Temporal changes in dissolved ¹³⁷Cs concentrations in groundwater and stream water in Fukushima after the Fukushima Dai-ichi Nuclear Power Plant accident

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 July 2013, after the earthquake and tsunami disaster. 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 (a fast flush of activity as a result of rapid washoff and a slow decline as a result of soil fixation and redistribution processes). The highest ¹³⁷Cs concentration recorded at Iboishi-yama was 1.2 Bq/L 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 0.10–0.21 and 0.69–1.5 y, respectively in the three catchments. The effective half-life of groundwater was 0.46–0.58 y at Koutaishi-yama and 0.50–3.3 y at Iboishi-yama. 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. - Highlights: • The highest stream water dissolved ¹³⁷Cs concentration recorded at Iboishi-yama was 1.2 Bq/L on August 6, 2011. • During the rainfall events, the concentration of dissolved ¹³⁷Cs concentration in stream water increased temporarily. • The dissolved ¹³⁷Cs concentration in groundwater at a depth of 30 m at Iboishi-yama ranged 0.0025–0.039 Bq/L. • The effective half-lives of stream water in the initial fast flush and secondary phases were 0.10–0.21 and 0.69–1.5 y. • The effective half-lives of groundwater was 0.46–0.58 y at Koutaishi-yama and 0.50–3.3 y at Iboishi-yama.