Oil pollution can enter marine waters from various sources, such as stormwater drainage from cities, untreated waste disposal from factories and industrial facilities, as well as unregulated recreational boating, onshore air pollution or natural seeps and by the heavy traffic in marine transport. The challenge is the prevention, detection, and removal of spills [1
]. The ability to detect oil in the aquatic environment depends on the oil form and weathering state, the amount of oil in seawater and the spatial scale of the oil spill. When oil is visible on the surface of seawater, space-borne remote sensing detection, or airborne detection [2
] are the most effective methods to assess the scale of the spillage. The advantage of remote sensing is the ability to observe events in remote and often inaccessible areas such as oil spills from ruptured pipelines. Moreover, remote sensing data through multi-temporal imaging allow obtaining information about the oil movement rate and direction, which is crucial in oil spill control. Remote sensing is based on a wide range of devices such as infrared video, photography from airborne platforms, thermal infrared imaging, and laser fluorosensors. In oil spill detection, airborne or space-born optical sensors are useful. After a very large oil spill in the Gulf of Mexico in 2010, various space-borne remote sensors were tested as possibly suitable for oil pollution detection and oil slick tracking [4
]. However, during darkness and under poor weather conditions, only Synthetic Aperture Radar (SAR) is reliable [2
When oil is invisible in an optical range, there is a broad spectrum of methods available for oil detection, such as ultraviolet, infrared or Raman spectroscopy [1
] and sensitive methods based on fluorescence phenomenon [12
]. Submersed oil (for example, oil-in-water emulsions) to a small extent manifests itself in the above-water upward radiance field, although detection is possible under certain lighting and weather conditions [14
] or using underwater sensors [15
Fluorescence spectroscopy also seems to be a sensitive method to study the complex structure of hydrocarbons with aromatic compounds responsible for oil fluorescence. However, seawater exhibits its own fluorescence spectra due to the presence of natural seawater components, referred to as Dissolved Organic Matter (DOM) [16
], Fluorescent Dissolved Organic Matter (FDOM) [17
], Coloured Dissolved Organic Matter (CDOM) [18
], phytoplankton [20
] and pigments [21
]. It was found that the fluorescent properties of seawater in the ultraviolet (UV) range change after contact with even a small amount of oil. The fact that the shape of the spectra of seawater fluorescence changes after contact with oil has already been demonstrated [13
] in studies of the ultraviolet field (excitation from 200 to 340 nm, emission 240–500 nm). Thus, the problem to be solved now is how to identify water that has been in contact with oil, based on fluorescence spectra.
The research described in this paper involves situations in which oil appears in the marine environment, but visual perception is unlikely. The oil may be invisible when it enters the water in small amounts but can become a threat over time. These events can occur, for example, in the case of unsealing underwater pipes [22
], oil leaking during port trans-shipment [23
], or leaks from wrecks [24
]. Fuel leaks or lubricating oil from ships cannot be ruled out even if they operate faultlessly, or when a failure was not noticed by the ship’s crew. In some parts of the sea, there may be natural seepage from the bottom of the sea [26
The paper reports on a study of the effectiveness of fluorometric index (FIo/w
) for water sampled from the sea at different points sampled several times in the warm season. It is, therefore, a continuation and extension of the authors’ previous paper [29
] that includes water from only one place. In the present paper reports on a study extracted from excitation-emission fluorescence matrix spectra (EEMs), fluorometric index (FIo/w
) for water sampled from the sea in five stations located along the coastal and port waters of the Gdynia region (Southern Baltic Sea) collected at four times in summer season (June and July) in this area.
The fluorometric index FIo/w was defined as a tool for oil pollution detection in seawater in such a manner that the oil pollution present in the sea is manifested by an increase in FIo/w. In the authors’ previous paper, the effectiveness of FIo/w was tested for seawater polluted by various kinds of oils and various oil-to-water ratios for one sampling point. The tests performed earlier and statistical calculations indicated a high similarity in the values of FIo/w independent of both the kind of oil and oil-to-water ratio.
In the paper, the test of the effectiveness of FIo/w was expanded, corresponding to different sampling points and several sampling times. The results presented in this paper concern seawater collected from five different stations in coastal waters four times during the summer season in the Baltic Sea (June–July) and seawater polluted by oil for different oil-to-water ratio (0.5–500 × 10−6). For the tests, only one oil was used (Petrobaltic-crude oil), taking into account the independence of FIo/w from the kind of oil. The results indicate the FIo/w value, starting from a certain threshold amount of oil (in this case it the oil-water-ratio is about 5 × 10−6), ceases to increase despite increasing the dose of this contamination for five different stations in coastal waters. The rising effect of FIo/w in the case of contact of water with oil was observed in each of the twenty examined cases. In summary, the results indicate the independence of FIo/w index from the time and point of sampling. By treating this fact as a good prognostic of the effectiveness and universality of the method, it is possible to extend the research to include water collected in other seasons and from a wider area. Moreover, the wavelengths proposed for the FIo/w index could be used in the future to design and build sensors for oil detection.