SIR,

Vertical dispersion of Chernobyl fall-out on Kesselwandferner. Oetzlal Alps, Austria

In the accumulation area of Kesselwandferner (Oetztal Alps, Austria; 3250 m a.s.l.), snow samples were taken within an area of 10 000 m² in the years 19862013;88. We have measured gross ß-activity and gamma-activity of samples in vertical profiles contaminated by radioactive fall-out from the Chernobyl accident. Profiles in 1986 cover a depth range from the surface down to 1 m, in 1987 down to 6 m, and in 1988 down to 11m (Reference Ambach, Rehwald, Blumthaler, Eisner and BrunnerAmbach and others, 1987, 1988). This allows us to study the displacement of radioactive contamination in firn.

The depth range contaminated expands from the surface to 1 m (1986) to the range from 3 to 6 m (1987), and to the range from 5 to 11m (1988). The upper boundary of the contaminated depth range migrates to greater depths due to net accumulation. Peak values of gross ß-activity in vertical profiles are reduced by radioactive decay and by leaching out from 149Bq/kg (1986) to 47 Bq/kg (1988). The upper boundary of the contaminated range serves as a significant marker, as the activity changes from the low natural background to high values. The lower boundary of the contaminated depth range is less clearly defined due to percolating melt water.

Whereas in 1986 samples ⁹⁵Zr, ⁹⁵Nb, ¹⁰³Ru ¹⁰³/¹⁰⁶Ru/Rh, ^110mAg, ¹²⁵Sb, ¹³⁴Cs, ¹³⁷Cs, and ¹⁴⁴Ce, could be detected by gamma spectroscopy, in 1988 only traces of ¹¹⁰Ag, ¹⁰⁶Rh, and ¹²⁵Sb were detectable without significance. The main gamma-activity results from ¹³⁴Cs and ¹³⁷Cs contributing 13 and 87% to total gamma-activity.