estimate what percentage of the total 51% dispersed was through natural dispersion and what percentage was dispersed through the use of dispersants since during a response to a major spill leaving an experimental control where no response is mounted is not a realistic option when environmentally sensitive sites are threatened.
However, NETCEN had carried out field trials using Forties Blend crude oil prior to the SEA EMPRESS spill (Lunel et al. 1995, Lunel & Davies 1996). These carefully controlled experiments with oil in the field used steady state releases of Forties Blend crude oil so that quantitative measurements of the dispersed oil phase could be made and also experimental controls could be set up which were not treated with dispersant. These experiments showed that 6% (a range 3-9%) of the Forties Blend will disperse naturally in the first 30 minutes after release in conditions typical of those at the SEA EMPRESS (Lunel et al. 1995, Lunel & Davies 1996). In these same steady state releases (Lunel et al. 1995, Lunel & Davies 1996) when Forties Blend was treated with the dispersant Dasic Slickgone NS (one of the dispersants used at the SEA EMPRESS incident) the total per-centage of Forties Blend dispersed in the first 30 minutes was on average 22% (a range 16-28%).
In the field trials natural dispersion accounted for 6±3% the chemical dispersion was around 16±9% out of the total of 22±6% dispersed. Thus, the ratio of natural : chemical dispersion in these field trials was 1 : 2.7 (range 1 : 1.8 to 1 : 8.3) under wind conditions typical of those at the SEA EMPRESS incident. If this same ratio of natural : chemical dispersion occurred at the SEA EMPRESS incident then of the 51 % dispersed 37% (range 29-45%) would be from chemical dispersion and 14% (range 6-22%) would be from natural dispersion.
It is not possible to give a definitive split between natural and chemical dispersion. However, the range of 6-22% dispersion from the natural dispersion process alone is consistent with our field trial experience and the output of the field validated model OSIS.
Estimated Mass Balance for Forties Blend for 29 March 1996
During this period some 446 t of dispersant was applied to enhance the rate of natural dispersion (the dis-persants used in decreasing order of volume sprayed were : Finasol OSR-51, Dasic LTSW, Dasic Slickgone NS, Dispolene 34S, Superdispersant 25, Enersperse 1583, Corexit 9500) . It was not possible to gather quantitative data at the spill on the relative effectiveness of different dispersants on Forties Blend crude. A notable feature of the spray response was the highly effective targeting achieved by the use of remote sensing aircraft positioned above the spray aircraft to direct the spray pattern. This operation is well tried and practised in the UK and allowed the DC3 aircraft in particular to target effectively ribbons of oil as narrow as 10-20 m. Since 446 t of dis-persant were used this implies that each tonne of dispersant resulted in the dispersion of 47 to 73 t of oil, with a mean around 60 t. Clearly this represents a highly successful dispersant operation.
Therefore, we estimate that if dispersants had not been used at the SEA EMPRESS incident the instead of the estimated 10,000-15,000 t of emulsion impacting the shoreline, 72,000-120,000 t would have impacted the South Wales coastline. Clearly this would have resulted in a significant increase in the 6,900 bird casualties, would have smothered the intertidal zone in thicker layers of emulsion for longer periods of time and would have provided a near-impossible task for the teams trying to re-open tourist beaches in time for Easter at the start of April.
OVERVIEW OF THE FATE OF THE HEAVY FUEL OIL
Heavy Fuel Oil (HFO) is used as the fuel for the majority of ships. Therefore, spills of HFO are not uncommon. Experience from around the world, including the NAKHODKA spill, has shown that despite the fact that spills of HFO are often not the largest spills in terms of volume, the impact of these spills can be significant because HFO does not disperse and biodegrade readily. Laboratory weathering studies carried out on the HFO from the SEA EMPRESS indicates that only around 2-5% will evaporate. From previous experience once would expect less
