For thirty years, reduction of urban stormwater pollution is a large environmental concern. Within the city, various urban emissions are carried by water runoff towards receiving bodies such as rivers. Knowledge's development and a better understanding of this transport for some pollutants are necessary for a better urban stormwater management. Thus two French research programs, the “Experimental Urban Catchment” and the “PIREN-Seine”, tried jointly to characterize the trace metals transport mechanisms and the evolution of their biogeochemical speciation. An experimental urban catchment was chosen and equiped in the Le Marais district in Paris in order to study the metallic pollution transfer from the atmosphere to the catchment outlet both by dry and wet weather. This 42 ha catchment, impervious at 90%, with a combined sewer network, is typical of an old town centre. Its measurement network (atmospheric fallout, roofs, courtyards, streets and catchment outlet) is probably unique. Trace metals were investigated during 16 months, from May 1996 to October 1997, and 20 rain events were intercepted. Bulk and dissolved concentrations, dissolved / particular distribution, and fluxes of cadmium, copper, lead and zinc show a significant evolution in the catchment area from up-to downstream, as well as a great temporal variability. Trace metals are predominantly deposited by dry fallout (about 80%), and mainly dissolved in rainwater (from 65 to 95%). Metallic runoff loads depend on the nature and characteristics of each urban surface. Trace metals concentrations in roof runoff were greater than any other site. Roof runoff are heavily contaminated because of a corrosion of the roof and gutter materials. At the catchment outlet, after transport within the sewer, bulk metal concentrations decrease (compared to roof runoff), and their become mostly particular. Moreover, the speciation distribution of particulate metals shows a decrease of metals lability. At the scale of a rain event, roof runoff is the main trace metals source. Moreover, inlet / outlet metallic mass balances show that a few quick physico-chemical processes take place in the combined sewer during a rain event. On one hand, runoff erodes of sewer sediments and contribute to the wet weather pollution measured at the catchment outlet. On the other hand, a part of these sewer sediments (not identified at present) seems to be a metal well. Some reactions (adsorption, precipitation, or complexation) between the dissolved metals and the sewer suspended particles sediments may control this mechanism.
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