Influence of Microtopography on the Accumulation of Radiocesium in a Waterside Land: A Case Study of a Secondary Branch of the Abukuma River Flowing through Fukushima Prefecture

Abstract

In order to better understand the influence of microtopography on the accumulation of radiocesium in a flood channel, we surveyed a secondary tributary of the Abukuma River, the watershed of which was highly contaminated with radiocesium. Fifteen surface soil samples were collected using 5 cm soil core samplers, and their radiocesium concentration, dry density, and grain size distribution were analyzed. Furthermore, the elevation of the soil sampling points was surveyed using a real-time kinematic global positioning system (RTK-GPS). The radiocesium concentrations ranged from 1.46×10 3 to 3.43×10 4 Bq·kg -1. Signifi cant positive and negative correlations were observed between elevation and radiocesium concentration, and between elevation and dry density, respectively. In addition, the proportion of the fine particle size fraction positively correlated with elevation. One possible reason for this observation is as follows: A large amount of radiocesium in particulate form runs off into the river from the watershed when the river swells and the fl ood channel is inundated. The sedimentation/accumulation of suspended particulate materials is facilitated on a riverbed with a higher elevation for the inundated area. On the other land, particulate materials rich in radiocesium tends to be fl ushed away from a riverbed at a lower elevation owing to a higher chance of contact with river water.