A novel approach to monitoring the Cs-137 contamination of forest soils in Bavaria, Germany

Latest incidents in Japan prove that nuclear accidents are still a hazardous threat to ecosystems worldwide. Regarding the radioactive fission product Cs-137, particularly forest soils are hot spots of long-term contamination. In the case of forest-rich Bavaria, one of the German states affected most severely by Chernobyl-born radionuclide deposition in 1986, this topic is of high relevance. Particular considerations also include the development of specific monitoring approaches to effectively determine and evaluate the distribution of Cs-137 in forests. Numerous previous studies have indicated that monitoring should take special account of the role of humus for Cs-137 mobility and bioavailability in this context. Official programmes in Bavaria, however, lack such considerations concerning the controlling influence of humus. Hence, a new monitoring project has been established across the Bavarian state forests, together with the Bavarian State Ministry of the Environment and Public Health, focussing on humus-controlling landscape parameters as crucial core criteria. The concept design divides state forests into several units of distinctive landscape types and subjects each of them to a strict filtering scheme in order to create a consistent selection of deciduous/coniferous as well as steep/flat monitoring sites in representative altitudes. Site area varies with altitude (low/high mountains: 30 × 30 m; lowlands: 50 × 50 m). Additional filters further consider site homogeneity, proximity to other official monitoring instruments, information supply, turbation, accessibility and regular distribution. This paper explains the transfer of these standards into a GIS-based application routine and discusses its feasibility. Besides, the established monitoring sites and first results on current Cs-137 loads are presented. In summary, a total of 48 monitoring sites were established (48 soil profiles, 96 soil monoliths, 96 composite samples). The GIS interface proved to be a helpful tool in this context. The presented monitoring succeeds in providing a comprehensive picture of the broad range of Cs-137 activities throughout Bavarian forest soils. Composite samples, taken according to official AVV-IMIS standards (∼ 30 random samples per site conjoined to one humus and mineral composite each), show that the average specific Cs-137 activity within the uppermost ∼ 30 cm varies between 31.78-1214.00 Bq kg^-1(humus) and 5.68-232.00 Bq kg^-1 (mineral soil). The results highlight characteristic spatial patterns and demonstrate that the specific Cs-137 activity in humus is still comparatively high. In all, the established data base provides a solid basis for further investigations on the driving factors of the vertical Cs-137 distribution in forest soils as well as on questions of bioavailability and effective soil-to-plant transfer. Beyond that, the presented approach can as well be transferred to any other forest soil contaminant of interest.