Long term monitoring of oil spills in European seas

International Journal of Remote Sensing Vol. 30, No. 3, 10 February 2009, 627–645 Long term monitoring of oil spills in European seas G. FERRARO*{, S. MEYER-ROUX{, O. MUELLENHOFF{, M. PAVLIHA{, J. SVETAK{, D. TARCHI{ and K. TOPOUZELIS{ {European Commission (EC), Joint Research Centre (JRC), Institute for the Protection and Security of the Citizen, Via E. Fermi, 21020 ISPRA (VA), Italy {University of Ljubljana, Faculty of Maritime Studies and Transport, Pot Pomorscakov 4, 6320 Portoroz, Slovenia (Received 8 June 2007; in final form 4 February 2008 ) Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 Accidental pollution at sea can be reduced but never completely eliminated; however, deliberate illegal discharges from ships can indeed be reduced by the strict enforcement of existing regulations and the control, monitoring and surveillance of maritime traffic. Despite this, operational oil discharges are common and represent the main source of marine pollution from ships. To analyse this problem, the Joint Research Centre (JRC) of the European Commission has focused its attention on the need to monitor in the long term sea-based oil pollution. This research aims, in particular, to map the oil spills, to identify the hot spots and to define the trends in all European seas. For this reason, JRC has collected all relevant data concerning sea-based oil pollution from different actors and archives. For the North and Baltic seas, data from aerial surveillance were used and, for this reason, all oil spills are real and confirmed. Conversely, the data for the Mediterranean and the Black Sea derive from oil spills detected by JRC in low resolution SAR (Synthertic Aperture Radar) satellite images from archives. For the Mediterranean and the Black Sea, these data represent the only source to draw some preliminary conclusions on marine oil pollution. This paper presents for the first time a comprehensive view of the long term monitoring of sea-based oil pollution in all the seas around Europe. The key conclusion of this study is that, if the data analysed are not homogenous, operational pollution in the seas around Europe seems to be slightly decreasing. 1. Introduction Oil marine pollution is a major threat to the ecosystems of European seas. The source of the oil pollution may be the mainland or directly at sea. Sea-based sources are mainly discharges coming from ships or offshore platforms. Other marginal seabased sources are leaks from wrecks and natural seeps. Oil pollution from sea-based sources can be accidental or deliberate. Fortunately, the number of marine accidents and the volume of oil released accidentally are on the decline. However, routine tanker operations can still lead to the release of oily ballast water and tank washing residues. Furthermore, fuel oil sludge, engine room wastes and foul bilge water, produced by all type of ships, also end up in the sea. In the last decade maritime transportation has been growing steadily, reflecting the *Corresponding author. Email: [email protected] International Journal of Remote Sensing ISSN 0143-1161 print/ISSN 1366-5901 online # 2009 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/01431160802339464 Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 628 G. Ferraro et al. intensified co-operation and trade in the European region and a prospering economy. More ships also increase the potential number of illegal oil discharges. Both oil tankers and other kinds of ships are among the suspected offenders of illegal discharges. In the North Sea, regular aerial surveillance to detect oil spills and to catch the polluters began in the 1980s. The eight countries bordering the North Sea work together under the Bonn Agreement and undertake aerial surveillance using aircraft equipped with remote sensors (RS). Data of observed oil spills are available from 1986. It should be stressed that in the North Sea there are many off-shore installations which are sources of sea-based pollution. Deliberate illegal oil discharges from ships have also regularly been observed within the Baltic Sea since 1988. A complex set of measures known as a Baltic Strategy has been implemented by the nine contracting parties to the Helsinki Convention. These measures include surveillance flights and improved usage of remote sensing equipment. As a result, a decrease in the number of observed illegal discharges has been identified over recent years in the North Sea and in the Baltic Sea, despite the rapidly growing density of shipping. Although the number of observations of illegal oil discharges has been decreasing it should be kept in mind that, for some areas, aerial surveillance is not evenly and regularly carried out and therefore there are no entirely reliable figures for all areas. For the North-East Atlantic, there are no data available on a regular basis concerning deliberate oil discharges from vessels. Moreover, this area is not defined ‘Special Area’ according to Annex I of the MARPOL Convention. Outside ‘Special Areas’, it is difficult to assess if visible oil discharges from ships are illegal. However, in this area there was, for the first time, operational use of satellite imagery during the Prestige accident in 2002. For the Mediterranean Sea and the Black Sea there are no data derived from regular aerial surveillance, so the only possible way to monitor these seas is the use of SAR satellite images from archives. The reliability of the satellite image analysis is not yet fully satisfactory and further investigations and validation activities are necessary. However, the use of archive satellite imagery is the only way to extract information for these seas. The Joint Research Centre (JRC) is carrying out a systematic mapping of the oil spills using satellite imagery in these two seas. This action helps to reveal the scale of the oil pollution problem, thus stressing the need for more concerted international action. 