Industrial and municipal (sewage) plants located at the coast often discharge substances into the sea that float on the sea surface and damp there the short water waves that are responsible for the radar backscattering.
Therefore they often become visible on radar images as dark patches. They consist of surface-active material whose chemical composition can vary considerably. Not only mineral (petroleum) oil is surface-active, but also a countless number of natural and artificial (i.e. chemically produced) substances. It can be supposed that in many cases these discharges are not known to the authorities and that some of them are illegal.
Space-borne SAR imagery can be helpful in disclosing these coastal discharges which can be quite harmful for the coastal environment.
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