4:15 PM - 4:30 PM
[HCG31-04] Radiocesium runoff forms and its temporal variation at two rivers in Iitate, Fukushima
Keywords:Radiocesium, Sediment, Soil erosion, Fukushima
The study sites were two watersheds in Iitate Village, Fukushima, Japan (Figure 1). The southern observation watershed (Hiso River watershed, 25.6 km2) has higher radiation levels compared to the northern Mano River watershed (10.8 km2). Forest accounts for close to 75% of the land area in both watersheds. Sediments containing radiocesium carried by runoff from surrounding lands into the rivers were monitored. A monitoring system is composed of a rain gauge, water level sensor, water velocimeter, turbidity sensor, and automatic water sampler (Figure 2).
Focusing on sediment particle size, more than 75% of the 137Cs was adsorbed on finer particles such as clay, silt, and fine sand which were occupied less than 50% of suspended sediment as shown in Figure 3. Correlations between suspended sediment concentrations (SSC) with 137Cs concentrations in storm waters are shown in Figure 4, and were approximately linear. These results mean radiocesium was discharged with the suspended sediment and organic matter. Comparing the two watersheds in the Figure 4, the slope of the regression line at Hiso was greater than that at Mano, which means radiocesium content in the suspended sediment at Hiso was larger than that at Mano. This also agrees with the distribution of radiocesium content in the topsoil shown in Figure 1. Comparing the slope of the regression lines from 2013 to 2016 in Figure 4, it decreased clearly with the lapse of time. The decreasing ratio for three years was 79% at Hiso and 83% at Mano. These ratios are much greater than the decay ratio of 6.7% estimated by physical half-life of radio-cesium. This high decreasing ratio might have been resulted due to the selective erosion and transportation of fine particles and organic matter in the hillslope and waterway. Monitored total amounts are summarized in Table 1. Hiso radiocesium losses were greater than those at Mano even though sediment yield was smaller, and this was due to the greater concentrations of radiocesium at Hiso. Spatially averaged 137Cs contents at Hiso and Mano were 1017 kBq/m2 and 421 kBq/m2. Thus, decontamination of radiocesium in topsoil by natural soil erosion processes may not be effective. Focusing the form of radiocesium in the river water, most of 137Cs was discharged as particulate form in the storm events. Its form accounted for greater than 95% of the total 137Cs amount at both watersheds.