Petrographic Characterization of Anthropogenic and Natural Organic Matter in Polluted Fluvial Sediments

Fossil fuel and combustion residues contribute to environmental degradation by contaminating soils and sediments. Such anthropogenic organic matter (OM) can serve an archival function, documenting the local history of industrial pollution. A 3 m fluvial sediment core was collected in an industrial area near Chicago, Illinois. Aliquots of dry, whole sediment sampled every 30-50 cm down the. core were mixed with petrographic epoxy, polished and examined under white and blue reflected light using a Leitz Ortholux II POL-MK microscope. Ages are estimated by extrapolation of sedimentation· rates computed for nearby cores using 137Cs dating (Cahill and Unger, 1993). In the upper sediments of the core (depths ≤76 cm) coal particles account for 24-34 I % by volume of the total solid OM. Coke comprises 17-21 vol. %, petroleum coke another 3-6 vol. % and char another 14-22 vol. % (Fig. 1). Fly ash, while predominantly an inorganic industrial waste product, is also readily detectable with reflected light microscopy and is abundant in the upper sediments. The OM of this interval is almost exclusively the product of local industrial activity, such as steel-making and oil refining. In contrast, the deeper sediments (130-307 cm) show a strong predominance of plant fragments (76-84 % by volume of the solid OM), with ancillary charcoal (15-20 vol. %) (Fig. 1). The sediments of this deeper interval archive the natural OM of the preindustrial environment. The onset of major industrialization is recorded in the transitional sample collected at 106 cm, containing 15 % coal, 4 % coke and 46 % plant fragments (by volume of the total solid OM) (Fig. 1). This depth interval corresponds approximately to the year 1903, in agreement with historical records. Interestingly, the amount of total solid organic carbon (determined after CH2Cl2-extraction of the sediment) remains relatively constant throughout the entire cored interval (6.7 ± 2.0 weight %, Fig. 1). The sedimentary record at this site clearly documents the loss of the pristine natural environment due to unregulated industrial development. Reference Cahill R. A. and Unger M. T. (1993) Evaluation of the extent of contaminated sediments in the West Branch of the Grand Calumet River, Indiana-Illinois, USA. Water Sci. Tech. 28:53-58