A study of cycling of 90Sr in a natural forest on the Canadian shield

Abstract

A study of the cycling of 90Sr in a natural forest growing over a shallow zone of low-level 90Sr-contaminated groundwater was carried out at the Chalk River Laboratories of AECL Research. The contamination is about 1 m below the surface under most of the site and evidence from earlier studies indicates that 90Sr is being brought to the surface by deeper rooting trees. Once it is on the surface, the 90Sr becomes available for uptake by other biota in the area. The study was initiated by taking an inventory of 90Sr in various compartments including: soil, leaf litter, wood and foliage. Samples of leaf fall, stemflow and throughfall were also collected in order to estimate the relative importance of these mechanisms in cycling 90Sr to the forest floor. Most of the 90Sr on or near the surface is in the leaf litter and the top 8 cm of soil, as well as in the vegetation. Downward migration of 90Sr through the unsaturated zone is slow. This may be due to strong retention by the soil; however, it could also be due to recycling of 90Sr by the vegetation. The accumulated inventory in the leaf litter is about equal to the input from leaf fall over a period of 3 years.