The pollutant
Evolution, behaviour and impact of different hydrocarbons on the coast.
The definition of clean-up techniques, the choice of equipment and the scaling of worksites depend heavily on the characteristics of the pollution. Other than the volume spilled, several factors influence the nature of the pollutant and more particularly the type of arrivals of oil on the coast.
Nature and evolution of the pollutant
When
oil is spilt at sea, it is subjected to different processes
which may alter its overall appearance and its physico-chemical
characteristics.
These processes are mainly:
- spreading
- evaporation
- emulsification
- dispersio
- dissolutio
- sedimentation
- biodegradation.
Classification of hydrocarbons
The different crude oils and refined products are made up of mixtures of hydrocarbons and various other components which largely influence the behaviour, evolution and impact of the pollutant, as well as the techniques used to treat it.
The multitude of existing products highlights the need for a simplified classification system to divide petroleum products into a limited number of categories. The proposed classification, into four classes (volatile, light, medium and heavy), is based on pollutant characteristics which have a significant influence on response strategies and techniques, in particular the viscosity and the tendency to adhere to surfaces.
Classification of hydrocarbons according to their behaviour and their impact on the coast
Types of arrivals on the coast
The pollutant arrives in varying forms on the coast. These forms can be classified in the following way:
- Massive arrivals in the form of thick, homogeneous slicks
(from a few millimetres to several centimetres thick) or
spread over a considerable surface area (several hundred
square metres), in a more or less continuous manner.
- Scattered arrivals which may differ in appearance and
can be subdivided into two categories:
- Deposited on the surface in the form of patches, patties, tar balls, festoons, stripes, foam, splashes, mottling, spots or even sheen (on the water) or a greasy film on the foreshore.
- Penetrated into the substrate (or naturally buried) in the form of layers of polluted sediment or alternate layers.
Furthermore, the pollutant can also be deposited on varied debris or become trapped in plant-based beachings.
Surveying
Surveying is an important stage as it allows the extent of the pollution to be assessed relatively accurately, before rationally allocating and correctly proportioning response efforts. The aim is therefore to rapidly provide as much relevant information as possible. For this, we recommend providing surveyors with survey forms so that field-based observations are reported in a standard form, ensure consistency in reports by restricting inaccurate and therefore unreliable subjective reports to a minimum.
Shoreline surveying procedures are described in detail in an operational guide on the assessment of shoreline pollution, produced by Cedre and entitled “Surveying Sites Polluted by Oil”.