Source apportionment of polycyclic aromatic hydrocarbons in a coastal lagoon by molecular and isotopic characterisation

The origin of polycyclic aromatic hydrocarbon (PAH) contamination in sediments of a coastal lagoon of the Adriatic Sea (Pialassa, Baiona, Ravenna, Italy) has been investigated. Concentrations, molecular distributions and stable carbon isotopic ratios (δ13C) of PAHs extracted from core sediments and soils have been determined by high pressure liquid chromatography (HPLC) with fluorescence and diode array detection (DAD), by gas chromatography–mass spectrometry (GC–MS) and isotopic ratio mass spectrometry (GC–IRMS). The total concentrations of PAHs in core sediments are highly variable, ranging from 30 to 112,000 ng/g and exhibit depth profiles similar to those registered for mercury and polyvinyl chloride (PVC). PVC was manufactured in the nearby industry from acetylene, in turn produced by partial combustion/pyrolysis of methane, the process considered responsible for PAH emission into the lagoon. In accordance, the molecular pattern of PAHs found in sediments is similar to that reported for the interior of fuel rich premixed flames of methane, being characterised by the dominance of pyrene, cyclopenta[cd]pyrene, benzo[ghi]fluoranthene, fluoranthene and by the presence of several five-membered ring PAHs, including high levels of 4H-cyclopenta[def]phenanthrene and minor amounts of dicyclopentapyrenes. The extremely low δ13C values of sedimentary PAHs (from −62‰ to −36‰) confirm that local biogenic methane is their major source. The distribution of the isotopic signatures suggests PAHs were introduced into the lagoon by direct discharge into the water body rather than by atmospheric fallout. The uncommon molecular and isotopic distribution of these PAHs is proposed as fingerprinting specific for pyrolysis of biogenic methane useful to source apportionment.