Formation of radioactivity enriched soils in mountain areas

A field study was carried out in the Mercantour Mountains at 2200 m altitude to investigate the processes of soil enrichment in atmospheric Chernobyl 137Cs. Soils with high 137Cs activities have been collected in the pasture areas with frequently measured 137Cs activity values of the order of 7000 Bq m−2. At some single spots (about 6% of the studied area), activity in soils reached 300 000 Bq m−2, which represents 44% of the 137Cs of the total area. Data further showed that spatial distribution of Cs depends widely on its origin: Chernobyl Cs is mainly concentrated in “enriched” soils, whereas older Cs and 241Am fallout from nuclear weapons tests (NWTs) and natural atmospheric 210Pb in soils is less heterogeneously distributed. In order to elucidate the processes which have led to the enrichment in Chernobyl 137Cs in the Alps in May of 1986, we have studied the repartition of atmospheric 7Be isotope (half-life=53.3 d) in the pasture compartments (soil, litter, grass, and snow). Snow 7Be data give evidence that fallout enrichment is related to snow accumulation (snow drift). The transfer of beryllium occurs rapidly to the grass and litter, where the strongest pollutant accumulations were measured. However, 7Be transport to the soil required more than 8 months.