Geomorphological assessment of sediment contamination in an urban stream system

Little is known about the influence of fluvial-geomorphological features on the dispersal of sediment-related contaminants in urban drainage systems. This study investigates the relation between reach-scale geomorphological c onditions and network-scale patterns of trace-element concentrations in a p artially urbanized stream system in East-Central Illinois, USA. Robust stat istical analysis of bulk sediment samples reveals that levels of Cr, Cu, Pb , Ni, and Zn exceed contamination thresholds in the portion of the watershe d in close proximity to potential sources of pollution-in this case storm-s ewer outfalls. Although trace-element concentrations decrease rapidly downs tream from these sources, substantial local variability in metal levels exi sts within contaminated reaches. This local variability is related to reach -scale variation in fluvial-geomorphic conditions, which in turn produces v ariation in the degree of sorting and organic-matter content of bed materia l. Metal concentrations at contaminated sites also exhibit considerable var iability over time. Analytical tests on specific size fractions of material collected at a high ly contaminated site indicate that Cr and Ni are concentrated in the 0.063 to 0.250 mm fraction of the sediment. This fraction also has elevated conce ntrations of Zr. SEM analysis shows that the fine sand fraction contains sh ards of stainless steel within a matrix of zircon sand, an industrial mater ial associated with a nearby alloy casting operation. Samples of suspended load and bedload at the contaminated site also have elevated amounts of tra ce metals, but concentrations of Ni and Cr in the bedload are less than con centrations in the bed material, suggesting that these trace elements are r elatively immobile. On the other hand, amounts of Cu and Zn in the bedload exceed concentrations in the bed material, implying that these trace metals are preferentially mobilized during transport events. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.