Integrated Use of n-Alkanes and PAH to Evaluate the Anthropogenic Hydrocarbon Sources and the Toxicity Assessment of Surface Sediments from the Southwestern Coasts of the Caspian Sea

Polycyclic aromatic hydrocarbon (PAH) compounds and normal alkanes form a large group of undegradable environmental contaminats. This study aims to determine the sources and distribution of oil pollution (PAH compounds and normal alkanes) in the sediments of the southwestern coastal areas of the Caspian Sea and to compare their levels with the relevant standards. For this purpose, 18 surface sediment samples were collected from depths of 10, 20, and 50 meters along two transects in the vertical direction located in the coastal areas of Sangachin and Hashtpar (Gilan Province). The samples were then examined using mass-spectrometric gas chromatography. The origins of n-alkanes were identified using CPI index (0.76-0.95), U/R (3.30‒6.57), and Pristane/Phytane (0.21‒0.42). The sources of PAHs were determined using the index ratios of LMW/HMW (1.93‒13.37), Phenanthrene/Anthracene (11.44‒ 16.7), Chrysene/Benzo (a) anthracene (4.69‒10/33), Fluoranthene/Pyrene (0.53‒0.69), and MP/P (0.05‒0.08). Results confirmed the dominant petrogenic source of the hydrocarbons found in the region. The total concentrations of 30 aliphatic hydrocarbons and PAHs in the sediments ranged from 823.8 to 3899.5 µg/g and from 626.95 to 3842.5362 ng/g, respectively. Comparison of the measured PAH concentrations with US sediment quality guidelines revealed that the levels of naphthalene, fluorine, Acenaphthylene, and Acenaphthene exceeded the ERLs at stations with depths of 50m in Sangachin and Hashtpar while comparisons with Canadian standards indicated that they were higher than PELs at all the stations sampled. A major point of great concern is the high concentration of naphthalene as the most toxic PAH compound, which naturally warrants due attention to adopt appropriate management programs.