Reduction of external exposure from deposited Chernobyl activity by run-off, weathering, street cleaning and migration in the soil

Relatively high concentrations of radionuclides from Chernobyl deposited with a heavy shower of rain in Southern Bavaria made it possible to measure the reduction of external exposures by run-off, weathering, street cleaning and migration in the soil. In situ spectroscopy showed that within the first few days about 60% of the caesium was removed from asphalt, concrete and granite pavements by run-off and street cleaning. About 70% of the rest disappeared from the urban surfaces with a half-life of about 80 days. Contamination on roofs was found to be higher than on streets. It is shown that the widely used analytical approximation for the reduction of the dose rate due to the migration of caesium in the soil under-estimates the exposure. (author)