The trace of metals : Use, emissions and sediment load of urban heavy metals
Abstract: Urban areas have been considered potential ecological hot spots for metal pollution. This is the result of three trends. First, the 20th century saw a rapid increase in the use of many materials, including metals, in the industrialized world. Second, urbanization has made towns and cities areas nodes of material flows. Third, emissions from production activities have received considerable attention and regulation, but emissions via consumption have largely been ignored, so for several metals they have become more important than emissions from production activities. These emissions largely occur from centres of population. Thus, metal pollutionin urban areas is a subject of increasing importance.Whereas most studies of environmental pollution have dealt with pollutants in the environment, a new research field has evolved that is devoted to earlier identification of the fluxes that cause pollution. A tool in this work is material flux analysis. This thesis presents an extendedmaterial flux analysis of five heavy metals, Cd, Cu, Hg, Pb and Zn, in the city of Stockholm, Sweden. This approach combines an analysis of the metal fluxes within the urban system, with an investigation of sediments, which aims at identifying the current sediment load of metals, and to reconstruct the historical load by using dated sediment cores.The results indicate that the use of Cd, Hg and Pb have been reduced during the last three decades. Thus emissions to the aquatic environment have been substantially reduced. However, metal concentrations in sediments from central Stockholm, as well as fluxes of metals to the sediments, arc highly elevated compared to other areas. Budget comparisons indicate that the area investigated in and around Stockholm has changed from a positive balance, indicating a net export of Cd, Hg and Pb to the Baltic in the 1970s, to a negative balance, indicating thatthere are unidentified sources of these metals. For Cu, the budget is approximately balanced, and for Zn the area seems to be an exporter to the Baltic Sea.It is argued that the unbalanced budgets of Cd, Hg and Pb highlight the importance of integrated approaches, and of including sediment loads in environmental monitoring. The results also stress the need for a better understanding of the urban biogeochemistry of heavy metals.
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