2. 2.1 Types of pollution Pollution from offshore platforms Offshore platforms can legally discharge oil at sea, according to detailed parameters. The main discharge associated with an offshore installation is produced water. However oil could also be released into the sea from oil on cuttings and from produced sand contaminated with oil, well clean-up fluids, releases during well abandonment and pipeline decommissioning. In addition to permitted operational discharges, spillages may occur where systems fail. Specific rules are set to prevent pollution from offshore activities in different regional agreements, such as the Baltic Convention on the Protection of the Baltic Sea Area of 1992, the Barcelona Convention Protocol for the Protection of the Mediterranean Sea against Pollution Resulting from Exploration and Exploitation Long term monitoring of oil spills in European seas 629 of the Continental Shelf and the Seabed and its Subsoil (adopted in 1994 but not yet in force), and the Convention for the Protection of the Marine Environment of the North-East Atlantic (known as the OSPAR Convention) of 1992. As an example, the OSPAR Recommendation 2001/1 for the Management of Produced Water from Offshore Installations establishes that no individual offshore installation should exceed a performance standard of 40 mg l21 (i.e. 40 mg of dispersed oil per litre or 40 parts per million—40 ppm) for produced water discharged into the sea. An improved performance standard of 30 mg l21 was to apply by the end of 2006. These discharge limits are based on the total weight of oil discharged per month divided by the total volume of water discharged during the same period. A maximum oil concentration of 100 mg l21 (100 ppm) is generally applied (OSPAR website). Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 2.2 Pollution from ships While it is well known that accidental pollution at sea can be reduced but never completely eliminated, illegal discharges from ships can indeed be eliminated by the strict enforcement of existing regulations and the control, monitoring and surveillance of maritime traffic. Detailed rules on oil pollution discharges from ships have been internationally agreed. Marine oil pollution by vessels, termed ‘operational oil pollution’, includes various types of oil and oily mixture discharge, as a result of the daily routine operations of ships. Some of these, such as oily ballast water and tank washing residues, relate to tankers only. When old tankers offload cargo and prepare to travel empty, they must take on large quantities of ballast water to maintain the proper balance of the ship. When the ballast water is discharged, oil residues are released as well. All tankers when switching cargoes have to wash and remove oil residues from hull walls. The remaining residue from tank washing should be stored in specific tanks (slops) and can be discharged only following strict regulations. All types of ship, however, may discharge pollution into the sea from engine room wastes, bilge waters and, in rare cases, used oil. Due to the low quality of ship fuel, only part of it is effective for propulsion. Before being burnt, some fuel must be centrifuged, generating residues which are stored in a specific sludge tank. The sludge should be emptied into harbour facilities. However in practice ships do not always unload in ports. Moreover, ships use a large amount of lubricant that often leaks and ends up in bilges. Bilge water, following strict parameters, can be legally discharged. The international legal regime concerning pollution from ships is defined in the International Convention for the Prevention of Pollution from Ships (MARPOL 73/ 78), which in its Annex 1 deals specifically with prevention of pollution by oil. MARPOL distinguishes between oil pollution outside or inside ‘Special Areas’. Almost all the seas around Europe have been designated Special Areas according to Annex 1 of MARPOL 73/78. Only the Norwegian Sea, the Bay of Biscay and the Iberian Coast are not covered by the Special Area status. In special areas, oil discharges from ships have been completely prohibited, with minor and well-defined exceptions. Without entering into details, we can say that discharges are allowed when the oil content of the discharged effluent does not exceed 15 mg l21 (i.e. 15 ppm). Outside special areas, requirements to legally discharge oily mixtures are less strict. As an example, oil discharges from cargo tanks are allowed with some parameters but the 15 mg l21 limit does not apply. 630 G. Ferraro et al. The 15 mg l21 limit is a key parameter because, as it is recognized by the International Maritime Organization (IMO), it is not possible to see oily mixtures at sea with oil content below 15 mg l21 (Resolution MEPC.61(34) of 9 July 1993 on Visibility Limits of Oil Discharges). This statement was based on scientific studies which confirmed that a discharge of oily mixtures with a concentration of 15 mg l21 can under no circumstances be observed, either visually or with remote sensing equipment (North Sea Directorate 1992). Therefore, not all visible (by the eye or by RS) oil spills are necessarily illegal. However, visible and/or detectable oil discharges from ships, observed in a MARPOL special area, are certainly illegal. Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 3. Monitoring systems—detection methodologies Vessels, airplanes and satellites are used to detect and monitor oil spills. The vessels, especially if equipped with specialized radars, can detect oil at sea, but they can cover a very limited area. The vessel, however, remains necessary for oil sampling. For example, in some EU Member States sampling is necessary for prosecution of the polluter. Aircraft are the most frequently used tool to detect and monitor oil pollution at sea. Observations by experienced aircrew are fully reliable in detection, classification and quantification of observed pollution. Aerial surveillance can be based on simple visual analysis of the aircrew, using for example the Bonn Agreement Oil Appearance Code, or can be executed with auxiliary RS tools. Airborne observations can be carried out using Side Looking Airborne Radar (SLAR) to locate the oil spill, infrared/ultraviolet sensors (IR/UV) to quantify the extent of the film, microwave radiometer (MWR) to measure the spill thickness, and laser-fluorosensor (LFS) to classify the oil type (Trieschman et al. 2003, Brekke and Solberg 2005). Among these methods, SLAR is the most used. Satellites equipped with Synthetic Aperture Radar (SAR) can provide information on the presence of oil at sea. Brekke and Solberg (2005) presented a detailed description of oil spill detection by satellite remote sensing in the world’s oceans. It should be remembered that satellite-borne SAR images do not allow the detection of oil spills if the sea surface is too rough or too smooth, i.e. in the case of winds approximately below 2 m s21 and above 10 m s21. Finally, satellite SAR images are unable to identify the pollution culprit (i.e. the name of the ship which polluted); whereas satellites can at best detect the position of the possible pollution culprit. 4. Oil spill databases JRC has collected in a database all available information concerning oil spill detections. The data used for the North Sea and for the Baltic Sea derive from specialized aerial surveillance. The number of detections and flight hours are available for these two seas. However, even the concept of flight hour is not always homogenous because it depends on the speed of the aircraft and the RS systems on board. Roughly we can say that an aircraft equipped with a SLAR flies at a speed of 335 km h21 and observes 40 km, i.e. it can cover 13400 km2 in one hour. However, RS in aerial surveillance was introduced only in the 1980s and even today surveillance is not always undertaken by aircraft with RS on board. Therefore, in this study, we cannot use the area coverage of the aircraft as a parameter; instead we have to use the simple number of flight hours. Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 Long term monitoring of oil spills in European seas 631 The data used for the Mediterranean and the Black Sea are derived from analysis of SAR satellite imagery. The datatype used in this study was mainly SAR uncalibrated low-resolution images, since these are the most targeted and cheap product for the application. A spatial resolution (pixel) of about 200 m appeared to be sufficient for statistical investigations of marine oil pollution (Gade and Alpers 1999, Gade and Redondo 1999, Gade et al. 2000). Data were provided by the European Space Agency (ESA). In order to be certain to the maximum possible degree that the detected spills were due to man-made activities rather than look-alike manifestations of natural phenomena, all the images were carefully analysed using a dedicated semi-automatic detection scheme, which includes, as final step, a decision by a skilled operator. Each identified spill was then registered in a database, together with information concerning its geographic position, the date and time of detection, the spilled area, its average contrast strength, and a vector describing its shape. It is important to underline that oil spills in the period 1999–2004 have been identified in archive images and the presence of oil at sea has not been confirmed by aerial or vessel surveillance. For this reason, even though only high confidence features have been taken into consideration, we prefer to term them ‘possible oil spills’. 5. 5.1 Results of oil spill detection in the European seas The North Sea The first regionally-developed framework to execute surveillance as an aid to detecting and combating pollution and to prevent violations of anti-pollution regulations is the Bonn Agreement (Bonn Agreement website). This international Agreement was signed in 1969, following some major oil spills, and entirely revised in 1983. The Agreement was developed to encourage the North Sea States to improve jointly their basic capacity for combating and monitoring oil pollution. The Bonn Agreement has been ratified by North Sea coastal States (Belgium, Denmark, France, Germany, the Netherlands, Norway, Sweden and the United Kingdom) together with the European Community. Ireland is in the process of acceding to the Agreement and Spain follows the activities with an observer status. The accession of Ireland will greatly enlarge the maritime areas covered by the Bonn Agreement, and this is in line with the MARPOL Convention (Annex I) which has already enlarged the North Sea Special Area to the Celtic Sea around Ireland. With its history of more than three decades of cooperation amongst North Sea states, the Bonn Agreement can be seen as the leading regional agreement in the field of surveillance and combating marine pollution. The Bonn Agreement Working Group on Operational, Technical and Scientific questions concerning counter Pollution Activities (OTSOPA) facilitates the implementation of the Agreement and executes the Work Programme approved by the Contracting Parties. Among the relevant activities in the field of surveillance, a special place should be given to the development of an Oil Appearance Code which is used to visually assess the quantity of oil at sea. In recent years the emphasis has been on the regional coordination of surveillance, including the opportunities offered by satellite surveillance. The results of aerial surveillance are jointly evaluated and regularly published on the Bonn agreement website. A recent study (Carpenter 2007) has further analysed the Bonn Agreement database. North Sea states organize many joint exercises and multi-national Co-ordinated Extended Pollution Control Operation (CEPCO) G. Ferraro et al. Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 632 Figure 1. Oil spills detected by aerial surveillance in the North Sea in the year 2005. Map produced by JRC for the Bonn Agreement. flights. This cooperation facilitates a common assessment of the amounts of oil discharged and gives to aircraft and crews of different nationalities experience of working together. Every year a map of the detected oil spills (figure 1) by aerial surveillance is produced by the Bonn Agreement in collaboration with JRC. Figure 2 shows the flight hours and the oil spills related to the North Sea. As an indicator to analyse the trend of pollution, the number of detected oil spills has been divided by the number of flight hours. However, it should be noted that reporting systems have changed during the years. Therefore, it is difficult to make direct comparisons between Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 Long term monitoring of oil spills in European seas 633 Figure 2. Total numbers for the North Sea: flight hours and observed slicks (1986–2005) and their ratio. years. For example, until 1999 (inclusive) flight hour data had been provided in absolute numbers but, from 2000, the ‘BA flight hour’ has been used. The BA flight hour is one hour of remote sensing over the sea at a standardized speed of 335 km h21. In 2003, the reporting format was changed once again to provide flight hour data in absolute numbers. Despite the changes in the reporting format, BA data have been of outstanding value for identifying trends in oil pollution levels in the North Sea since 1986. In the North Sea it has been possible to compare data from the analysis of satellite images with the data coming from aerial surveillance (Ferraro et al. 2006a). Unfortunately at present JRC has performed the analysis of satellite imagery only for the years 2000 and 2001 and this does not allow comparison of possible trends. However some preliminary conclusions can be drawn. There is a strong coincidence of the areas where concentrations of oil spills are detected; these areas can be correlated with the main maritime routes and areas where offshore platforms are installed. Since the images were probably covering the North Sea more homogeneously than aerial surveillance, which mainly focuses on specific routes, it could be inferred that the selection of the routes was correctly chosen to cover the most critical areas. A study was carried out investigating the possibility of correlating the oil spills detected in satellite images from archives with spills observed by aircraft (Gérôme 2004). However, it is almost impossible to assess the number of matching possibilities because there is too much uncertainty about the persistence and drifting of an oil spill. 5.2 The Baltic Sea In 1974, all the sources of pollution around the Baltic Sea were made subject to the Helsinki Convention (HELCOM website) signed by the Baltic coastal states. In the Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 634 G. Ferraro et al. light of political changes and developments in international environmental and maritime law, a new convention was signed in 1992 by all the states bordering the Baltic Sea (Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia and Sweden) and the European Community. After ratification, the Convention entered into force on 2000. The Convention covers the whole of the Baltic Sea area, including inland waters as well as the waters of the sea itself and the seabed. Measures are also taken in the whole area of the Baltic Sea to reduce land-based pollution. The governing body of the Convention is the Helsinki Commission—Baltic Marine Environment Protection Commission—also known as HELCOM. HELCOM works to protect the marine environment of the Baltic Sea from all sources of pollution through intergovernmental cooperation. Among the HELCOM Groups which propose and implement policies and strategies, HELCOM Response focuses specifically on coordination of aerial surveillance to provide a complete picture of sea-based pollution around the Baltic, and to help identify suspected polluters. The results of aerial surveillance are jointly evaluated and regularly published on the HELCOM website. The HELCOM States endeavour to fly, as a minimum, twice per week over regular traffic zones including approaches to major sea ports as well as in regions with regular offshore activities and once per week over the regions with sporadic traffic and fishing activities. Twice a year, several Baltic Sea states jointly organize surveillance flights (24 to 36 h): one covering the southern part of the Baltic Sea, and another flight over waters further north. HELCOM facilitates these CEPCO flights (Co-ordinated Extended Pollution Control Operation). Every year a map of the detected oil spills (figure 3) is produced by HELCOM aerial surveillance. Figure 4 shows oil spills that have been detected within the Baltic Sea since 1989. From 1999, the number of observed illegal oil discharges gradually decreases every year (from 488 in 1999 to 293 in 2004). A decrease in the number of observed illegal discharges, despite rapidly growing density of shipping, increased frequency of the surveillance flights and improved usage of remote sensing equipment, illustrates the positive results of the complex set of measures known as a Baltic Strategy implemented by the Contracting Parties to the Helsinki Convention. Also, an increased amount of waste delivered to the Baltic Sea ports illustrates that more and more ships would rather deliver oil