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

The use of stable Pb isotopes for tracing Pb contamination within the environment has strongly increased our understanding of the fate of airborne Ph contaminants within the boreal forest. This paper presents new stable Pb isotope (Pb-206/Pb-207 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 Ph 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 Pb-206/Pb-207 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 Pb-206/Pb-207 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 Pb-206/Pb-207 ratios of the mor layer suggest that the upward transport of Ph as a result of plant uptake is small (<0.04 mg m(-2) a(-1)) in comparison to atmospheric inputs (similar to 0.5 mg m(-2) a(-1)) and annual losses with percolating soil-water (similar to 2 mg m(-2) a(-1)); consequently, the Ph 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 Ph than streams from forested catchments, as indicated by Pb concentrations about three times higher and lower Pb-206/Pb-207 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 Ph in deeper mineral soil layers remains uncertain. (c) 2008 Elsevier Ltd. All rights reserved.