Determining sources of the 137Cs concentration in seawater at Fukushima Daiichi Nuclear Power Plant using Antecedent Precipitation Index

Radiocesium (¹³⁷Cs) was one of the radioactive materials released from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March 2011. Highly ¹³⁷Cs contaminated water from groundwater to the sea was reduced after installation of the sea-side impermeable wall as a countermeasure against contaminated water in October 2015. As a result, ¹³⁷Cs contamination in water from other sources became more prominent and the levels of ¹³⁷Cs concentration in seawater was correlated with rainfall fluctuation. To determine the source of contamination, we estimated the fluctuation patterns of ¹³⁷Cs concentration in seawater, groundwater level, and discharge from the channels using the Antecedent Precipitation Index (Rw) method.
The results indicated that the fluctuation in seawater collected near the 1-4 Units had strong agreement with the 3 day half-life of Rw. The half-life is shorter than that estimated by groundwater level (7 to 30 day). Therefore, the ¹³⁷Cs concentration in seawater was influenced by relatively faster runoff than the deep groundwater flow. We also made the spatial distribution map of ¹³⁷Cs concentration in seawater to determine the sources of contamination. It showed that the ¹³⁷Cs contaminated area was the highest at “south- inside the intake of 1-4 Units” where the outlets of the K and BC discharge channels are located. In particular, the concentration of ¹³⁷Cs in the channel K was found to correlate with the concentration of ¹³⁷Cs in seawater near the 1-4 Units (average of R² = 0.5). These results indicate that the concentration of ¹³⁷Cs in seawater inside the FDNPP port can be estimated by the Rw method and that the source of the contamination could be determined using the half-life.