Oil in the environment

The elements which evaporate during the first few hours and are photo-oxidised or biodegraded before and during operations represent between 30 and 50% of a crude oil spill.

Containment and recovery operations at sea and onshore help recover around 20% of the remaining pollutant, or 40% at the very best.

It can be deduced therefore that at least a third and in some cases up to half, of the volume of crude oil spilled subsists in the environment. It does so in three forms: dissolved for a small part made up of the soluble compounds, settled on the seafloor and adsorbed by the sediments for the heavy compounds, and in the form of particles for the rest.

This oil which is missing from response calculations is therefore not in the form of a large, thick, continuous slick drifting under the surface and threatening the shoreline. Rather, the initial slicks are scattered by the swell, wind and currents.

Persistence of heavy fuel oil on the shoreline

One of the general public’s main concerns in the aftermath of the Erika was the persistence of fuel oil which had not been removed from the shoreline.

As it was a substance with very low biodegradability, there was a risk of finding it years later. The subject was therefore broached in the follow-up programme of ecological and ecotoxicological consequences of the oil spill (www.suivi-erika.info).

Studies showed that 3 years after the incident residual fuel oil remained on rocks in the form of thin layers, broken down on average by 66%, and in the form of patties and crusts, on average 55% broken down, with noticeable variations
in the environment. Within riprap where the fuel oil was sheltered from light, it remained viscous, broken down by less than 20 %.

The break down of the fuel oil, as expected, was very low. However, there was significant photo-degradation where the oil was exposed to solar rays, in particular in fine layers.

It would have been far different if the Erika had been carrying a light product: little to no pollution would have remained on the shoreline after 1 year.

This weathered oil interferes with species and ecosystems by a simple mechanical effect, like asphalt debris from roads being swept away by a flood and dispersed in the natural environment.

In large quantities in the sand on a beach, or the mud of a marsh, weathered oil can affect seasonal beach growth and depletion cycles, causing localised effects on the wildlife.

The dissolved fraction, droplets adsorbed by the sediments, as well as particles in suspension can still contain perceptible proportions of aromatic compounds, with non negligible toxicity. It is these elements in particular which could contaminate organisms in the long term, by breaking down respiratory and digestive cells, in particular in organisms which filter the water or sediments. If hydrocarbons in suspension subsist in large quantities, they can prolong abnormal concentrations in shellfish, causing their collection to be banned.

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Summary of fate of oil calculated by mass

Amoco Cadiz: light oil, high level of evaporation
Prestige: heavy fuel oil, high level of recovery at sea and in the wreck

 

Residual fuel oil from the Erika on a bed of vegetated stones

Oiled slabs of granite for a study of oil persistence on rocky substrates

At this stage, natural and chemical dispersion has begun its work. The oil, freed of its evaporable and rapidly photo-oxidisable and biodegradable components, is fragmented into scattered patties, tar balls and particles.

Certain slicks drift in the open sea, out of reach of recovery operations at sea. Others are beached, covered by sediment or forgotten, deliberately or otherwise, by shoreline clean-up operators. In this case, the oil is weathered, semi-solid to solid and mainly made up of asphaltenes - non toxic, long-chain hydrocarbons.

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Public fears

Dramatic media representation of spills can fuel public worries about the risk of contamination of organisms in the long run:

• fear of alterations in behaviour, metabolism, growth and reproductive faculties of plants and animals which come into contact with the oil
• fear of bioaccumulation of certain hydrocarbons, causing an amplifying effect in the food chain, eventually reaching consumers of sea produce
• fear of development of benign or malignant tumours in the organisms affected and their descendants
• fear of potential chromosomal aberrations liable to affect future generations
• fear of long term impact on the health of operators participating in shoreline clean-up and bird rescue.

Scientific compilations like those on page 95 provide objective answers to those worries, but they do so in words more suitable for scientists that for the general public. During the crisis, that information must be translated into easily understandable answers, specially adapted to the incident concerned and provide in real time. Whenever there is a recognised risk, in-depth studies should subsequently be implemented.