We measured activity concentrations of radiocesium (134Cs and 137Cs) in seawater samples collected in North Pacific coastal and offshore areas of Japan within several months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in March 2011, including archived seawater samples whose radiocesium concentrations were previously reported to be below detection limits. By merging 329 new data with published results, we succeeded in reconstructing the temporal changes in activity concentrations and inventories of FNPP1-derived radiocesium in the coastal and offshore areas within several months after the accident for the first time. 137Cs directly-discharged from the FNPP1 was transported eastward within the coastal area about 250 km from the FNPP1 during two months after the accident due to complex movements of coastal surface currents. The eastward speed was calculated to be about 5 cm s-1. Eastward transport of 137Cs to the offshore area more than 600 km away from the FNPP1 along the north flank of the Kuroshio Extension Current was faster (about 9 cm s-1) and probably more dominant in the eastward transport. The total inventory of directly-discharged 137Cs in early April 2011 was estimated to be 3-6 PBq approximately, which agrees with the smaller estimates in previous studies (2-6 PBq).

¹³⁵Cs/¹³⁷Cs is a potential tracer for radiocesium source identification. However, due to the challenge to measure ¹³⁵Cs, there were no ¹³⁵Cs data available for Japanese environmental samples before the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. It was only 3 years after the accident that limited ¹³⁵Cs values could be measured in heavily contaminated environmental samples. In the present study, activities of ¹³⁴Cs, ¹³⁵Cs, and ¹³⁷Cs, along with their ratios in 67 soil and plant samples heavily and lightly contaminated by the FDNPP accident were measured by combining γ spectrometry with ICP-MS/MS. The arithmetic means of the ¹³⁴Cs/¹³⁷Cs activity ratio (1.033 ± 0.006) and ¹³⁵Cs/¹³⁷Cs atom ratio (0.334 ± 0.005) (decay corrected to March 11, 2011), from old leaves of plants collected immediately after the FDNPP accident, were confirmed to represent the FDNPP derived radiocesium signature. Subsequently, for the first time, trace ¹³⁵Cs amounts before the FDNPP accident were deduced according to the contribution of global and FDNPP accident-derived fallout. Apart from two soil samples with a tiny global fallout contribution, contributions of global fallout radiocesium in other soil samples were observed to be 0.338%–52.6%. The obtained ¹³⁵Cs/¹³⁷Cs database will be useful for its application as a geochemical tracer in the future.

Six years after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, data for ²³⁶U and ²³⁶U/²³⁸U have remained limited to a few heavily contaminated samples. In the present study, activities of ²³⁶U, ²³⁹Pu, and ²⁴⁰Pu, along with other U isotopes in 46 soil samples both heavily and lightly contaminated by this accident were measured by inductively coupled plasma–mass spectrometry (ICP-MS) and triple-quadrupole ICP-MS. The ²³⁶U activities and ²³⁶U/²³⁸U atom ratios in these soil samples were in the range of (0.469–24.4) × 10⁻⁵ Bq kg⁻¹ and ((0.099–1.35) × 10⁻⁷), respectively. Higher ²⁴⁰Pu/²³⁹Pu atom ratios (0.245–0.312) and ²³⁸Pu/^239+240Pu activity ratios (0.859–1.62) indicated Pu contamination originated from this accident and global fallout in some samples. For those soil samples along with black substances collected along roads in Fukushima Prefecture, high linear correlations were presented between ²³⁶U activities and ^239+240Pu activities (Pearson’s r = 0.755, p < 0.01), and between ²³⁶U activities and ²³⁸Pu activities (Pearson’s r = 0.844, p < 0.01). The analysis of these soil samples confirmed the release of ²³⁶U, although in trace amounts, during the FDNPP accident.