Capping of Contaminated Marine Sediments : Ebullition of Biogenic Gas and its Impact on the Flux of PAH

Abstract

The risk associated with exposure to contaminants has, in recent years, drawn attention to the fate and transport of these contaminants in shallow marine sediments. It has been suggested that the transport of Hydrophobic Organic Contaminants (HOC), such as Polycyclic Aromatic Hydrocarbons (PAH) from the sediments surface, may be enhanced by the existence of mechanisms which complement the diffusive and advective fluxes. Gas bubbles released from the sediment, through the ebullition of biogenic gas is one of these possible mechanisms. Ebullition may theoretically increase PAH transport in the diffusive boundary layer (DBL), by introducing additional advection through water filled bubble voids and through the partitioning of PAH to the gas-water interface of the rising bubbles. This enhanced transport may reduce the effectiveness of remediation techniques, such as the isolation of contaminated sediments by engineered capping. Therefore, a series of one dimensional (1-D) diffusion tests have been conducted, using contaminated marine sediment from Oslo Fjord. Half of the tests had a carbon source added to the sediment to initiate ebullition. The setup consisted of some tests with only sediment, both with and without ebullition and also some with the sediment covered by a capping layer (gravel – 0-2 mm), also with and without ebullition. Tests consisting of only capping material and only sea water were also performed. Ebullition was observed 30 – 60 days after initiation, generating gas filled fissures in the sediment. The diffusive flux of 10 PAH compounds from the sediment or capping phase, through a 1 – 2 cm layer of sea water, was then measured over a period of 7 months. Results from the uncapped tests showed that the diffusive flux of PAH fitted a linear model described by Fick’s first law and increased from 2.1×10-4± 0.4×10-4μg/cm2/day, for those tests without ebullition, to 3.0×10-4± 0.4×10-4 μg/cm2/day for those tests with ebullition. The capped tests showed no significant increase in the diffusive flux of PAH attributed to ebullition. This finding is positive and confirms that the ebullition of biogenic gas from marine sediment, in a laboratory experiment, has no impact on the effectiveness of capping as a remediation method.