waste to ports than illegally discharge into the Baltic Sea (HELCOM website). Although the number of observations of illegal oil discharges has been decreasing over the last five years, it should be kept in mind that for some areas aerial surveillance is not evenly and regularly carried out and therefore there are no reliable figures for all areas. The use of low resolution SAR satellite imagery from archives for the Baltic Sea is not entirely satisfactory (Ferraro et al. 2006a). The available 1300 images for the year 2000 were analysed and 100 oil spills were detected. This number seems slightly low compared with data obtained from aerial surveillance in the framework of the Helsinki Convention. In particular, in the year 2000, 476 oil spills were detected by aerial surveillance (4809 flight hours). One of the possible explanations of the result of the analysis of the Baltic Sea images is related to the type of images used. As mentioned above, the JRC study was developed using un-calibrated low-resolution images and probably this type of image is not appropriate to detect oil spills in the Baltic region. Finally, we have to remember that the Baltic Sea has special weather conditions (in particular, icy waters in the northern part) during part of the year and low resolution images may not be the best tool to detect oil spills in these conditions. 635 Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 Long term monitoring of oil spills in European seas Figure 3. Oil spills detected by aerial surveillance in the Baltic Sea in the year 2005. Map available on the HELCOM website. 5.3 The North-East Atlantic For the North-East Atlantic, there are no data available on a regular basis concerning deliberate oil spills. As an additional problem, this area is not designated Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 636 G. Ferraro et al. Figure 4. Total numbers for the Baltic Sea: flight hours and observed slicks (1988–2004) and their ratio. a ‘Special Area’ according to Annex I of the MARPOL Convention. This means (see above) that visible (by the eye or by RS) oil discharges from ships are not necessarily illegal, because ships can legally discharge higher amounts of oily mixtures outside ‘Special Areas’. Even if it has not been yet developed an adequate surveillance system to detect marine pollution in the North-East Atlantic, this area has to be considered of major interest for the European Union. In fact, France, Portugal and Spain have declared an Exclusive Economic Zone (EEZ) in the Atlantic. As the Portuguese islands of Azores and Madeira and the Spanish Canaries Islands also have their own EEZ, the area in the North-East Atlantic which falls under the jurisdiction of the EU Member States is extremely wide (figure 5). In particular, it is interesting to note that the surface of the waters under jurisdiction of EU Member States in the North-East Atlantic is bigger than the whole Mediterranean Sea. Due to the dimension of the area to be monitored, the use of new technologies and in particular of low resolution SAR satellite imagery in wide swath (400 km6400 km) is a key challenge in the years to come. On the other hand, in this area there was, for the first time, operational use of satellite imagery during the Prestige accident in 2002 (Fortuny et al. 2004). JRC, in support of the Monitoring and Information Centre (MIC) of the European Commission, interpreted 169 radar satellite images over the areas affected by the Prestige Tanker accident. JRC produced geo-located radar images with the indication of likely spilled areas. For each likely spilled area a level of confidence (low, medium, high) was also provided. On average, the time delay from the image acquisition to the delivery of the final results to MIC was about nine hours. In practice, the bottleneck in this process was the delivery of the images by the image Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 Long term monitoring of oil spills in European seas 637 Figure 5. Area under jurisdiction of EU Member States in the North-East Atlantic. provider. It must be noted that a great number of the images were acquired by the ESA ENVISAT satellite, which was in its commissioning phase and the image delivery scheme was not yet fully operational. Consequently, it can be concluded that the overall efficiency was quite good. 5.4 The Mediterranean Sea Operational pollution from ships is a major problem within the Mediterranean region. While accidental pollution rarely occurs within the Mediterranean waters, operational pollution is a common practice in this basin, representing the main source of marine pollution from ships. Furthermore, the increase of maritime traffic crossing the basin renders the situation even more worrying (REMPEC 2002). The lack of a regular surveillance service has determined the absence of data on verified spills due to illegal discharges from ships in the Mediterranean. Moreover, this lack of surveillance can encourage the discharge of dirty ballast waters or oily sludge. Due to the large extent of the basin a surveillance system based only on aerial patrolling can be difficult to implement, for several different reasons. In this respect, satellite surveillance may represent a valid complement because of its ability to provide global coverage, including of remote areas. Satellite surveillance still has a number of limitations, such as an uneven spatial coverage, a quite sparse number of acquisitions in time and a residual number of false alarm cases. Nevertheless, comprehensive studies based on the systematic analysis of space imagery have turned out to be a unique source of information for an overall assessment of the problem. This kind of information, once available, helps to identify the areas at major risk of operational pollution, to which particular attention