堤外地における放射性Cs集積に及ぼす微地形の影響: 福島県を流れる阿武隈川2次支流を事例として

摘要

堤外地における放射性Cs集積に関わる微地形(標高差)の影響を明らかにすることを目的とし，阿武隈川2次支流である石田川の堤外地において，10m程度の区画から表層5cmの底質試料を15地点より採取し，放射性Cs濃度，乾燥密度，および，粒度分布を測定するとともに，試料採取地点の標高をRTK-GPSを用いて測量した。その結果，採取試料の放射性Cs濃度は1·46×10~3～3.43×10~4 Bq·kg~(-1)であるとともに，試料採取地点の標高（最大比高74cm)は，放射性Cs濃度，細粒分割合，乾燥密度を有意に説明することができた。その理由として，放射性Csは主に出水時に懸濁態で流出し，出水時に冠水する範囲においては，植生の繁茂する高水敷で懸濁物質の沈降•堆積が起こりやすい一方，標高の低い地点においては，放射性Csを含んだ懸濁物質は水流により洗脱されてしまう可能性が考えられた。%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×l0~3 to 3.43×l0~4 Bq·kg~(-1). Significant 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: Alarge amount of radiocesium in particulate form runs off into the river from the watershed when the river swells and the flood 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 flushed away from a riverbed at alower elevation owing to a higher chance of contact with river water.