Quantifying spatial distribution of 137Cs in reference site soil in Asia

Fallout radiocaesium (137Cs) has been used to trace soil migration (i.e., by erosion and redistribution) and soil-to-crop transfer levels of radiocaesium in agricultural fields. For soil erosion studies, reference site settings (initial 137Cs inventory, Bq m−2) are critical. Typically, non-disturbed and flat grasslands have been selected as reference sites. However, it will be difficult to justify the 137Cs concentrations for these reference sites in the future because already almost 50 years have passed since the heaviest annual global fallout was reported and 137Cs has been redistributed in some of the selected reference sites. It would be helpful if empirically derived initial 137Cs inventory data were available for the sites for comparison. In this study, we focused on areas in Asia and northern Australia where the major source of 137Cs is global fallout. We carried out a literature survey, and a total of 83 inventory datasets of global fallout 137Cs were collated for those areas in the region extending from 72.85° E to 141.35° E and from 12.43° S to 49.19° N. These data included global fallout deposition data in Japan obtained before the Fukushima Daiichi Nuclear Power Plant accident. We found a good correlation between the logarithm of global fallout 137Cs concentrations at reference sites and latitudes (R = 0.805, p < 0.001). From the results, we concluded that in the future it will be difficult to measure 137Cs in the low latitude areas (20° S to 20° N) due to its low concentrations in soil; rather future studies using ultra-low background gamma spectrometry detectors installed in underground laboratories would be necessary.