236U and radiocesium in river bank soil and river sediment in Fukushima Prefecture, after the Fukushima Daiichi Nuclear Power Plant accident

Abstract

Almost 8 years after the Japanese Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, data for ²³⁶U and ²³⁶U/²³⁸U have mainly remained limited to only a few heavily contaminated samples. In the present study, activities of ²³⁶U, ¹³⁴Cs, and ¹³⁷Cs, along with ²³⁴U, ²³⁵U, ²³⁸U, in 15 river bank soil and 10 river sediment samples, were measured by ICP-MS/MS and γ spectrometry. The ¹³⁴Cs activities and ¹³⁴Cs/¹³⁷Cs activity ratios (decay-corrected to March 11, 2011) in these 15 river bank soil samples were from 74.8 to 3.88 × 10⁵ Bq kg^-1 and from 0.944 to 1.02, respectively; and in these 10 river sediment samples were from 87.1 to 1.86 × 10⁵ Bq kg^-1 and from 0.904 to 0.990, respectively. The ²³⁶U activities and ²³⁶U/²³⁸U atom ratios in these soil samples were in the respective ranges of (0.139-17.6) × 10^-5 Bq kg^-1 and (0.259-3.83) × 10^-8; and in these sediment samples were in the respective ranges of (0.884-27.0) × 10^-5 Bq kg^-1 and (1.12-5.04) × 10^-8. For one river sediment core sample, ¹³⁴Cs and ²³⁶U activities decreased with the depth indicating ¹³⁴Cs and ²³⁶U accumulated in the river sediment with time. Unlike ¹³⁴Cs, no clear evidence of FDNPP accident-derived ²³⁶U has been found in this study, although further monitoring is encouraged to establish the background database on ²³⁶U/²³⁸U for its potential application as a tracer in environmental studies.

Keywords: (134)Cs/(137)Cs; (236)U/(238)U; ICP-MS/MS; River system; Source identification.