Natural lead concentrations in pristine boreal forest soils and past pollution trends: A reference for critical load models

Knowledge of natural, prepollution concentrations of heavy metals in forest soils and temporal trends of soil pollution are essential for understandin g present-day pollution (ecotoxicological assessments) and for establishing realistic goals for reductions of atmospheric pollution deposition (critic al loads). Soils not exposed to deposition of atmospheric pollution no long er exist and, for example, present lead (Pb) pollution conditions in northe rn European soils are a consequence of nearly 4,000 years of atmospheric po llution. We use analyses of Pb concentrations and stable Pb isotopes (Pb-20 6/Pb-207 ratios) Of ombrotrophic peat and forest soils from southern Sweden and a model for Pb cycling in forest soils to derive an estimate for the p repollution concentration of Ph in the mor layer of boreal forest soils and to back-calculate Pb concentrations for the last 5,500 years. While the pr esent-day concentrations of the mor layer are typically 40-100 mu g g(-1) ( 0.25-1.0 g m(-2)), Pb concentrations of pristine forest mor layers in Swede n were quite low, less than or equal to 0.1 mu g g(-1) (less than or equal to 1 mg m(-2)). Large-scale atmospheric pollution from the Greek and Roman cultures (ca. 0 AO) increased Pb concentrations to about 1 mu g g(-1). Lead (Pb) concentrations increased to about 4 mu g g(-1) following the increase of metal production and atmospheric pollution in Medieval Europe (ca. 1000 AD).