SUMMER OUTDOOR RADON VARIATIONS IN CANADA AND THEIR RELATION TO SOIL-MOISTURE

In the summers of 1990 and 1991, outdoor Rn-222 detectors were installed for 3 mo in 78 communities across Canada. The 1990 measurements showed large regional variations with the average outdoor levels for the provinces of Manitoba (59 Bq m(-3)) and Saskatchewan (61 Bq m(-3)) exceeding the average indoor levels of 55 Bq m(-3) for the U.S. The variations in the 1990 Rn-222 levels showed little correlation with the uranium concentration of the ground and were attributed to changes in soil moisture content. All measurements above 30 Bq m(-3) were found to be in dry climatic regions where the annual precipitation was less than 550 mm. In addition, most of the sites with high Rn-222 levels were on glacial lake clays. A soil moisture modeling program showed that because of the low precipitation, these clays were drying out in the summer allowing fractures to develop and Rn-222 to migrate easily through the ground. The much lower 1991 measurements for Manitoba (10 Bq m(-3)) and Saskatchewan (15 Bq m(-3)) were believed to be due to the increased precipitation in the prairie regions in the summer of 1991 compared to the unusually dry summer of 1990.