11:30 AM - 11:45 AM
[MAG38-10] Radioactive Cs-137 discharge by Dissolved water, Suspended Sediment and Coarse Organic Matter from Headwater Catchment
Keywords:Cs-137, Dissolved water, Suspended sediment, Coarse organic matter, Headwater catchment, Discharge
The concentration of dissolved 137Cs in groundwater and stream water in the headwater catchments in Yamakiya district, located ~35 km north west of Fukushima Dai-ichi Nuclear Power Plant (FDNPP), was monitored from June 2011 to December 2014. Also 137Cs concentration in suspended sediments and coarse organic matter such as leafs and blanches were monitored. Groundwater and stream water were sampled at intervals of approximately 2 months at each site. Intensive sampling was also conducted during rainstorm events. Compared with previous data from the Chernobyl NPP accident, the concentration of dissolved 137Cs in stream water was low. In the Iboishi-yama catchment, a trend was observed for the concentration of dissolved 137Cs in stream water to decline, which could be divided into two phases by October 2011 (after 200 days after the accident).
The highest 137Cs concentration recorded at Iboishi-yama was 1.2 Bq/L at the peak on August 6, 2011, which then declined to 0.021-0.049 Bq/L during 2013 (in stream water under normal water-flow conditions). During the rainfall events, the concentration of dissolved 137Cs in stream water increased temporarily. The concentration of dissolved 137Cs in groundwater at a depth of 30 m at Iboishi-yama displayed a decreasing trend from 2011 to 2013, with a range from 0.039 Bq/L to 0.0025 Bq/L. The effective half-lives of stream water in the initial fast flush and secondary phases were 0.1-0.2 y and 1.0-2.2 y, respectively, in the three catchments. The trend for the concentration of dissolved 137Cs to decline in groundwater and stream water was similar throughout 2012-2013, and the concentrations recorded in deeper groundwater were closer to those in stream water. The declining trend of dissolved 137Cs concentrations in stream water was similar to that of the loss of canopy 137Cs by throughfall, as shown in other reports of forest sites in the Yamakiya district.
The 137Cs concentration in suspended sediments showed 10000-45000 Bq/kg and 137Cs concentration in organic matter showed 1000-12000 Bq/kg. The 137Cs concentration in organic matter showed declining trend through the observation period and the trend was similar to that of dissolved 137Cs concentration in stream water. In contrast 137Cs concentration in suspended sediment did not showed a clear declining trend in every catchment (before the decontamination works). After the decontamination works in Iboishi-yama catchment, the 137Cs concentration in suspended sediment declined largely to 200-300 Bq/kg. The 137Cs concentration in organic matter and dissolved 137Cs concentration in stream water did not declined largely.
In conjunction with discharge data and turbidity data, 137Cs discharge flux were calculated for three components (dissolved water, organic matter, suspended sediments). As a result, it is shown that more than 98% of the 137Cs discharge was due to the discharge by suspended sediments. It is also suggested that dissolved 137Cs concentration have relationship with 137Cs concentration of organic matter and so it is still important to clarify the process of 137Cs migration through dissolved water and organic matter.