Trace metal speciation and availability in a contaminated freshwater wetland located in Chicago, Illinois

In order to determine the effects of metal contamination on wetlands, it is necessary to gain an understanding of trace metal fate and speciation. In this study, the chemical speciation of iron, zinc, lead and manganese was investigated in a contaminated urban wetland located in a former industrial area of Chicago IL. Particular attention was paid to the formation and stability of metal sulfide phases in the wetland sediment. The aquatic system studied is impacted by steel slag waste burial within the surrounding area. It is representative of many urban wetlands located around the southern end of Lake Michigan.; Metal speciation was studied in this system using x-ray absorption spectroscopy (XAS) and chemical extraction methods. XAS methods included Zn and Fe speciation analysis by spectral deconvolution using reference standard phases and analysis of Fe pre-edge structural features. Speciation results from these analyses were compared to those obtained by Tessier sequential extraction and acid volatile sulfide determination.; XAS experiments showed that Fe and Zn are substantially associated with sulfide in the pond sediments. Zinc sulfide accounts for about 50 to 60% of the total zinc present. Laboratory studies conducted on anoxic pond sediments showed significant changes in zinc speciation as a result of oxidation, but long term zinc remobilization from sulfide phases is limited by the formation of secondary mixed carbonate hydroxide precipitate phases. Chemical extraction techniques were much less reliable than XAS at determining trace metal speciation in these sulfide dominated sediments.; In addition to furthering the development of an XAS based speciation scheme that allowed the analysis of dilute metal samples in complicated environmental matrices, this work has important implications for wetland management and creation in metal contaminated areas. The results suggest that wetlands may be a viable means of treating metal contaminated soils and waters, provided that anoxic conditions are maintained. Low zinc and lead accumulation in site vegetation indicates that metal sulfide formation is limiting the availability of these elements. Submergent and floating aquatic plants, however, represent a potential pathway for metal accumulation towards higher trophic level organisms and should be controlled at these sites.