# Potentially Toxic Trace Elements in the Urban Soils of Santiago de Compostela (Northwestern Spain)
## 4 Discussion
The lithological origin of these elements was very clearly manifested in our study for Cr and Ni, which are typically higher in mafic and ultramafic rocks, such as amphibolites and serpentinites [44], and high concentrations of these elements have been found in cities where these geological substrates are present, like Torino, Antalya, Gibraltar, Addis Ababa, or Baltimore [2]. In our study, both elements were highly correlated between them and not to other metals (Table 5) and presented a similar spatial distribution (Figure 3), with concentrations significantly higher in soils developed over amphibolites (Figure 2). In addition, significant correlations were found with edapheric properties typically associated with amphibolite soils, such as clay and Fe compounds, in which these soils are richer than soils developed over schists, genesis, or granites [45, 46]. In the case of As, which concentrations in the soils of Santiago de Compostela are much higher than in other cities in the world [2, 3], an important lithological component exist. This is due to the geochemical enrichment in As in the region of Galicia, as shown by the high background levels (Table 2) and average concentrations in natural soils that are as high as 21 mg kg\({}^{-1}\) (Table 3, [36]). Anthropic sources, such as fossil fuel combustion, especially coal, can also contribute to increasing As concentrations in urban soils [47]. In addition, there are two points in the city that present much higher As levels than the rest (soils 8 and 15, with respectively 138 and 205 mg As kg\({}^{-1}\)), corresponding to recently urbanized areas. Since there is no relation of these points to any known source of As emissions, the reason for such high concentrations remains uncertain. The availability of information about the presence and concentrations of trace elements in urban soils is a valuable tool for the management of urban ecosystems and for land planning in the cities in general. Since the objective must be to reduce the potential risks for human health and the environment that these elements represent, practices must be adopted that aim at limiting the transfer of pollutants from soils to other environmental compartments. Contrarily to urban soils in many other cities, the soils in Santiago de Compostela are acid [30], which would increase the potential mobility of trace element since their solubility is higher at acid pH values [44]. Besides, erosion and transport of soil particles can be important processes of pollution of water courses and other soils, due to the high rainfall regime in the area, in combination with the topography of the city. In this sense, practices oriented at conserving and increasing soil organic matter levels can be a decisive factor to reduce these risks: on the one side, the high affinity of trace elements, in particular Pb and Cu, for organic matter in these acid soils [48, 49] leads to a reduction of their mobility with increasing organic matter contents; on the other side, organic matter improves soil aggregate stability and permeability, increasing infiltration and reducing runoff losses and erosion. Finally, land use in the city should be planned carefully and taking soil trace element concentrations into account. This is especially significant for the development of urban agriculture projects, where the risk that trace elements represent is higher due to their potential transference to the food chain through crops [42; 50; 51; 52]. In this sense, soils should always be analyzed before developing new urban gardens so the areas with the highest levels of pollutants can be avoided. In addition, regular monitoring of trace elements in the soils of these areas is also necessary, since initial pollutant concentrations are likely to increase with time due to the use of some amendments, fertilizers, and agrochemicals [53].