Long-term Trends of Plot-scale Cs-137 Wash-off on Various Land Uses in the Area Affected by the Accident of Fukushima Dai-ichi Nuclear Power Plant

Abstract:

Understanding plot-scale Cs-137 wash-off is indispensable for predicting Cs-137 fate. However, there are only a few numbers of long-term observations and hence lack of information on temporal trends of Cs-137 wash-off leaves uncertainties on the predictions. This study shows approximately three years’ observation results of plot-scale Cs-137 wash-off from various land uses after the accident of Fukushima Dai-ichi Nuclear Power Plant. Erosion plots (22.3 m × 5.0 m) were established in 2011 and 2013 on two uncultivated farmlands (FL-A1, FL-B1), two cultivated farmlands (FL-A2, FL-B2), three grasslands (GL-A, GL-B, GL-C) and one Japanese cedar forest (JCdF) in Kawamata town. Eroded sediments as well as the data of rainfall and vegetation cover were collected almost every two weeks. At the end of observations, we took soil samples from three to nine points on each plot with scraper plates (30 cm ×15 cm) to obtain Cs-137 depth distributions. Collected sediment and soil samples were dried, weighed and measured with HPGe type gamma detectors. We calculated β_eff, ‘effective’ relaxation depth (g/m²), based on Cs-137 depth distributions. Annual soil loss ranged from 0.04 to 75 t/ha/year and the largest was from FL-A2 followed by FL-B2, FL-A1, FL-B1, GL-A, GL-B, JCdF and then GL-C. Sediment amount weighted mean values of Cs-137 concentration normalized by initial deposition density ranged from 0.0062 to 0.084 ((Bq/kg)/(Bq/m²)) and the largest was from JCdF followed by FL-B1, FL-A1, GL-A, FL-A2, FL-B2, GL-B, and then GL-C. Decreasing trends of Cs-137 concentration were found on FL-A1 and FL-B1, whereas no trend was found on other plots due to constant values on FL-A2 and FL-B2 and due to variable values on GL-C, GL-B, GL-C and JCdF. Depth distributions of Cs-137 on FL-A1 and FL-B1 showed exponential distributions with low inventories on their upslope points and deeper penetrations with high inventories on their downslope points. Mean values of β_eff ranged from 2.1 to 24.5 g/cm² and positively correlated with annual soil loss on uncultivated farmlands and grasslands. These results indicate that active soil erosion on uncultivated slopes results in decreasing trends of Cs-137 concentration of sediments and possibly enhances decreases of radiological dose by removing surface soil of high Cs-137 content and mixing them with subsoil.