should be given. Considering the small number of accidental pollution events compared to the operational ones within the region, it becomes evident that the use of satellites would be more relevant in monitoring illicit discharges from ships. Studies carried out by JRC (Pavlakis et al. 2001, Bernardini et al. 2005, Tarchi et al. 2006, Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 638 G. Ferraro et al. Topouzelis et al. 2006, Ferraro et al. 2006a, 2006b, 2007a, 2007b), based on the analysis of a large number of SAR images, detected a significant number of possible spills within the Mediterranean Sea. To assess the distribution of sea-based oil pollution, the number of possible detected spills has to be compared to the total number of satellite images analysed. It is also necessary to precisely account for the images which only partially cover a sea area. To this aim, we present the coverage area in terms of square degrees observed per year, where only sea surface is taken into consideration. A square degree is a square having approximately 110 km per side, i.e. about 60 nautical miles. Due to the fact that all meridians join at the poles, the square degrees are not all equal: their average size tends to decrease towards the North. Table 1 summarizes the results obtained for the whole Mediterranean for the period 1999–2004. For 2003 the results are preliminary and will be verified further. In total 18947 SAR images were analysed and 9299 possible oil spills were detected. The cumulative result is shown as a point-like map in figure 6. In this map, each possible spill is represented by a dot at the location of the spill centroid (average position). The corresponding coverage of analysed images for the same period is displayed in figure 7. The majority of spills are located beyond the 12 nautical mile limit of territorial waters, probably indicating deliberate intention to avoid risk of legal action. With reference to figure 7, it should be noted that a low number of images were available for Libyan coastal waters, while, by contrast, many images were available for the seas surrounding Italy. As a consequence the results are skewed somewhat towards these areas. However, the variation of coverage may be considered quite smooth and the problem almost disappears when sub-areas of limited size are taken into consideration. Figure 8 displays the oil spill density obtained by merging the information from the two previous maps, i.e. by normalizing the number of observed possible oil slicks in a given area with the total number of observations available for that area. This procedure basically removes any bias effect and accounts for uneven coverage of the area. In summary, the map can be employed to understand the spatial distribution of possible oil slicks and to identify hot-spot areas. As may be expected, the spill distribution appears to be highly correlated with the major shipping routes. Concentrations appear in the Ionian Sea, the Adriatic Sea, the Messina Strait, the Sicily Channel, the Ligurian Sea, the Gulf of Lion and east of Corsica. All over the region, however, the spills show considerable spatial dispersion. The whole set of detected possible oil spills was then analysed in terms of seasonal variations. The general trend shows that the number of detections has a maximum Table 1. Yearly coverage and possible oil slicks detected over the whole Mediterranean basin in the period 1999–2004. Year Coverage (square degrees) Possible oil spills 1999 2000 2001 2002 2003 2004 Total 1382 3642 2495 1840 2289 3885 15533 1638 2297 1641 1401 897 1425 9299 Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 Long term monitoring of oil spills in European seas 639 Figure 6. Possible oil spills detected in the Mediterranean Sea during the period 1999–2004. during the summer months. Such behaviour has been often observed in similar studies and can basically be explained by the fact that during summer the mean wind speed is lower, thus determining a higher visibility of oil pollution (Gade et al. 2000). However, in particular areas, such as the sea between Corsica, Sardinia and the seas around Italy, the increased number of summer detections shows a systematic distribution well correlated with local main maritime routes. This fact suggests that the increase in number of detections may also be due to the increment of maritime traffic (ferries) during the tourist season. Figure 7. Coverage of SAR images analysed in the present study for the Mediterranean Sea during the period 1999–2004. Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 640 G. Ferraro et al. Figure 8. Oil spill density for the Mediterranean Sea for the period 1999–2004. It is usually important to provide an estimation of the oil quantity which is spilled to the sea on a yearly basis. Such a precise estimation would require, in addition to the spilled area, an accurate knowledge of the spill thickness. No information concerning the thickness can be retrieved from radar observations and no direct calculation of the corresponding total volume of oil can be made from these data. However, some authors (Parker and Cormack 1984, Hollinger and Mennella 1973, Brown et al. 1995, Pavlakis et al. 2001) estimate a minimum oil spill thickness which can be seen in a radar image. For example, Parker and Cormack (1984), after experimental investigations of controlled mineral oil spills in the open sea, concluded that a spill thickness of 0.1 mm was a threshold for imaging with an airborne SLAR. Making the extreme assumption that all the detected spills are very thin, a minimum overall quantity of oil could be deduced. Moreover, in a recent study (REMPEC 2002) it has been estimated that up to some 100 000 tons of oil and oily waters enter the Mediterranean Sea every year due to operational pollution. Considering these figures, and taking into consideration the distinct hydrological and ecological characteristics of the basin, as well as its extensive coastline (45 000 km) and high concentration of specially protected areas, the situation in the Mediterranean Sea is raising a big concern. In the effort to define a possible indicator of the trend of the detected oil spills in the different years in relation to the area analysed, a table has been produced (figure 9). The oil spill density has been calculated by dividing the number of detected oil spills by the area coverage. For the Mediterranean Sea, it seems that for the period 2000–2002 the density remains constant but in the years 2003–2004 there is a significant reduction in density. 