Can the contamination of urban street sediment be used as an indicator for traffic density? A case-study in the city of Leuven, Belgium

摘要

The concentration, in urban street sediment, of 13 chemical elements was measured in the city of Leuven, Belgium. A total of 77 locations were sampled in pedestrian zones, streets with limited traffic, streets with much traffic, and on the ring road that has very busy traffic. The data were compared to NO_2 concentrations measured by the CurieuzeNeuzen-Vlaanderen project, a large-scale study with 20,000 sampling points all over Flanders (the northern part of Belgium). NO_2 is a frequently used indicator for traffic pollution. In Leuven the highest enrichment (strongest pollution) was measured for Cu, Sb, Pb and Zn. These elements could be related to brake wear and tire wear, respectively. For Cu and Zn the concentrations in Leuven exceed those in most of the other cities investigated in the literature. Moderate enrichment was measured for Cd, Cr, Ni and S. The other elements (As, Co, Fe, Mn and V) showed concentrations close to the background value in unpolluted soil. The Integrated Pollution Index (IPI) for Leuven is 8.15, which, according to criteria proposed by the literature on street sediment, classifies Leuven, on average over its surface area, as a "very highly polluted" city. As expected, for Cu and Sb the highest contamination is found on the ring road and the busy traffic circulation loops. For Zn and Pb, on the other hand, the highest contamination occurs in the city center, in the pedestrian zone where no traffic is allowed except for buses and taxis. We hypothesize that this is a result of historic accumulation of these elements at the time traffic was still allowed in this zone. In Leuven the chemical composition of street sediment did not correlate to the NO_2 concentrations. This study shows that measurements of current pollution by traffic are not sufficient to determine the health risk because much exposure to toxic substances may be caused by resus-pension, by traffic or wind, of substances that have accumulated in the city over time, sometimes decades ago, when regulations were much less stringent then today.