The fate of 137Cs in coniferous forests following the application of wood-ash

Abstract

In the future, it may become common practice in Swedish forestry to recycle wood-ash, a waste product of the combustion of bio-fuel. As a consequence of the Chernobyl radioactive fallout in 1986, large areas of central Sweden were contaminated. Application of recycled wood-ash, originating from contaminated areas, to a previously uncontaminated forest, risks an increase in the concentration of radioactive 137Cs. We measured 137Cs radioactivity in different parts of coniferous forests in seven field experiments. Measurements of radioactivity were made 5–8 years after an application of wood-ash equivalent to 3000 kg ha −1. The sites, in a north–south transect across Sweden, have a background radioactivity ranging from 0 to 40 kBq m −2, the higher levels are mainly a result of the Chernobyl fall-out. Depending on its origin, the radioactivity of the applied wood-ash ranged from 0.0 to 4.8 kBq kg −1, corresponding to 0.0–1.44 kBq m −2. In autumn 1999, samples were taken from the soil, field vegetation, needles and twigs and the levels of 137Cs determined. In addition, soil samples were analysed for extractable K. The highest 137Cs concentration was found in the soil. At six of the seven sites there were no statistically significant effects of wood-ash application on 137Cs activity. This was despite the fact that the wood-ash had, in one case, added the same amount of radioactivity as the background. However, at one site with intermediate 137Cs deposition (10–20 kBq m −2), there was a statistically significant decrease in 137Cs radioactivity in the soil, needles and twigs from the plots treated with wood-ash. The decrease in radioactivity was partly due to the fact that one of the main constituents of wood-ash is K, which is antagonistic to 137Cs. Based on our results, it appears that application of wood-ash containing 137Cs does not necessarily increase the 137Cs radioactivity in plants and soil. However, some of the observed effects could be a result of the low number of replicates used in this study.