INFILTRATION OF RADIONUCLIDES WITH LOW MOBILITY (CS-137 AND CO-60) INTO A FOREST SOIL - EFFECT OF THE IRRIGATION INTENSITY

To obtain information on the effect of the rain intensity during the wet deposition of radionuclides we investigated the infiltration of Cs-137 Co-60 (as chlorides) into a forest soil using undisturbed, unsaturated soil columns (30 cm length, 20 cm in diameter; sandy podsol under pine). The depth profiles of the activities were measured as a function of time with a scanner technique. The total amount of irrigation was always 14 mm per rain event, its intensity was adjusted with a rain simulator to 2 mm/h (slight to moderate rain) or 30 mm/h (heavy rain shower). At the low rain intensity the organic top layer of the soil (0-4 cm) acted as a very effective sink for Cs-137 and Co-60. At the high rainfall intensity, however, only 70% of Cs-137 was retained by the organic layer, while the rest penetrated to a depth of 14 cm. Co-60 which is bound more strongly and rapidly by humic substances was retained more effectively by the organic layer (c.90%). In the mineral horizon of the soil, the two radionuclides exhibited the opposite behaviour. Cs-137 was present there in a rather immobile state, while Co-60 was at least on a short-term scale rather mobile, and consequently found to a comparatively large extent even in the effluent of the column. The above observations demonstrate that the sorption properties of a radionuclide have even at high rainfall intensities a considerable effect on its infiltration in the soil.