Today, the use of standard tools of petroleum organic geochemistry is the most convenient way to characterize and quantify contamination of environment by fossil fuel. Good knowledge of natural processes that alter the hydrocarbons during biodegradation, weathering, oxidation or simply evaporation, can also be applied to predict the fate of a pollution. For instance, organic geochemistry played an important role in the environmental assessments of oil spills (e.g.Exxon Waldez or Deep Water Horizon). Oil spill identification is usually conducted by a specialized laboratory in order to find the source of a spill. Guidelines on recommended methods for sample collection, handling and analysis are well established (e.g. NORDTEST Oil Spill Identification system). At TRCQ, we use a variety of geochemical techniques in order to: *make the distinction between naturally occurring hydrocarbons and anthropogenic pollution; *make the distinction between crude oil and refined hydrocarbons; *assess the origin of oil spills, oil slicks and gas seeps--these techniques can be used for the characterization of the origin of fluids in the case of well integrity issues; *monitor the fate of spilled oil and its alteration; *map the spatial distribution of hydrocarbon pollutants in sediments and aquifers and their evolution through time. Advanced geochemical techniques are used: *high resolution gas chromatography (HRGC) for fingerprinting; *HRGC coupled with mass spectrometry (HRGC/MS) for fossil biomarkers analysis; *compound specific isotope analysis (CSIA), a sophisticated technique that consists of measuring the carbon and hydrogen stable isotope ratios of individual compounds (C1 to C30) separated by GC; the stable isotope ratio of individual components depends on their source and their alteration, and CSIA is the only technique able to correlate the gas seep to its source. In this paper, we show the use of organic geochemistry to characterize the oil spill pollution at seabed occurring in 1998. GC-MS analyses of the alkane fractions has allowed delineating three groups of samples: *samples dominated by a petroleum signature; *samples in which petroleum occurs with a subordinate land-derived contribution; *samples in which petroleum is associated to a recent marine input--within this group one may generate a subgroup in which some terrestrial debris have been seen in minor amount.


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