Temporal variation and provenance of thorium deposition observed at Tsukuba, Japan

Abstract

Temporal variations of monthly thorium (Th) deposition observed at Tsukuba, Japan during the period 1990–2007, comparing with plutonium deposition, was studied. The monthly 232Th deposition as did 239,240Pu, varied according to season and inter-annually. In particular, 232Th deposition increased significantly in spring coinciding with Asian dust (Kosa) events. The 230Th/232Th activity ratios vary according to sources and can therefore be used to differentiate between locally derived and remotely derived 232Th. The 230Th/232Th activity ratios in deposition samples showed large variability with high ratios occurring in early spring. These high 230Th/232Th ratios in deposition can be attributed to local dust storms, especially in early spring, that cause resuspension of soils from cultivated fields which are characterized by high 230Th/232Th activity ratios. The results reveal that both locally and remotely derived 232Th deposition showed seasonal variations with maxima in spring, although the remotely derived fraction is dominant rather than the locally derived one. The 232Th deposition maxima later in spring is attributable to the remotely derived fraction, corresponding to the Kosa events. Annual 232Th deposition exhibited an increasing trend, suggesting the presence of sources other than soil dust such as fly ash from increasing coal burning.