Fire as an agent in redistributing fallout super(1) super(3) super(7) Cs in the Canadian boreal forest

The presence of fallout super(1) super(3) super(7) Cs in the boreal forest and the effect of fire in redistriburting super(1) super(3) super(7) Cs were studied in the remote region of Wood Buffalo National Park, N.W.T., Canada. Results of a preliminary study of five burned (the fire occurred in 1981) and five unburned stands conductd in 1986 revealed that super(1) super(3) super(7) Cs concentrations were higher in the surface soil of the burned stands than in the unburned ones. In 1989, a comprehensive study was conducted, in which one burned and one unburned white spruce stand were sampled in greater detail. The latter investigation also revealed a difference in the distribution of super(1) super(3) super(7) Cs within the burned stand compared to the unburned one. Specifically, in the unburned stand, the highest super(1) super(3) super(7) Cs concentration was identified in 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 as part of the super(1) super(3) super(7) Cs bound to the above-ground matter and concentrated it in the ash layer of the burned surface soil. An additional ecologically important finding in our study was that significantly lower total super(1) super(3) super(7) Cs load was observed in the burned stand compared to the unburned one. Hence, our data not only provide evidence that super(1) super(3) super(7) Cs is being redistributed within the burned stand to the surface soil, but also that part of the super(1) super(3) super(7) Cs is lost due to fire, presumably contaminating other ecosystems. Volatilization and fly-ash during the fire, and runoff (e.g. from snow melt) after the fire are the most likely mechanisms for the super(1) super(3) super(7) Cs removal. These findings point to fire as an agent of super(1) super(3) super(7) Cs secondary contamination for initially unaffected systems, as well as for those previously contaminated.