FIRE AS AN AGENT IN REDISTRIBUTING FALLOUT CS-137 IN THE CANADIAN BOREAL FOREST

The presence of fallout Cs-137 in the boreal forest and the effect of fire in redistributing Cs-137 were studied in the remote region of Woo d Buffalo National Park, N.W.T., Canada, Results of a preliminary stud y of five burned (the fire occurred in 1981) and five unburned stands conducted in 1986 revealed that Cs-137 concentrations were higher in t he surface soil of the burned stands than in the unburned ones. In 198 9, a comprehensive study was conducted, in which one burned and one un burned white spruce stand were sampled in greater detail. The latter i nvestigation also revealed a difference in the distribution of Cs-137 within the burned stand compared to the unburned one. Specifically, in the unburned stand, the highest Cs-137 concentration was identified i n the epiphytic lichens and in the mosses, whereas in the burned stand , the highest concentration was measured in the surface organic soil. These results indicate that fire caused the mobilization of part of th e Cs-137 bound to the above-ground matter and concentrated it in the a sh layer of the burned surface soil. An additional ecologically import ant finding in our study was that significantly lower total Cs-137 loa d was observed in the burned stand compared to the unburned one. Hence , our data not only provide evidence that Cs-137 is being redistribute d within the burned stand to the surface soil, but also that part of t he Cs-137 is lost due to fire, presumably contaminating other ecosyste ms. Volatilization and fly-ash during the fire, and runoff (e.g. from snow melt) after the fire are the most likely mechanisms for the Cs-13 7 removal. These findings point to fire as an agent of Cs-137 secondar y contamination for initially unaffected systems, as well as for those previously contaminated.