Implications of a multi-layered nepheloid system for hydrocarbon distributions and transport in a microtidal Mediterranean estuary

Abstract

Abstract The origin and fate of non-aromatic hydrocarbons (NAH) and polycyclic aromatic hydrocarbons (PAH) were studied at the land-sea boundary in the microtidal estuary of the Rhone River. The sampling strategy was based on the collection of particles in the multi-layered nepheloid system which develops off the river mouth and extends towards the continental shelf. After separation by normal phase HPLC, hydrocarbons were analyzed by gas chromatography and gas chromatography/mass spectrometry. The distributions of NAH and PAH among the various nepheloid layers are discussed in terms of abundances, compositional patterns and related diagnostic criteria. Advection within the benthic nepheloid layer ( bnl ) appeared to constitute an important transport pathway for hydrocarbons, and particularly for PAH. This layer showed a preferential enrichment in fossil fuel related PAH, and to a lesser extent in n-alkanes, whereas pyrolytic inputs were essentially transported within the surface layer. The qualitative decoupling of hydrocarbons between the two layers strongly supported the hypothesis that processes such as flocculation, aggregation and interactions between different phases are the dominant mechanisms leading to the formation of the bnl . A comparison between the hydrocarbon distributions in the BNL and in the underlying surface sediments confirmed the distinct character of the BNL and suggested that resuspension does not significantly contribute to the BNL material.