Temporal variation of iodine-129 in rainwater at Tsukuba before and after the Fukushima Daiichi Nuclear Power Plant accident

Rainwater has been sampled on a monthly basis at the University of Tsukuba, Japan, since 2009. The serious accident at the Fukushima Daiichi Nuclear Power Plant in March 2011 resulted in the atmospheric emission of large quantities of radionuclides. Monthly I-129 deposition was monitored for 19 months before and 41 months after the accident to estimate its influence at Tsukuba. Before the accident, I-1(29) concentration in rainwater was in the range (0.19-2.6) x 10(8) atoms L-1. There was a rapid increase after the accident, with the highest I-129 concentration recorded in March 2011 as (5.4 +/- 0.8) x 10 (10) atoms L-1. It took approximately 1 y for the I-129 concentration to return to pre-accident background levels, due to continuing resuspension and deposition. Measured decreases in I-129 concentration in rainfall showed good agreement with double-exponential modeling.