Reverse trajectory study of oil spill risk in Cyclades Islands of the Aegean Sea

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Abstract

The impact of hypothetical oil spills in Cyclades Islands (Aegean Sea, Eastern Mediterranean) is investigated by a reverse trajectory method whereby tracers have been back-tracked using surface current and wind data from 2013 to 2018. Results show that the westernmost islands of the archipelago (Kithnos, Kea, Andros) are at high risk all year round due to the proximity to the western oil tanker route (Bosphorus to Western Mediterranean). The arrival time of oil to these islands is less than 1 day, with the involvement of the entire perimeter of each island. The eastern islands are exposed to oil coming from the eastern oil tanker route (Bosphorus to Suez). In the summer, oil can reach the northern coast of Amorgos and Donousa in less than 1 day and Naxos in less than 2 days due to persistent ‘meltemi’ and induced surface current. Oil surveillance (for example by SAR) should focus all year round on the western route and on the eastern route north of the archipelago during the summer.

Introduction

Shipping and maritime transport of oil and hazardous substances are a matter of concern for coastal sites and marine habitats of the Mediterranean Sea. The Mediterranean Sea is still the shortest route connecting Asia and Europe, and bears 1/3 of the global seaborne oil traffic (Madrid et al., 2016). Gibraltar and Bosphorus are two of the narrowest and busiest straits in the world, and the Suez Channel has recently been widened (Zodiatis et al., 2016). According to the International Tanker Owners Pollution Federation Limited (ITOPF), collision is the first cause of accidents resulting in an oil spill (26%) and high traffic rates increase the risk of an accident. In addition, the risk of oil pollution is augmented by the illicit discharge along the main tanker routes, more frequent in international waters.

In the Eastern Mediterranean, the Aegean Sea is crossed by oil tanker routes connecting Bosphorus with Gibraltar in the west and Suez in the east (Fig. 1a). This study investigates the risk of oil pollution in the Cyclades archipelago of the Aegean Sea (Fig. 1b), which hosts several coastal sites of primary historical, ecological and touristic importance. The aim of the study is to provide two results: first, the identification of sea areas around the archipelago where an oil spill would be most dangerous; second, the identification of the islands most exposed to the risk of contamination due to oil spill. In this study, the pathways of the oil have been simulated by meteo-oceanographic data from 2013 to 2018, and the technique employed is the backward-in-time integration of Lagrangian trajectories, which is specific for the prevention of oil spill risk. The results account for the consideration that the oil spill could occur in proximity of the two oil tanker routes encompassing the archipelago.

Previous studies aimed to the same purpose have been produced in North Sea (Chrastansky and Callies, 2009), Gulf of Finland (Soomere et al., 2010, Delpeche-Ellmann and Soomere, 2013) and in protected areas of the Mediterranean Sea, for instance in the Strait of Bonifacio (Olita et al., 2012a), in the Sicily Channel (Ciappa and Costabile, 2014) and in the Levantine basin (Alves et al., 2014). These studies, crucial for oil spill contingency planning, are also relevant for satellite surveillance acting as a deterrent to discourage the practise of illegal discharge (Pavlakis et al., 2001, Brekke and Solberg, 2005). Oil spill satellite surveillance is based on Synthetic Aperture Radar (SAR) (Alpers and Huhnerfuss, 1988, Garcia-Pineda et al., 2020) and is operated over the European Seas by the CleanSeaNet (CSN) service of the European Maritime Safety Agency (EMSA). Today, about 13 images per day are acquired over the Aegean Sea by the COPERNICUS Sentinel-1 SAR constellation (S1A and S1B satellites), and other SAR constellations provide an equivalent number of images per day. Considering the large amount of data acquired by current and future earth observation systems, priority in oil spill surveillance should be assigned to the sea areas where an oil spill could occur and from where winds and currents quickly push the oil towards specific marine protected areas.

The paper is structured as follows: technique and data used are described in Section 2; the results for the whole archipelago and for the most exposed islands are illustrated in Section 3. Technique and results are discussed in Section 4 and findings are summarized in Section 5.

Section snippets

Lagrangian trajectories and backtracking

In oil spill modelling, trajectory methods are commonly used to investigate the oil drift pattern induced by wind and currents, and trajectories can be tracked forward-in-time or backward-in-time (Spaulding, 2017).

Forward-in-time tracking consists of Lagrangian elements (LE) which are released at the source of the oil spill and are tracked forward-in-time up to their final destination. This is the most straightforward approach to forecast the evolution of known oil spills and is used in

Results

The simulations were carried out according to the following scheme: Lagrangian tracers were released from each of 457 receptor points at intervals of 40 min from 1st of January 2013 to 31st of December 2018, with a total amount of 6.0 106simulated trajectories per year. The tracers were tracked backward-in-time for 5 days by integrating the equation of motion backward-in-time with a time step of 5 min, moved by the wind (fields updated every 6 h) and the sea current (fields updated every day)

Discussion

During an oil spill, part of the oil can be released at several depth levels of the water column or near the bottom and part is released at sea surface, depending on the nature of the accident (sinking of oil tanker, with or without hull splitting; collision between oil tankers, occurrence of fires). After the first few hours, the oil floating at the sea surface is exposed to the elements and undergoes weathering processes specific for the type of oil (Fingas and Fieldhouse, 2004). Oils forming

Conclusions

The risk of oil pollution in Cyclades Islands in the Aegean Sea due to two main oil tanker routes, the western route connecting Bosphorus Strait with the western Mediterranean Sea and the eastern route connecting Bosphorus to Suez Channel, is investigated in this study by oil pathways simulated by Lagrangian tracers tracked for 5 days backward-in-time around the archipelago for a six-year timeline, from 2013 to 2018. The results are that major western islands such as Kithnos, Kea and Andros are

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The author acknowledges EU institutions including Copernicus, Marine Environment Monitoring Service (CMEMS), which allowed free access to the data used in this study.

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