USING ENVIRONMENTAL FORENSICS TO DISTINGUISH LEGACY AND ONGOING CONTAMINATION IN URBAN WATERWAYS

Sediments in urban waterways worldwide are characteristically contaminated by a large variety of inorganic and organic substances due to deliberate or inadvertent discharges. Among organic contaminants, hydrocarbons are particularly ubiquitous as a consequence of the intensive use of fossil fuels, typically in sediment concentrations orders of magnitude greater than manufactured compounds such as polychlorinated biphenyls (PCBs) and pesticides. The environmental forensic approach permits source fingerprinting based on the distribution of hydrocarbons present at a given site. A key distinction is that between the "petrogenic" signatures of hydrocarbon mixtures derived directly from crude oil and petroleum product spills and the "pyrogenic" from fuel combustion products, in particular polycyclic aromatic hydrocarbons (PAHs) typically dispersed as airfall deposits. Coal tar residues persist to this day as a distinctive contaminant legacy of formerly common manufactured gas plants. Sites near active and former factories may be locally contaminated by more unusual materials, such as resins used in plastics manufacturing.

In urban sediments contaminant hydrocarbons most often coexist with complex mixtures of biologically-derived organic compounds, which may in turn be signatures of other types of environmental stress. As indicators of a particularly vexing problem, sewage-derived steroids, fatty acids, and surfactants are incorporated into sediments from discharges of inadequately treated urban waste or via combined sewer overflows (CSOs) after heavy rainstorms. Water bodies enriched in nutrients from treatment plant discharges or fertilizer runoff may suffer seasonal algal blooms, which in turn leave their own molecular signatures in bottom sediments.

Molecular characterization of sediments by pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) permits the recognition of all the above-mentioned types of organic pollutants in a single analysis. This can include contaminants not yet recognized by government regulators and overlooked by conventional analytical methods. The method also permits the assessment of the degree to which the "natural" organic matter co-occurring in the sediment is predominantly aquatic or terrestrial in origin.