137Cs reference site characteristics in Hong Kong - some considerations

Abstract

The distribution of 137Cs is being assessed at input reference sites in Hong Kong. This work suggests that, in certain landscapes, difficulties in identifying potential reference sites and subsequently confirming site stability exert some constraints on 137Cs measurement programmes. While the value of guidelines on site and sample selection has been reaffirmed, concern has emerged about the vertical distributions that may be taken to indicate reference site stability. The decline of 137Cs concentrations down a reference soil profile are often reported to be exponential. Recently, a more complex profile with depleted 137Cs totals at the soil surface has been reported and discussed. Thus there is no clear standard for a reference profile, though the profile has been the only empirical control on a reference site's suitability after selection. This study presents the depth distribution and spatial variation of 137Cs areal activities at three reference sites in Hong Kong. The three sites have different spatial variability and different depth distributions, and serve to illustrate the difficulty of using vertical depth distributions as a confirmation of site stability. One possible independent check is the expected 137Cs total from rainfall records; another is numerical modelling of the predicted profile given the physical and chemical controls on soil 137Cs in reference site profiles.