Mechanisms of 137Cs migration in coniferous forest soils

Field and laboratory studies demonstrate two processes involved in Cs-137 migration in coniferous forest soil. The data illustrate that the effects of leaching and litter accumulation combine, over the time interval since deposition, to achieve the observed distribution of Cs-137 in the soil profile. Today, under normal field conditions the rate of Cs-137 leaching from forest humus is low and even extreme environmental conditions do not increase greatly the extent of Cs-137 mobilisation by leaching. A three-phase model of Cs-137 migration in the organic horizons of coniferous forest soils is proposed. The phases are discrete but at any point in time the behaviour of some portion of deposited Cs-137 Will be characteristic of one of the three phases. In the first phase Cs-137 intercepted by the canopy is washed to the forest floor where a proportion (20-40% is reported) rapidly percolates through the organic horizons reaching the underlying soil; the remainder is retained in the organic horizons. In the second phase, which our study estimates to last about 5 years, the Cs-137 contained within the Ol and Of horizons moves to the Oh horizon. Accumulation of relatively uncontaminated litter on top of the Ol horizon contributes to this migration, while Cs-137 mobilised from the Ol and Of horizons by decomposition is leached to the Oh horizon. The Cs-137 which reaches the Oh horizon, either by leaching or by decomposition and burial, enters the third phase in which the pace of migration is greatly reduced. Decomposition is slowest in the Oh horizon and the accumulation and turnover of Oh material is also very slow. At the site investigated in this study the bulk of the deposited Cs-137 is in this third phase and we suggest that this phase will persist for many years to come. (C) 2000 Elsevier Science Ltd. All rights reserved.