Features of the spread of organic compounds in the Black Sea after the accidental spill of fuel oil.

The paper discusses the consequences of the accidents with tankers Volgoneft-212 and Volgoneft-239 in the Black Sea on December 15, 2024. The post-emergency study had showed that the water area near Anapa had been the most polluted. The decrease in hydrocarbon concentrations observed 2.5 months after the accident was due to both their physical weathering on the shore and in the water, and the bacterial degradation. As a result, the concentration of chloroform-extractable bitumen decreased, as well as low-molecular alkanes and polyaromatic hydrocarbons (PAHs) in its composition. However, the composition of hydrocarbons remained dominated by oil-derived compounds in the suspended matter of bottom waters and in surface sediments. After 3.5 months, light n-alkanes up to n-C17 and naphthalenes were almost negligible in the oil lumps. In the surface sediments, the concentrations of aliphatic hydrocarbons also declined. In the Kerch Strait, they decreased from 71 to 53 μg/g (dry weight) and from 1.15 to 0.86 % Corg normalized; in the Anapa area, they dropped from 31 to 25 μg/g (dry weight) and from 5.63 to 3.06 % Corg-normalized. The content of n-C17 and odd high molecular weight homologues increased. In contrast, PAH concentrations slightly rose in the Anapa area mainly because of the increase in phenanthrene content. In the area of the Kerch Strait and Crimean Peninsula, they went up driven by pyrogenic PAHs. This suggests inputs from vessel traffic and shipping related activities as these areas are high traffic maritime routes. The toxicity index in the sediments of Anapa area decreased from an average of 82 % in March to 63 % in April 2025, while the total toxicity level declined from 0.33 to 0.07. Concentrations of individual PAHs did not exceed the Canadian interim quality guidelines for marine surface sediments.