Distribution of 137Cs in water leachates of forest humus

The distribution of 137Cs in water extracts of organic layers of forest soils was investigated using an ultrafiltration method. Samples were taken from two sites in the Bavarian Alps. The different horizons of the organic layers were extracted by column elution with water and 50 mmol/litre CsCl solution. Water extracts were fractionated using membranes with the molecular weight cutoff of 500, 1000 and 3000 Dalton to determine the molecular weight distribution of dissolved organic carbon (DOC). The activity of 137Cs associated with the different DOC-fractions was measured by direct gammaspectrometry. Cesium-137 in the fraction smaller than 500 Dalton was defined as ‘ionic’. The results indicated a change of the binding sites depending on the degree of decomposition and humification in the profile. Organic bound 137Cs was identified in all extracts. High mobilities of DOC and 137Cs were observed in the same horizons, fractionation showed then low-molecular-weight DOC and 137Cs mainly in ionic form. In horizons with high-molecular-weight DOC the portion of organic bound 137Cs ranged up to 40%.