Preliminary observations on the spatial variability of fallout Caesium-137 at reference sites in Hong Kong

Abstract

Caesium-137 (¹³⁷Cs) tracing of sediment movement depends substantially on a valid assessment of ¹³⁷Cs reference values. Recent research suggests that ¹³⁷Cs is more spatially varied than previously assumed. The range of ¹³⁷Cs values at and between reference sites, and environmental controls on ¹³⁷Cs behaviour are prime topics for study. The present research investigates reference sites in the monsoonal tropics of Hong Kong. There was doubt about the use of ¹³⁷Cs in such areas, sustained by the paucity of ¹³⁷Cs studies in the Tropics and the potential lack of ¹³⁷Cs here due to global air circulation patterns. This paper shows that the technique is applicable in the Hong Kong environment. As rainfall controls ¹³⁷Cs fallout, a large range of mean annual rainfall within a small area makes Hong Kong a good location to test ¹³⁷Cs variability. the profile reflecting ¹³⁷Cs movement in the soil. The results illustrate the difficulties of using a simple model of profile distribution as confirmation of a site’s stability. They also confirm the difficulty of finding sites that have experienced no significant disturbance in the past four decades. Ten core samples and the profile point value were used to assess ¹³⁷Cs variability across the input sites. These data permit an evaluation of sampling methodology. The present results suggest that a large sample number is required to express the mean input value at a site. If this is not to impede routine use of the ¹³⁷Cs technique, further investigations of sampling methodology are needed.