwest of the power plant). Two sets of slopes and lakes, respectively in pasture and forest catchments, were examined. Snowpack thickness, soil infiltration, surface runoff volume, soil and sediment physicochemical properties, Cs concentration of precipitation, meltwater, and rainwater, and lake siltation rates were determined. The total radioactive Cs content in precipitation was 0.7-7.4 Bq·L-1 and was below the Japanese standard (10 Bq·L-1). Total radioactive Cs was at the allowable level in water flowing down the pasture catchment slope (0.1-9.2 Bq·L-1) during snowmelt and rainfall, as well as in pasture (0.9-8.8 Bq·L-1) and forest (0.7-5.2 Bq·L-1) catchment lake water. There was no soil erosion (surface runoff) in the forest catchment. Soil losses in the pasture catchment were 23 due to rainfall and 9 kg ha-1 yr-1 following spring snowmelt. After snow-melt, a 0.5 and 0.2 mm thick layer of silt was deposited in pasture and forest catchment lakes, respec-tively, and 1.4 and 0.6 mm were deposited during the rainfall period. Average siltation rates were 1.9 and 0.8 mm·yr-1 for pasture and forest catchment lakes, respectively. The upper layer of lake bottom sedi-ments is represented mainly by silt fractions (2-50μm), with high organic matter (4.0-5.7％) and radiocesium (1100-1600 kg·ha-1) contents.