Identification of natural, anthropogenic and petroleum hydrocarbons in aquatic sediments

Abstract

Complex distributions of hydrocarbons occur in most aquatic sediments. Total concentrations can range from a few parts per million in non-polluted intertidal and oceanic areas to parts per thousand in heavily contaminated estuarine, lake and near-shore environments. Iatroscan TLC-FID provides a quick assessment of the total hydrocarbon load, but capillary GC, HPLC and GC-MS are essential for obtaining detailed composition data from which information on possible sources can be derived. Alkenes from microalgae, such as n-C 21:6, n-C 17:1 and unusual C 25 and C 30 isoprenoid alkenes, are often the most abundant single components in sedimentary hydrocarbon distributions. Some hydrocarbons are also produced from diagenetic transformation of functionalised lipids present in the sediment. Long-chain alkanes typical of plant waxes can be important constituents, even in marine sediments far from the coast. These distributions display a strong predominance of odd chain lengths, whereas n-alkane distributions in oils usually show little or no predominance of either odd or even chain lengths. However, the presence of this type of distribution in a sediment may not indicate petroleum contamination since biological sources for similar distributions are now recognised. Petroleum-derived residues are common in coastal and estuarine areas, particularly those near urban or industrial centers. This contamination is readily seen in capillary gas chromatograms of the alkanes as an unresolved complex mixture (UCM). The source of the oil can often be deduced from characteristic distributions of biomarker steranes, rearranged steranes, hopanes and methyl hopanes determined by capillary gas chromatography-mass spectrometry. Methyl hopanes are major polycyclic alkanes in oils from carbonate source rocks, such as those from the Middle East, but are uncommon in Australian oils. GC-MS fingerprinting techniques show that lubricating oils are a major source of hydrocarbon pollution in many estuaries and coastal areas around Australia. It has been estimated that natural oil seeps may also contribute as much as 10% of the hydrocarbons in the global marine environment. Examples of this include major oil seepage in the Gulf of California and the widespread occurrence of bitumen strandings on South Australian beaches. Examples from marine and estuarine environments around Australia are presented to illustrate the use of modern analytical techniques to identify, quantify and determine the origins of hydrocarbons in aquatic sediments.