Once the main part of the pollution has been cleared away and all risks
of new arrivals of pollutant eliminated, the
final clean-up phase can begin. Even if the sea naturally completes
the operation, final clean-up by man is necessary when:
• the estimated timeframe for self-cleaning is incompatible with the economic or aesthetic constraints of the site (e.g. a popular tourist site during the pre-summer or summer season)
• the pollution may have a major impact on living, natural or cultivated resources or may become a source of chronic contamination.
The basic principle of final clean-up is to take advantage as far as possible of natural clean-up processes and only to recreate these processes where they prove to be of limited efficiency. The main mechanical, chemical and biological self-cleaning processes are:
• cleaning by wave action, the impact freeing fresh oil from surfaces as well as, on highly exposed sites, scouring the residues by abrasion of pebbles and rocks
• mixing polluted sediment by waves, separating the oil trapped in the sediment and placing it in suspension
• washing fluid oil through the sediment by forced percolation, with receding waves or the outgoing tide
• the effect of ultraviolet rays which destroy hydrocarbon films
• the activity of bacteria and micro-organisms which are capable of breaking down hydrocarbons.
Sections of vegetation, in dunes or on rocks, may be polluted to a varying extent. Intervention in such areas is delicate and should be subject to a decision and specific recommendations made by expert botanists, who generally supervise the worksite. The term “botanical worksite” encompasses the sometimes extremely painstaking and time-consuming operations of scraping, brushing, scything, manual sand screening, suction and collection. The aim is to remove the maximum amount of pollutant without damaging the plant cover or the soil more than it is already damaged.
Botanical worksite, final clean-up, Le Croisic (France)
A wide range of techniques is available and the choice depends on the site characteristics, the extent of the pollution, the nature of the pollutant and the required quality of clean-up. The determination of the techniques to be used, and the degree of cleanliness to be attained should take into account, amongst other factors, the ecological sensitivity of the site and its immediate surroundings.
As for rocks and port infrastructures, low pressure washing with cold water should be carried out on ecologically sensitive sites. High pressure washing, if necessary with hot water, can be used on non sensitive sites. Non toxic solvents can be used for port facilities. Sheet or bulk sorbents can be used on rocks and in shore pools. In the most exposed areas, the best solution is often to leave nature to do the job. As we will see later, the breakdown of hydrocarbons in this case is rapid and practically complete.
On stony beaches, after initial removal of oil, films of oil remaining on the surface should be washed using pressurised cold water and swept into recovery trenches on the lower foreshore. In the case of buried oil, either flooding can be carried out to free the pollutant and cause it to rise to the surface, or drains can be dug to accelerate natural washing, for example using the technique of surfwashing. Stones can even be washed offsite, for instance in a drum adapted from a concrete mixer drum.
High pressure washing
Cleaning using an oleophilic roller
When to stop cleaning
The level of clean-up required and the urgency of completing it are dictated by the ecological sensitivity of the site, its uses and the season. Some members of the general public will always demand that the site be cleaned of the last trace of pollutant. However, this “spotless” cleaning, although satisfying, can cause more damage to the environment than the pollution itself. It is therefore necessary to assess the advantages and disadvantages of the available techniques and not to dismiss the option of natural completion.
In practice, any pollutant which may be remobilised, and constitutes a potential source of recontamination, should first be removed wherever possible. Once this risk has been eliminated, we must then question the utility of further intervention. Except in particular cases, such as popular tourist beaches, the aim is not to remove all traces of oil, but rather to provide the environment with the most favourable conditions for rapid reinstatement of populations and restoration of socio-economic activities, ensuring that the remaining pollutant is not harmful to the ecological niche or the site’s uses.
Clean-up by manual scything
On sandy beaches, flushing,
flooding, surfwashing and drainage techniques can be used. On dry sand,
a second screening phase (possibly manual) can be carried out to complete
initial clean-up. On damp sand, oleophilic
rollers can be used. Finally, the technique of tilling frees fluid oil trapped
in sediment and encourages natural break down. This technique can be used
to complement screening operations.
Marshes and mudflats are areas in which oil tends to accumulate and cause serious ecological impact, through a toxic effect for light oils (chemical action) and by oiling and choking vegetation in the case of heavy oils (physical effect). Response in such cases is delicate due to difficult accessibility, low load-bearing capacity and limited circulation. Furthermore, it could have disastrous consequences for the ecosystem.
Oleophilic roller pushed by a small caterpillar tracked vehicle
In very sheltered sites where clean-up operations would lead to additional
damage due to the ecological fragility of the biotopes in question, not
intervening can be a justifiable solution. Non-intervention should be a
carefully considered and well argued decision in order to be justifiable
in the face of public opinion and the media. This option does not consist
of simply “doing nothing”. It involves close monitoring of the
site and of the evolution of the pollutant.
Once clean-up is finished, all or part of the polluted vegetation may be scythed or the crusts of weathered oil scarified to facilitate the reinstatement of vegetation. This must be carried out extremely carefully, under rigorous ecological control. Finally, all access routes created for the purposes of response and all waste storage facilities should be cleaned and restored to their original state.
Assessment of the level of clean-up to be attained
Recovery on land