# Potentially Toxic Trace Elements in the Urban Soils of Santiago de Compostela (Northwestern Spain)
## 4 Discussion  
Trace element concentrations are expected to be higher in urban soils than in natural and agricultural soils because pollution sources are usually more abundant in urban areas, including industrial facilities, waste disposal, and waste management activities, concentration of traffic and population, etc. Most trace elements are emitted to the atmosphere from these sources and associate to dust that is later deposited in the surface, reaching the soils where they accumulate because of their affinity for soil components that reduce their mobility [37]. In this sense, studies in cities all over the world have consistently shown that urban soils present higher concentrations of potentially toxic elements than surrounding non-urban soils, in particular if they are affected by industrial activities [2; 3]. This was also observed in our study: in general, urban soils in Santiago de Compostela present trace element concentrations higher than the regional background values (Table 2) and the average levels in natural soils (Table 3 [36]), with different situations among the six elements considered. In a first group, Pb, Cu, and Zn concentrations exceed these values in 75%, 51%, and 45% of the soils analyzed, respectively, whereas Cr, Ni, and As concentrations are closer to those of natural soils, surpassing background levels in 20%, 4%, and 13% of the soils, respectively. Comparison of total trace element concentrations in urban soils in Santiago to other studies shows that Cu, Pb, and Zn concentrations are overall lower than those found in much larger and industrialized cities like Chicago, Baltimore, or New York City [2; 38; 39; 40]. However, average Ni or Cr concentrations are similar to these cities whereas As levels are higher in Santiago de Compostela. This can be explained by the predominance of different factors controlling the presence of each element in urban soils: Cu, Pb, and Zn would be mainly related to anthropogenic activities, so their concentrations in soil will  
Figure 4: Left: IGeo summary results. Right: Comparison of concentrations in soils with the Generic Reference Levels (GRL) established in regulations [32]. increase in parallel to the size and level of industrialization of the city. In turn, Cr, Ni, and As concentrations in urban soils are most frequently controlled by lithological factors [2]. In this study, soil Pb, Zn, and Cu concentrations were clearly associated, as shown by their similar spatial distribution (Figure 3) and their mutual significant correlations (Table 4), so it is likely that the enrichment observed for these elements has a common anthropogenic origin. Their concentrations are higher in low valley landforms in the city center: areas that are prone to receive and concentrate pollutants by runoff from higher areas. This is particularly evident for Pb, which is marked consistently in the literature as the trace element that most typically accumulates in urban soils as a result of human activities [2, 3, 41, 42], with traffic as the main cause of the emissions of this element, in addition to paints, glass and batteries [43]. Zinc emissions in urban areas are also associated to vehicle emissions (from example from tire abrasion), alloys, batteries, and metallic parts in buildings, such as gutters whereas the main sources of Cu in urban soils are alloys and electrical wiring [44]. Besides, in our study a non-negligible source of pollution is the common use of copper sulfate as fungicide in urban gardens, a typical agricultural practice in the area due to the climatic conditions that foster the development of fungal diseases. In contrast to Cu, Pb, and Zn, literature shows that Cr, Ni, and As levels in urban soils are mainly controlled by lithology, although this does not mean that anthropic sources such as alloys, paints and dyes, waste incineration, coal and oil combustion do not contribute [2].