2:45 PM - 3:00 PM
[MAG41-05] Study on deposition of 134Cs and 137Cs from atmosphere to freshwater environment using a mountainous lake, Lake Haruna in Gunma Prefecture, Japan
Keywords:lake sediment, sedimentation rate, radiocesium
This study investigated the deposition of 134Cs and 137Cs at a mountainous lake, Lake Haruna, which is located at 1084 m height in Gunma Prefecture, Japan. The lake has a small inflow stream and one outflow river with watershed of relatively high radiocesium accumulation area. The lake sediments were collected at two sites from northern part (HR1) and central part (HR2) of the lake by using a gravity corer in September 2015. The sediment core samples were sectioned for 1 cm interval. The sediment core was also collected at HR3 from central part of the lake in October 2017. After freeze-drying of the sectioned samples, the radioactivity of 134Cs and 137Cs were measured by a low background Ge detector of Low Level Radioactivity Laboratory, Kanazawa University and shows as Bq/kg in dry-sediment basis at the sampling date.
Radioactivity of 134Cs and 137Cs ranged from D.L. to 2,922 Bq/kg and from D.L. to 34,000 Bq/kg, respectively. Two peaks of the radioactivity are observed for the HR1 sediments at 2-3 cm and 5-6 cm depth in core. One distinct peak of the radioactivity of 134Cs and 137Cs is observed at 3-4 cm depth in the core of HR2. Maximum of the radioactivity is also detected for the HR3 sediment core (2,920 and 23,760 Bq/kg) at 4-5 cm depth. The sedimentation rate based on 210Pb for the HR1, HR2 and HR3 was 0.14, 0.031and 0.026 g/cm2/y, respectively. The higher sedimentation rate at HR1 shows two peaks, but lower sedimentation rate at HR2 and HR3 show one peak of radiocesium concentration. However, the inventory of 134Cs is almost constant (2.1-2.3 x104 Bq/m2) for the three sampling sites. Two major deposition was observed in the middle of March and the early of April 2011 from the monitoring data of air gamma-ray dose rate at Takasaki and the fallout analysis of radiocesium at Maebashi in Gunma Prefecture. These results suggest that the accumulation of rediocesium at Lake Haruna mainly results from atmospheric deposition and shows short-term deposition to the bottom sediment at early stage of the accident.