THE ORIGIN AND DYNAMICS OF CS-137 DISCHARGE FROM A CONIFEROUS FOREST CATCHMENT

The turnover of radioactive caesium was studied experimentally and the oretically in a forested catchment that was covered by snow during the wet deposition of radioactive nuclides from the Chernobyl accident. T he study spans from 1 week before to 8 years after the deposition even t. A fraction of the catchment is covered by a mire (16%). From the ed ge of the mire a stream channel runs to the outlet of the catchment. T wo phases of decreasing activity concentration in the stream water wer e found in addition to a positive influence of runoff on the activity concentration in the stream. The half-lives for Cs-137 in the stream w ater corresponding to the early and the later phase were estimated by non-linear regression to be 6.5 days and 4 years, respectively. During the first phase, which corresponded to the initial snow melt in 1986, 6.8% of the deposition was lost from the catchment, whereas the slow secondary loss during the following 8 years was 1.8%. The main contrib ution to the yearly discharge of Cs-137 occurred during spring and aut umn when the areal contribution to saturated surface runoff was highes t. The remaining deposition in soil a few years after the fallout was significantly lower in the mire than in the surrounding forest. By usi ng the calculated activity concentration of Cs-137 in the stream water together with the remaining deposition in the different biotopes and information on stream flow for the catchment it was concluded that the loss originated from the mire. During the initial phase 44% of the de position was lost from the mire, and during the following years the ye arly loss was 30% from the fraction that constantly undergoes saturate d surface runoff and 2% from the drier fractions of the mire. Until th e end of the study it was not possible to demonstrate any loss from th e recharge areas (podzol and cambisol soils), which means that physica l decay will govern the decrease in activity in these areas. (C) 1997 Elsevier Science B.V.