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 behavior 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. Three input sites representing a wide range of rainfall totals are reported here. The uneroded status of an input site is tested by producing a profile of 137Cs totals in 2 cm increments; the form of 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.