5.5 The Black Sea A special assessment for the Black Sea has been performed for the period 1999–2004 (however 2003 is missing). As for the Mediterranean, the analysis performed in the Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 Long term monitoring of oil spills in European seas 641 Figure 9. Total numbers for the Mediterranean Sea: area covered by SAR imagery, possible oil spills (1999–2004) and their ratio. Black Sea is of special interest due to the lack of a regular aerial surveillance over this basin. For the years 2000, 2001, 2002 and 2004 approximately 3165 frames acquired by the ERS-1, ERS-2 and ENVISAT satellites have been analysed and 1227 possible spills have been detected. Table 2 summarizes, for every year, the number of analysed SAR images and the number of detected oil spills. The number of images in the table includes also the cases where part of the frame contains land. In figure 10 the possible oil spills are compared to sea area covered by satellite imagery. The map displayed in figure 11 provides an overview of illicit discharges from vessels in the area of the Black Sea. This kind of representation does not take into account the uneven distribution of the observations and it is necessary to proceed to an unbiased evaluation of the hot spots. Figure 12 provides the coverage of SAR images available for detection and analysed for the years 2000, 2001, 2002 and 2004. Figure 13 displays the oil spill density as obtained by merging the information from the two previous maps, i.e. by normalizing the number of observed possible oil slicks in the Black Sea with the total number of observations available for that area. The Table 2. SAR images analysed and spills detected for the years 2000, 2001, 2002 and 2004 in the Black Sea. Year SAR images analysed Spills detected 2000 2001 2002 2004 Total 710 519 422 1514 3165 255 249 200 523 1227 Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 642 G. Ferraro et al. Figure 10. Total numbers for the Black Sea: area covered by SAR imagery and possible oil spills 2000–2004 and their ratio. analysis of the image of the Black Sea reveals an evident concentration of oil spills along the main maritime routes: Bosporus Strait–Odessa, Bosporus Strait– Novorossiysk and Bosporus Strait–Azov Sea. Moreover a concentration of oil spills was detected in the area north of the Bosporus Strait and in the Marmora Sea. Figure 11. 2004. Possible oil spills detected in the Black Sea for the years 2000, 2001, 2002 and Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 Long term monitoring of oil spills in European seas 643 Figure 12. Coverage of SAR images analysed in the present study for the Black Sea for the years 2000, 2001, 2002 and 2004. 6. Conclusions First of all it is important to underline that this analysis was performed using nonhomogeneous data. For the North and Baltic seas, data from aerial surveillance were used and, for this reason, all oil spills were confirmed. However, in these northern seas, we do not have data concerning the spatial coverage (i.e. the detailed plans of the flight routes), so it has not been possible to normalize the area coverage with the number of spills. Figure 13. Oil spill density for the Black Sea for the years 2000, 2001, 2002 and 2004. Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 644 G. Ferraro et al. Conversely, the data for the Mediterranean and the Black Sea derive from oil spills detected in archive satellite imagery. These spills are therefore to be considered as ‘possible oil spills’ because they have not been confirmed by an aircraft and or a vessel. However, the analysis using satellite data allowed the possibility of creating density maps of oil spills comparing the area coverage with the number of spills. These maps allow identification of hot spots. The key conclusion of this study is that if the data are not homogenous, the operational pollution in the seas around Europe seems to be decreasing. The best record concerns observation of detected spills in the Baltic Sea. Since 1999 the ratio of spills to flight hours is continuously decreasing. A similar positive trend has to be registered for the North Sea where the ratio of spills to flight hours has dropped off more than 50% comparing the nineties with the last five years. A steady decrease has been registered from 2003. It is possible also to register a slightly positive trend for the Mediterranean Sea; the ratio spill/area coverage has dropped off approximately 50% from 2000–2002 to 2003–2004. The trend also seems positive for the Black Sea; however the analysis of the satellite imagery for 2003 and 2005 will provide a better overview of the situation for this sea. We think that the positive trend in the decrease of oil pollution, at least for the areas closer to the European coasts, could be related to the implementation of the Directive 2000/59 on port reception facilities which entered into force in December 2002. In conclusion, it seems necessary to continue to monitor the trend in the seas around Europe and in particular to reassess the situation