Gas chromatographic analysis of freshwater lipid compounds from Guadarrama river in areas with high human pressure

Abstract

Increasing demand of urban areas in Spain during the last decade has caused a series of environmental problems that are reflected in the quality of riverine waters. Many rivers in the Iberian Peninsula, that formerly received runoffs from agricultural areas, are at present taking rising inputs of urban effluents, frequently not subjected to previous wastewater treatment. These waters often transport anthropogenic molecules, which in several cases can be successfully distinguished from products derived from natural sources, such as fluvial organisms, in this case mainly benthonic. In this study series of organic compounds in waters from 16 summer sampling stations along the low basin of Guadarrama river (Madrid) are identified using gas chromatography in order to discriminate between natural or anthropogenic origins. This river basin, subjected to predominantly agricultural management until the 80s, is presently occupied by ca. 1 million inhabitants. The organic fraction was extracted with methylene chloride, methylated with trimethylsilyldiazomethane and analysed in a Varian Saturn 2 gas chromatographic-mass spectrometric system equipped with a 10 m x 0.22 mm i.d., cross-linked OV-101 column; these chromatographic conditions being specifically fitted for a detailed identification of high-molecular weight signature compounds in water. The chromatographic analysis (Fig. 1) showed a complex composition made up mainly of C15- C33 (mostly C17—C29) n-alkenes with odd-C number preference, suggesting their origin in epicuticular waxes from vascular plants. Fatty acids (mainly C14—C34) were in very low amounts, with a bimodal distribution and major peaks for palmitic and stearic acids; the odd-C numbered fatty acids typical in terrestrial plants were practically lacking. In addition, all samples showed a conspicuous series of wax esters composed almost exclusively of C74—C32 palmitates, the triacontyl hexadecanoates dominating in most samples. In conclusion, the overall interpretation of the chromatogram suggests that the information on the balance between inputs from terrestrial and aquatic organisms is displayed mainly by indices such as the relative amount of waxes and the chain length and carbon preference index of the alkyl series, mainly fatty acids. On the other hand, few possibilities to monitor the origin of the organic matter from different samples on the basis of the occurrence of diagnostic biomarker compounds derived from the autochthonous trophic system.