The biogeochemistry of atmospherically derived Pb in the boreal forest of Sweden

Abstract

The use of stable Pb isotopes for tracing Pb contamination within the environment has strongly increased our understanding of the fate of airborne Pb contaminants within the boreal forest. This paper presents new stable Pb isotope (206Pb/207Pb ratio) measurements of solid soil samples, stream water (from a mire outlet and a stream draining a forest dominated catchment) and components of Picea abies (roots, needles and stemwood), and synthesizes some of the authors’ recent findings regarding the biogeochemistry of Pb within the boreal forest. The data clearly indicate that the biogeochemical cycling of Pb in the present-day boreal forest ecosystem is dominated by pollution Pb from atmospheric deposition. The 206Pb/207Pb ratios of the mor layer (O-horizon), forest plants and stream water (mainly between 1.14 and 1.20) are similar to atmospheric Pb pollution (1.14–1.19), while the local geogenic Pb of the mineral soil (C-horizon) has high ratios (>1.30). Roots and basal stemwood of the analyzed forest trees have higher 206Pb/207Pb ratios (1.15–1.30) than needles and apical stemwood (1.14–1.18), which indicate that the latter components are more dominated by pollution derived Pb. The low 206Pb/207Pb ratios of the mor layer suggest that the upward transport of Pb as a result of plant uptake is small (<0.04 mg m−2 a−1) in comparison to atmospheric inputs (∼0.5 mg m−2 a−1) and annual losses with percolating soil-water (∼2 mg m−2 a−1); consequently, the Pb levels in the mor layer are now decreasing while the pool of Pb in the mineral soil is increasing. Streams draining mires appear more strongly affected by pollution Pb than streams from forested catchments, as indicated by Pb concentrations about three times higher and lower 206Pb/207Pb ratios (1.16 ± 0.01 in comparison to 1.18 ± 0.02). To what extent stream water Pb levels will respond to the build-up of Pb in deeper mineral soil layers remains uncertain.