Environmental effects of ash recycling on the incidence of 137Cs in vegetation and soil. Final report

Recycling of wood-ash from combustion of bio-fuel could in the future become a common practice in Swedish forestry. As a consequence of the Chernobyl radioactive fallout in 1986 large areas in central Sweden became contaminated. There is a risk that addition of wood-ash originating from these contaminated areas will increase the activity of 137Cs in a forests if added to a non-contaminated area. We report here of measurements of 137Cs activity in different coniferous forest compartments from seven field experiments. The activity measurements were made 5-8 years after wood-ash application. The field experiments, in a north-south transect in Sweden, have received a background activity ranging 0 - 40 kBq m-2 as a consequence of the Chernobyl fallout. The applied wood-ash had an activity ranging between 0 - 4.8 kBq kg-1 depending on origin. Samples of the various compartments, taken in autumn 1999, included samples of soil, field vegetation, needles and wood (both twigs and stem wood). The soil and the field-vegetation were analysed using a NaI detector while the samples obtained from the trees were analysed on a GeLi detector. In addition, the soil samples were also analysed for K, by extraction with 1M ammonium lactate. The highest 137Cs activity concentration was, by far, found in the soil, the activity found in the field-layer and the trees was in general lower. At six of seven sites there were no statistically significant effects of the wood-ash application on radiation, despite the fact that the wood-ash had, at least theoretically, in one case doubled the activity. However, at one site (Vindeln), with an intermediate 137Cs deposition (10-20 kBq m-2) from Chernobyl, there was a statistically significant decrease in 137Cs activity in the soil as well as in needles and twigs at the wood-ash amended plots. This occurred despite that the wood-ash had added a considerable activity (0.63 kBq m-2) to the site. The same ash applied to another site also decreased the activity, but in that case only in the H-layer. Some of the effects could be explained by the fact that the wood-ash application in some cases had decreased the weight of the organic horizon. The decrease in activity is most likely due to the fact that one of the main constituents of wood-ash is K, an ion that is antagonistic to 137Cs. At present, 5-8 years after the wood-ash application, a difference in K content between control and wood-ash amended plots was however detected only in one case. The general absence of an effect is not unlikely since K is a highly mobile ion in soil. To summarize, our results show that an application of 137Cs containing wood-ash does not increase the 137Cs activity of the plants