Caracterización geoquímica de los ciclos de elementos traza en ambientes urbanos

The results from several research projects carried out in Madrid and Avilés (Spain), Oslo (Norway), and Ostrava (the Czech Republic) between 1990 and 1997 have been used to outline a geochemical model of urban environments that focuses on the circulation of particulate material in the city. Three types of particulate material and corresponding different fractions have been considered in the model: suspended particles (size fractions <2.5 μm and 2.5-10 μm), settled particles or street dust (size fraction <100 μm), and soil particles (size fraction <100 μm). Samples were analysed by ICP-AES or ICP-MS, depending on each particular study. One of the geochemical cycles considered in the model could be adequately traced with the element Ag, which seems to be an excellent tracer of the use of composted sewage sludge as soil amendment in urban soils. More generally, a combination of elements (multielemental tracers) is needed to follow the circulation of particulate material along different pathways in an urban environment, i.e. Pb-Cu-Zn-Ba, to evaluate the contribution of traffic, and Zn-Cd to characterise the influence of corrosion. Lastly, it is hard to distinguish individual sources of trace elements, and only broad categories of origins, i.e. anthropic/urban, natural, and mixed/altered, can be safely determined. In this regard, Ga, La, and Y are good indicators of the natural component of urban particulate materials, and typical "urban" elements, i.e. Cu, Pb and Zn, can be used to trace the influence of the city on the natural environment around it.