The Origin and Behavior of a Flood Capping Layer Deposited on Contaminated Sediments of the Saguenay Fjord (Quebec)

Abstract

The upper section of the Saguenay Fjord was impacted by a catastrophic flood in July 1996. Contaminated sediments were capped by a layer of clean silty post-glacial sediments with background levels of trace metals and polycyclic aromatic hydrocarbons (PAHs). The capping layer was characterized by geotechnical and geochemical methods and its biological recolonization was monitored by annual sampling of the macrofauna. The strong dominance of surface deposit feeders Cirratulidae and Ampharetidae was observed at most stations in the first 2–3 years followed by carnivorous annelids such as Lumbrineridae and Nephtidae species indicating a well recolonized benthic habitat in the Baie des Ha!Ha!. The presence of benthic fauna was a major factor in modifying the density of sediments by physical mixing and irrigation, and in changing the surface roughness. The slope stability of the capping layer is considered as very good except in limited deltaic sectors at the head of the Baie des Ha!Ha! The new layer showed a good efficiency to isolate contaminated sediments from the sediment/water interface. Although manganese and iron were remobilized as the new layer became anoxic, mercury, arsenic and PAHs showed a very limited mobility through the flood layer which allowed geochemists to calculate the present fluxes of toxicants to the Baie des Ha!Ha! without interferences from older contaminated sediments trapped below the flood layer. The present flux of PAHs to bay (0.9 ng.cm-2.g-1) is about 300 times lower than the 1974 flux and 8 times lower than the 1986 flux.

Highly persistent butyltins in northern marine sediments: A long‐term threat for the Saguenay Fjord (Canada)

“…As much as 50 cm of postglacial clays were deposited, which buried the old contaminated sediment and, thus, provided a flood capping layer that benthic species rapidly recolonized [11]. Ha!…”

“…in only a few hours. As much as 50 cm of postglacial clays were deposited, which buried the old contaminated sediment and, thus, provided a flood capping layer that benthic species rapidly recolonized [11]. This event offered a unique opportunity to measure the possible diffusion of TBT from the previously contaminated sediment to the new sediment layer and to assess the very recent input of TBT from shipping activities.…”

Présence des hydrocarbures aromatiques polycycliques (HAP) dans les compartiments biotiques et abiotiques de la rivière et du fjord du Saguenay

“…Ha! (station 9), où la crue éclair de 1996 a apporté une épaisse couche de nouveaux sédiments d'origine terrigène recouvrant entièrement l'ancien lit sédimentaire (PELLETIER et al, 1999(PELLETIER et al, , 2003. Tous les échantillons biologiques furent entreposés à -20 o C dans des pots de verre pré-nettoyés à l'acétone/hexane jusqu'au moment de l'analyse.…”

“…(PELLETIER et al, 1999;VIGLINO et al, 2004). Le pic de HAP a été enfoui sous cette couche de sédiments propres et semble y être stable à long terme (PELLETIER et al, 2003) En résumé, on constate que les concentrations de HAP dans les sédiments de surface en 2002 sont de 10 à 30 fois plus 1 Identification des sites correspondants dans l'étude de MARTEL et al (1987) faibles que celles publiées par MARTEL et al (1987) pour des échantillons de 1982 (Tableau 2). En assumant que les taux de sédimentation n'ont pas changé, on peut estimer que les apports actuels en HAP aux sédiments de surface du fjord sont aussi 10 à 30 fois inférieurs à ceux calculés par MARTEL et al (1987) pour 1980, soit de l'ordre de 60 ± 30 kg • an -1 pour l'ensemble du fjord.…”