water. The mass of oil recovered at the refinery from at sea operations is estimated at 1,500±300 t. The oil was then reprocessed at the Texaco refinery by bleeding in the oil at very low rates so that the sea salts incorporated into the oil did not contaminate the catalysts in the refining process. This accounts for around 2% of the oil.
Oil recovered from the shoreline - 2% (range 1.5-2.5%) :
The oil recovered from the shoreline was composed of 2,500 t of liquid emulsion of 20 % oil reprocessed at the refinery (the mass of oil was measured at the refinery as 500±200 t) ; 3,500±500 t of oiled waste at 10% to landfarm ; and 7,800±200 t of oiled sand at 5% oil to landfarm. Therefore, of the 72,000 t only around 1,250±250 t (2%) was recovered from the shoreline.
Oil remaining on the shoreline - 5% (range 3-7%) :
The estimate of the volume of oil remaining stranded comes from extrapolating from the relatively few sites extensively studied during the earlystages of the spill. We estimate that around 5-9% of the oil released impacted the shoreline, 2 % was recovered leaving 3-7 % stranded on the shoreline on 29 February 1996.
Dispersion - by difference = 51% (range 43-59%) :
The rapid dilution of the dispersed oil together with the large volumes over which the oil is distributed means that it is not possible to generate an effective monitoring programme which can accurately quantify the dispersed oil budget. However, fluorometry measurements at sea showed that the dispersion process was a significant process in determining the fate of the spilt oil.
Using the figures available at present, if 40%±5% evaporated, 2%±0.5% was recovered at sea, 2%±0.5% was recovered from the shoreline and 5%±2% was left stranded on the shoreline after cleanup of the initial bulk oil, then by difference around 51%±8% of the oil is likely to have dispersed through a combination of natural and chemically enhanced dispersion.
INFLUENCE OF THE RESPONSE OPERATION ON THE FATE OF THE SPILT OIL
Once oil has been released into the environment the effects of the oil will be seen to some extent by all com-partments. The role of response to the spill is to provide a net environmental benefit (Baker 1995). Over time natural processes break the oil into small droplets and biodegrade the oil. In summary, there are 2 main approaches to oil spill response.
・ To fight these natural process and concentrate the oil and remove it from the environment.
・ The second is to enhance the rates of the natural processes of dispersion and degradation.
The first approach has the advantage of removing oil from the environment following a spill but the disad-vantage that it is fighting natural processes which are tending to disperse and dilute the surface oil. The second approach of relying on natural processes can be effective in reducing the environmental impact of a spill. For example, The conclusion of the 2 year study on the environmental effects of the BRAER was that there was no significant environmental impact. At most spills where the weather conditions are not as extreme as the BRAER there is a net environmental benefit in enhancing natural processes.
Mechanical recovery at sea
It is generally agreed that a maximum of around 10% of the oil spilt can be recovered by mechanical recovery at sea. In the BRAER spill where storm conditions existed throughout the incident no oil was recovered from the sea surface while during the EXXON VALDEZ response which relied heavily on mechanical recovery 8% of the surface oil was recovered (Wolfe et al.1994). As we have described above, despite the unfavourably high wind speeds at the SEA EMPRESS, mechanical recovery (using 4 mechanical recovery systems) removed 1.5-2.5% of the oil spilt. This prevented some 3,500-6,000 t of emulsion impacting the shoreline.
Dispersant application
The main strategy of dispersant use at the SEA EMPRESS was to enhance the natural dispersion process in order to remove oil from the sea surface and thereby reduce the potential environmental impact to sea birds, coastal waders, intertidal vertebraes and invertebratea and the many amenity beaches in the area. It is clearly difficulto to
