MERCURY AND LEAD PROFILES AND BURDENS IN SOILS OF QUEBEC (CANADA) BEFORE AND AFTER FLOODING

Forest soils and their flooded equivalents were sampled in the Cabonga (southern Quebec) and La Grande (northern Quebec) hydroelectric compl exes and analyzed at I-cm intervals for organic C, N, Fe, Mn, Hg, and Pb. In the La Grande region, thin Cladina sp. podzols and thicker Pleu rozium sp. podzols are characterized by lower Hg burdens (2500 and 510 0 mu g . m(-2), respectively) than the thicker but less dense soils ty pical of wetter environments (6000 to 8000 mu g . m(-2)). These differ ences suggest that a fraction of Hg is leached downslope from uplands (podzols) and partially accumulates in poorly drained soils (wetlands) below. Unlike Pb, Hg showed no apparent decrease in atmospheric trans port over 1000 km from south to north, away from the industrialized re gions. Upon inundation, the oxidation-reduction potential drops rapidl y but there was no noticeable decrease in the organic C burden, with t he exception of flooded podzols where the surficial organic layer may undergo erosion. In flooded wetland soils, Hg, Pb, and Fe display a pa rtial redistribution towards the sediment-water interface, but the Hg burdens of these soils remain comparable to those of their unflooded e quivalents. In contrast, the surficial erosion of the flooded podzols may represent one pathway for Hg to enter organisms in the reservoir.