SAR imagery and Seatrack Web as decision making tools for illegal oil spill combating — a case study

The number of marine pollution arising from illegal oil discharges from ship tank or bilge pumping is much greater than those spectacular ship accidents. Illegal spills are mainly detected on essential navigation routes. In every country, marine surveillance agencies are responsible for oil spill combating and on identification of illegal polluters. They rely on information that has been provided on potential oil spills by responsible institution. The first information is usually provided by satellite remote sensing. The decisions about oil combating action is taken based not only on SAR imagery, but checking confirmation from aerial surveillance and using oil spill modeling, also. SAR imagery and aerial surveillance does not provide information about the type of spilled oil, which is important input information for oil spill modeling. Different types of oil have different behavior in water and may affect the decisions about which oil combating activities should be taken. The aim of this study was to show how different type of oil behaves in water according to the Seatrack Web oil drift model, which is the main modeling tool of Estonian Border Guard who is responsible in oil combating. Current study is based on illegal oil spill accident that happened in the eastern Baltic Proper on 10 April 2010. Potential oil pollution was detected on SAR image at 9:08 UTC. Consecutive SAR image was obtained at 9:40 UTC showing no significant change of the slicks area and shape in such a short time. Oil pollution was also confirmed by aerial surveillance at 11:10 UTC. Report that was based on visual observations said that it was probably a bilge water, which started to vanish due to ship traffic. The pollution was also recorded by Side Looking Aperture Radar (SLAR). Seatrack Web model (STW) was used for the forecast of oil slick drift. The input of light and medium oil was chosen. The results showed rather different results about oil drift as well as about oil fate. In case of medium oil 20% of oil was expected to evaporate within a couple of hours and the rest stayed in water surface. In the case of light oil 20% was evaporated, but the rest of the oil was expected to disperse in to the water column and emerge on the surface in time to time. The light oil was simulated to drift to the NWW, while medium oil to the SW. Laboratory analyses of the sample that was taken at 14:30 UTC showed that heavy fraction of the oil (hydrocarbons C16-C36) was maintained in the water until then. In conclusion, this study shows that the information about the type of spilled oil is needed as soon as possible after the oil detection to make appropriate decision on oil combating activities.