Fire as an agent in redistributing fallout137Cs in the Canadian boreal forest

The presence of fallout137Cs in the boreal forest and the effect of fire in redistributing137Cs 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 conducted in 1986 revealed that137Cs 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 of137Cs within the burned stand compared to the unburned one. Specifically, in the unburned stand, the highest137Cs 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 of part of the137Cs 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 total137Cs load was observed in the burned stand compared to the unburned one. Hence, our data not only provide evidence that137Cs is being redistributed within the burned stand to the surface soil, but also that part of the137Cs 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 the137Cs removal. These findings point to fire as an agent of137Cs secondary contamination for initially unaffected systems, as well as for those previously contaminated.