Distribution of (210)Pb and (210)Po in boreal forest soil

Abstract

Vertical distribution and activity contents of (210)Pb and (210)Po were investigated in forest soils of Scots pine-dominated (Pinus sylvestris L.) stands from seven different locations in Finland. The mean total inventory in the soil profile, up to 20cm, of (210)Pb was 4.0kBqm(-2) (range3.1-5.0kBqm(-2)) and (210)Po 5.5kBqm(-2) (range 4.0-7.4kBqm(-2)), the organic soil layer containing 45% of the total inventory of both nuclides. In both the organic and the mineral layers the (210)Po/(210)Pb ratio was close to unity indicating a radioactive equilibrium between them. In the litter layer there was, however, a clear excess of (210)Po suggesting that polonium is recycled via root uptake from the root zone to the ground surface. The activity concentration (Bqkg(-1)) of (210)Pb clearly correlated with organic matter and the Fe, Al and Mn concentrations in soil indicating that radioactive lead is associated both with humic substances and the oxides of iron, aluminium and manganese. Radioactive lead was also seen to follow the behavior of stable lead. No systematic correlation between polonium and soil properties was seen.