| The study of hydrophobic organic compounds (HOCs) in sediments is useful in determining fate and transport characteristics of similar substances. Specific compounds or groups of compounds have been found to maintain their molecular integrity while under transport in sediments, making them good molecular markers, including polycyclic aromatic hydrocarbons (PAHs), alkanes and polychlorinated biphenyls (PCBs).; Surface sediment samples were collected in a longitudinal transect from various locations in several recognized hydrogeographic zones of the Potomac River, and two Potomac tributaries. These sediments were subjected to physical and chemical analyses to quantitatively determine sediment characteristics and hydrocarbon content, and to deduce source identities. Moisture content, particle size, and organic carbon, hydrogen and nitrogen content were determined. Samples were Soxhlet extracted and selectively fractionated to separate selected polycyclic aromatic hydrocarbons (PAHs) and alkanes for further analysis by gas chromatography-mass spectrometry (GC-MS), and to separate PCBs for further analysis by gas chromatography, with electron capture detection (GC-ECD). A parallel column, dual-detector technique was used in an attempt to increase resolution of coeluting congeners, a common problem in PCB analysis.; High levels of HOCs in the Anacostia River (urban watershed) were shown to translate to higher levels in the Potomac River downstream from their confluence. Similarity in the ratios of PAHs throughout the watershed supports atmospheric deposition as a secondary source. With the exceptions of retene and perylene, molecular weight ratios supported major inputs of combustion-generated, anthropogenic PAHs. Alkanes are traceable primarily to natural sources, but some petrogenic input was noted, particularly in the Anacostia, as supported by Carbon Preference Index (CPI) values. The C23 n-alkane, tricosane is enriched in sediments associated with certain marsh flora, notably Sphagnum species, and may be further enriched by the extensive use of sphagnum moss in residential and commercial landscaping.; The parallel-column, dual-detector technique for PCB analysis was noted to produce differing chromatographic elution patterns, although coelution was still found to be a major concern. The method was found to give little improvement to the single-column, single-detector technique commonly employed. |
