Radioactive Fallout and Accumulated Level of Soil Surface Contamination with 90Sr and 137Cs around St. Petersburg in 1954-1999

Abstract

Systematic data on radioactive atmospheric fallout in 1954-1999 and accumulated levels of contamination of the soil-vegetable cover with ⁹⁰Sr and ¹³⁷Cs around St. Petersburg, estimated on 1998, are presented. Comparative analysis is made of these data and directly obtained data on the vertical distribution of these radionuclides in the surface horizon (0-25 cm) of soddy-podzolic sandy loam soil: the levels of surface contamination with ^90Sr and ¹³⁷Cs were found to be 580±90 and 6500±600 Bq m^-2 on 1998. Before April 28, 1986 the ⁹⁰Sr and ¹³⁷Cs radioactivity in the investigated region corresponded to the global level. After Chernobyl accident the contamination levels with ⁹⁰Sr and ¹³⁷Cs increased, respectively, from 1200 to 1280 (by 6.3%) and from 2400 to 7050 Bq m^-2 (by a factor of 2.9). The ⁹⁰Sr and ¹³⁷Cs vertical distributions throughout the soil profile are considerably different: ¹³⁷Cs is practically totally fixed in the surface horizon, its activity being exponentially declining; whereas the stock of ⁹⁰Sr in horizons 0-10 and 0-25 cm represents only 22 and 59% of the integral accumulated inventory of ⁹⁰Sr in the soil (990 Bq m^{- 2}). The activity ratio of ¹³⁷Cs to ⁹⁰Sr in the atmospheric fallout in 1963-1985 was 1.85±0.42, and in river water in the same period, ca. 0.18, suggesting higher mobility in the soil of ⁹⁰Sr as compared to ¹³⁷Cs, and, therefore, higher rate of self-cleaning of the surface soil horizon with respect to ⁹⁰Sr.