Publication Abstract

Characterisation of the surface microlayer chemical contamination after shipping accidents

C. Guitart, P. Frickers, J. Horillo-Carballo, R.J. Law and J.W. Readman

A contamination survey was conducted after the beaching of the stricken cargo ship MSC Napoli in Lyme Bay on the south coast of Devon (UK). A grid of 22 coastal and offshore stations was sampled to investigate the extent of spilled oil and to screen for chemical contamination, as well as to evaluate the behaviour of the oil at the air-sea interface. Samples were collected from the sea surface microlayer (SML) and from subsurface waters (SSW) at each station. The fuel oil spilled (IFO 380) was also analysed. The determination of oil-related hydrocarbons (aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAH), terpanes and steranes) and the screening for other harmful chemicals on the inventory of the MSC Napoli in the seawater samples, was performed by PTV-GC/MS using large volume injection (LVI) techniques. These afforded investigation of sources and enrichments of petroleum hydrocarbons in the sea surface microlayer (SML). Rather than a single source, oil fingerprinting analyses of the samples revealed a mixture of three types of oil in the sea surface microlayer: heavy fuel oil, lubricating oil and a lighter oil (probably diesel). Enrichment factors (EF) in the SML (EF = C_SML/C_SSW) were calculated and, in the vicinity of the ship, approached 2000,  declining with distance away from the wreck. These factors represent approximately a 1000-fold enrichment over typical coastal SML enrichments and reflected a clear petrogenic origin of the contamination (as demonstrated, for example, by a Fl/Py ratio < 1). In addition, the spatial transport and fate (i.e. air-sea exchange processes and water column diffusion) of the oil-related hydrocarbons in the sea surface microlayer were investigated. Essentially, near the wreck, the SML was highly enriched in oil forming a visible sheen, both disrupting the normal air-sea water exchange processes and generating a downward diffusion flux of contaminants from the SML to the SSW. This was reflected by a higher occurrence of naphthalene relative to alkyl-naphthalenes in the SSW  compared to the SML. The higher concentrations and different sources of oil found in the SML in comparison to those found in the SSW, indicate that if only subsurface water samples are investigated in isolation, the true extent and impact of a spill could be underestimated. It is important to simultaneously evaluate contamination in the sea surface microlayer during emergency response.

Reference

C. Guitart, P. Frickers, J. Horillo-Carballo, R.J. Law and J.W. Readman (2008) Characterisation of the surface microlayer chemical contamination after shipping accidents. Environmental Science and Technology 42; 2275-2282