Mercury remobilization in Saguenay Fjord (Quebec, Canada) sediments: Insights following a mass-flow event and its capping efficiency

A mass-flow event triggered by the 1996 flood in the Saguenay region buried the mercury-contaminated indigenous sediments at the head of the Saguenay Fjord under up to 50 cm of postglacial deltaic sediments. The vertical distributions of total mercury and methyl-mercury in the sediments and pore waters were measured in box cores recovered from the Saguenay Fjord within and outside the affected area prior to and on six consecutive years after the flood. The total solid mercury (THgs) profiles show that remobilization was limited and most of the mercury remobilized from the contaminated, indigenous sediments was trapped below or slightly above the former sediment-water interface by authigenic acid-volatile sulfides (AVS). Nonetheless, a small fraction of the remobilized mercury diffused into the flood layer, some of it was methylated and/or scavenged by organic matter and AVS. Elevated solid-phase methyl-mercury concentrations, [MeHgs], at depth in the sediment are correlated to peak AVS and THgs but, in the absence of elevated dissolved methyl-mercury concentrations, [MeHgd], the higher [MeHgs] may reflect an earlier episode of Hg methylation, the product of which was scavenged by the AVS and buried. Throughout the sediment cores, sediment-water partitioning of MeHg and Hg(II) appears to be controlled in great part by the AVS and residual organic matter content of the sediment.