Record of the Fukushima nuclear accident in unsaturated soil water and the fate of nuclear contamination

The ³H and ³⁶Cl radionuclides, released during the Fukushima nuclear accident in 2011, contaminated forested environments. However, the opportunity to periodically observe these radionuclides was lost due to the catastrophic conditions caused by the earthquake and tsunami associated with the accident. The concentrations of these released nuclides in precipitation and their fates are critical when considering the effects of internal exposure. Given that ³H and ³⁶Cl are reliable hydrological tracers, records of their continuous depositional flux within unsaturated soil layers could provide valuable insights into the contamination levels during the initial years after the accident. Such records are indispensable for understanding the extent of contamination. Using a borehole drilled in 2014 at Koriyama, 60 km from the accident site, we reconstructed the deposition record of atmospheric ³H and ³⁶Cl following the accident. The contributions of ³H and ³⁶Cl from the accident were determined to be 1.4 × 10¹³ and 2.0 × 10¹² atoms m⁻², respectively, at this site. Unlike approaches based on radionuclide migration analysis, the ³H and ³⁶Cl concentrations in precipitation during the approximately six weeks following the accident were accurately recovered (607 Bq/L and 4.74 × 10¹⁰ atoms/L, respectively) by analyzing the depositional flux in the unsaturated soil column from depths of 0 m to 4.25 m. Both the ³H and ³⁶Cl concentration profiles were reassessed at the site in 2016. By that time, both soluble radionuclides had mostly been flushed out of the unsaturated soil zone due to rainfall over the 5.6 years since the accident. Although ³H concentrations in the unsaturated soil water and shallow groundwater had returned to less than 6 tritium units (TU), the ³⁶Cl concentration had not yet returned to the natural cosmogenic background deposition level. In contrast, ¹²⁹I was primarily found in the litter layer and the soil near the ground surface.