Wash-Off Effects in Urban Areas

Abstract

The reduction of the activity distributed in urban areas in three Austrian cities after a radioactive fall-out, by run-off and wash-off effects from stabilised surfaces and the resulting dose reduction to the population were investigated four years after the Chernobyl fall-out to predict the long-term external exposure of the population. The measurements were performed in cities with different fractions of dry and wet deposition after the Chernobyl accident in order to determine whether any differences in radionuclide removal with regard to wet and dry fall-out was observable. High resolution in situ gamma spectroscopy was employed to measure the gamma flux from 137Cs and 134Cs at points over stabilised surfaces, which was then compared with undisturbed grass surfaces. The average reduction of the plane activity on stabilised surfaces amounted to a factor of 10 ± 5 compared to the original deposition after the fall-out. Asphalt showed the highest reduction factor (11.4), concrete less (8.1), stone slabs and cobblestone only about (4.5) and gravel virtually no reduction (1.1). Only very little variation of this reduction with dry or wet deposition was observed.