Record of ³H and ³⁶Cl from the Fukushima nuclear accident recovered from soil water in the unsaturated zone at Koriyama

The opportunity to measure the concentrations of H-3 and Cl-36 released by the Fukushima nuclear accident in 2011 directly in rain was lost in the early stage of the accident. We have, however, been able to reconstruct the deposition record of atmospheric H-3 and Cl-36 following the accident using a bore hole that was drilled in 2014 at Koriyama at a distance of 60 km from the accident. The contributions of H-3 and Cl-36 from the accident are 1.4x10(13) and 2.0x10(12) atoms m(-2) respectively at this site. Very high concentrations of both H-3 (46 Bq L-1) and Cl-36 (3.36x10(11) atoms L-1) were found in the unsaturated soil at depths between 300 and 350 cm. From these, conservative estimates for the H-3 and Cl-36 concentrations in the precipitation in the similar to 6 weeks following the accident were 607 Bq L-1 and 4.74x10(10) atoms L-1, respectively. A second hole drilled in 2016 showed that H-3 concentrations in the unsaturated soil and shallow groundwater had returned to close to natural levels, although the Cl-36 concentrations were still significantly elevated above natural levels.