Experimental wildfire induced mobility of radiocesium in a boreal forest environment

Abstract

Wildfires are expected to increase with warmer climate, which can contribute to the mobility and the resuspension of long-lived and potentially hazardous radionuclides. The release of ¹³⁷Cs during combustion of dried litter, forest floor organic soil, and peat was investigated in a small-scale experimental set-up. Combustion conditions were varied to simulate different wildfire scenarios, and the fuels were dried organic material collected in a boreal environment of Sweden that was contaminated following the Chernobyl accident in 1986. The combustion-related release of ¹³⁷Cs to the air was on average 29% of the initial fuel content, while 71% of the initial ¹³⁷Cs remained in the ashes after the combustion. Peat and forest soil had the highest releases (39% and 37%, respectively), although these numbers should be viewed as potential releases since authentic wildfire combustion of these fuels are usually less effective than observed in these experiments. These results indicates that the ¹³⁷Cs has migrated downwards in the organic material, which imply potentially significantly more ¹³⁷Cs emissions in severe wildfires with intense combustion of the organic vertical profile in peatbogs and forests. More ¹³⁷Cs tended to be released during intense and efficient combustion processes, although no significant differences among combustion intensities were observed. The generated experimental data was used in an emission scenario to investigate the possible range in ¹³⁷Cs emissions from a wildfire. Our study shows that a severe wildfire in a contaminated area of 10,000 ha could potentially release up to 7 TBq of ¹³⁷Cs. This is the first laboratory study to investigate ¹³⁷Cs release upon varying combustion conditions using real fallout contaminated organic material obtained from a boreal environment.

Keywords: Boreal forest environment; Combustion experiments; Radiocesium; Wildfire emissions.