Organic geochemistry plays a major role in the environmental assessments of water quality. Today, the tools used in exploration and production of hydrocarbons combined with good understanding on natural processes which form and alter the hydrocarbons during biodegradation, weathering, oxidation and evaporation, can be applied to predict the fate of pollution in water. BTEX (Benzene, Toluene, Ethylbenzene and Xylene) are very toxic compounds and are normally present in significant concentrations in petroleum. They are pollutants in groundwater and surface water due to their high solubility. Pollutant identification is usually conducted by specialised laboratories in order to determine the relationship between hydrocarbons in water samples and suspected source of pollution. Guidelines on recommended methods for sample collection, handling and analysis are well established. For instance, analysis of BTEX in headspace gas of water sample collected in closed vessels can be performed by using: *High-resolution gas chromatography (HRGC) *HRGC coupled with mass spectrometry (HRGC/MS) Beyond to the recommended methods, more advanced techniques can be used for the pollution assessment and its behaviour: *Solid phase micro extraction gas chromatography mass spectrometry (SPME-GCMS). This simple technique is able to sample and analyze 1 ppt only of benzene in water (1 ppt = 1 mg of benzene in 1000 m³ of water). This technique is very accurate and quantitative owing to the addition of known trace amount of internal standards. *Compound specific isotope analysis (CSIA). This sophisticated technique consists in measuring the carbon and hydrogen stable isotope ratios of individual compounds. The stable isotope ratio of individual components depends on their source and their alteration. In this paper, we will demonstrate state of the art technology to measure BTEX in waste water and its behavior during the natural biodegradation and weathering process. It was well demonstrated that biodegradation increases the concentration of ¹³C and ²H isotopes in the residual non-biodegraded hydrocarbons. This paper shows that the use of SPME-CSIA and SPME-GCMS enable us: *To monitor trace levels of BTEX. *To give evidence of the biodegradation of BTEX owing to their strong and specific isotope fractionation. *To calculate biodegradation rate of BTEX by using isotope fractionation.


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