after the analysis of the satellite imagery of the year 2005 for the Mediterranean and the Black Sea. References BERNARDINI, A., FERRARO, G., MEYER-ROUX, S., SIEBER, A. and TARCHI, D., 2005, Atlante dell’inquinamento da idrocarburi nel Mare Adriatico. European Commission, EUR 21767 IT. BONN AGREEMENT WEBSITE. Surveillance reports are available online at: http://www. bonnagreement.org/eng/html/surveillance/surveillance.html (accessed 8 September 2008). BREKKE, C. and SOLBERG, A.H.S., 2005, Oil spill detection by satellite remote sensing. Remote Sensing of Environment, 95, pp. 1–13. BROWN, C., FRUHWIRTH, M., FINGUS, M., VAUDREUIL, G., MONCHALIN, J., CHOQUET, M., HEON, R., PADIOLEAU, C., GOODMAN, R. and MULLIN, J., 1995, Oil slick thickness measurement: a possible solution to a long-standing problem. Proceedings of the Eighteenth Arctic Marine Oil Spill Program Technical Seminar, Ottawa, Ontario (Environment Canada), pp. 427–440. CARPENTER, A., 2007, The Bonn Agreement Aerial Surveillance programme: Trends in North Sea oil pollution 1986–2004. Marine Pollution Bulletin, 54, pp. 149–163. FERRARO, G., TARCHI, D., FORTUNY, J. and SIEBER, A., 2006a, Satellite monitoring of accidental and deliberate marine pollution. In Marine Surface Films: Chemical Characteristics, Influence on Air–Sea Interactions, and Remote Sensing, M. Gade, H. Hühnerfuss and G.M. Korenowski (Eds) (Heidelberg: Springer), pp. 273–288. FERRARO, G., BERNARDINI, A., MEYER-ROUX, S. and TARCHI, D., 2006b, Satellite monitoring of illicit discharges from vessels in the French Environmental Protection Zone (ZPE) 1999–2004. European Commission, EUR 22158 EN. FERRARO, G., BERNARDINI, A., DAVID, M., MEYER-ROUX, S., MUELLENHOFF, O., PERKOVIC, M., TARCHI, D. and TOPOUZELIS, K., 2007a, Towards an operational use of space imagery for oil pollution monitoring in the mediterranean basin: a demonstration in the Adriatic Sea. Marine Pollution Bulletin, 54, pp. 403–422. Downloaded By: [Ferraro, G.] At: 08:10 19 March 2009 Long term monitoring of oil spills in European seas 645 FERRARO, G., BULGARELLI, B., MEYER-ROUX, S., MUELLENHOFF, O., TARCHI, D. and TOPOUZELIS, K., 2007b, The use of satellite imagery from archives to monitor oil spills in the Mediterranean Sea. In Remote Sensing of the European Seas, V. Barale and M. Gade (Eds) (Heidelberg: Springer), pp. 371–382. FORTUNY, J., TARCHI, D., FERRARO, G. and SIEBER, A., 2004, The use of satellite radar imagery in the Prestige accident. Proceedings of the International Conference Interspill 2004, 14–17 June 2004, Trondheim Norway. Proceedings on CD Rom. Available online at: http://serac.jrc.it/index.php?option5com_docman&task5docclick& Itemid5141&bid52&limitstart55&limit55 (accessed 8 September 2008). GADE, M. and ALPERS, W., 1999, Using ERS-2 SAR images for routine observation of marine pollution in European coastal waters. The Science of the Total Environment, 237/238, pp. 441–448. GADE, M. and REDONDO, J.M., 1999, Marine pollution in European coastal waters monitored by the ERS-2 SAR: a comprehensive statistical analysis. Proceedings of IGARSS 1999, 2, pp. 1375–1377. GADE, M., SCHOLZ, J. and vON VIEBAHN, C., 2000, On the detectability of marine oil pollution in European marginal waters by means of ERS SAR imagery. Proceedings of IGARSS 2000, 6, pp. 2510–2512. GÉRÔME, N., 2004, Oil spill detection using ERS-2 SAR images: Comparison with the aerial surveillance in the North Sea 2000. Master Thesis, Royal Military Academy, Brussels; in collaboration with JRC, Brussels. HELCOM WEBSITE. Surveillance reports are available online at: http://www.helcom.fi/shipping/ waste/en_GB/surveilance (accessed 8 September 2008). HOLLINGER, J.P. and MENNELLA, R.A., 1973, Oil spills: measurements of their distributions and volumes by multifrequency microwave radiometry. Science, 181, pp. 54–56. NORTH SEA DIRECTORATE, 1992, The Netherlands, Visibility Limits of Oil Discharges, Rijswijk, IMO document: MEPC 33/INF.28. OSPAR WEBSITE. Available online at: http://www.ospar.org. OSPAR Recommendation 2001/ 1 for the Management of Produced Water from Offshore Installations. Available online at: http://www.ospar.org/documents/dbase/decrecs/recommendation/01-01e_ consol%20Produced%20water.doc (accessed 8 September 2008). PARKER, H.D. and CORMACK, D., 1984, Evaluation of Infrared Line Scans (IRLS) and Side Looking Airborne Radar (SLAR) over controlled oil spills in the North Sea. In Remote Sensing for Control of Marine Pollution, J.M. Massin (Ed), pp. 237–256, NATO Challenges of Modern Society, Plenum, New York, USA, 6. PAVLAKIS, P., TARCHI, D., SIEBER, A., FERRARO, G. and VINCENT, G., 2001, On the monitoring of illicit discharges – a reconnaissance study in the Mediterranean Sea. European Commission, EUR 19906 EN. Available online at: http://serac.jrc.it/midiv/ pub/jrc_illicit_study.pdf (accessed 10 October 2007). REMPEC, 2002, Protecting the Mediterranean against maritime accidents and illegal discharges from ships, Malta. Available online at: http://www.rempec.org/admin/ upload/publications/WS%20BROCHURE%20(low%20res).pdf (accessed 8 September 2008). TARCHI, D., BERNARDINI, A., FERRARO, G., MEYER-ROUX, S., MUELLENHOFF, O. and TOPOUZELIS, K., 2006, Satellite monitoring of illicit discharges from vessels in the seas around Italy 1999–2004. European Commission, EUR 22190 EN. TOPOUZELIS, K., BERNARDINI, A., FERRARO, G., MEYER-ROUX, S. and TARCHI, D., 2006, Satellite mapping of oil spills in the Mediterranean Sea. Fresenius Environmental Bulletin, 15, pp. 1009–1014. TRIESCHMANN, O., HUNSANGER, T., TUFTE, L. and BARJENBRUCH, U., 2003, Data assimilation of an airborne multiple remote sensor system and of satellite images for the North and Baltic sea. Proceedings of the SPIE 10th International Symposium on Remote Sensing Conference: Remote Sensing of the Ocean and Sea Ice, 5233, pp. 51–60.