Mercury and lead profiles and burdens in soils of Quebec (Canada) before and after flooding

Abstract

Forest soils and their flooded equivalents were sampled in the Cabonga (southern Quebec) and La Grande (northern Quebec) hydroelectric complexes and analyzed at 1-cm intervals for organic C, N, Fe, Mn, Hg, and Pb. In the La Grande region, thin Cladina sp. podzols and thicker Pleurozium sp. podzols are characterized by lower Hg burdens (2500 and 5100 μg∙m−2, respectively) than the thicker but less dense soils typical of wetter environments (6000 to 8000 μg∙m−2). These differences 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 transport over 1000 km from south to north, away from the industrialized regions. Upon inundation, the oxidation–reduction potential drops rapidly but there was no noticeable decrease in the organic C burden, with the exception of flooded podzols where the surficial organic layer may undergo erosion. In flooded wetland soils, Hg, Pb, and Fe display a partial redistribution towards the sediment–water interface, but the Hg burdens of these soils remain comparable to those of their unflooded equivalents. In contrast, the surficial erosion of the flooded podzols may represent one pathway for Hg to enter organisms in the reservoir.