# Potentially Toxic Trace Elements in the Urban Soils of Santiago de Compostela (Northwestern Spain)
###### Abstract  
With the objective of increasing information inorganic pollutants in urban soils in Spain, we studied the presence of Cu, Pb, Zn, Ni, Cr, and As in 55 soils in the city of Santiago de Compostela (northwestern Spain). The soils were developed over diverse parent materials (granites, genesis, schists, and amphibolites) and present different land uses, urban grasslands, urban forests, urban allotment gardens, and peri-urban agricultural soils. Total trace element concentrations, analyzed by XRF of ground samples, were correlated to physicochemical properties of the soils, and the influence of land use, lithology, and location on the degree of pollution was explored. In most soils, trace element concentrations followed the sequence Zn (55-484 mg kg\({}^{-1}\)) \(>\) Pb (20-566 mg kg\({}^{-1}\)) \(>\) Cr (17-277 mg kg\({}^{-1}\)) \(>\) Cu (17-188 mg kg\({}^{-1}\)) \(>\) As (13-205 mg kg\({}^{-1}\)) \(>\) Ni (11-91 mg kg\({}^{-1}\)). The concentrations were overall higher than regional backgrounds, but not high enough to class the soils as contaminated according to the Spanish regulation. Accordingly, the geoaccumulation index values indicate that most soils present low to moderate pollution levels. Among the elements studied, Cu, Pb, and Zn were correlated between them, with their highest concentrations happening in soils of the green areas in the city center; Cr and Ni concentrations were related to lithology of the parent material, with the highest concentrations in soils developed over amphibolite; finally, As concentrations are higher in two precise points without a clear connection to a known source of pollution. urban soil; pollution; geoaccumulation; metals +
Footnote †: journal:  
Academic Editor: [PERSON]  
Received: 20 April 2021  
Accepted: 1 May 2021  
Published: 6 May 2021  
consequence, urban soils are commonly polluted by organic and inorganic substances, including heavy metals (Cu, Pb) and metalloids (As), polycyclic aromatic hydrocarbons (PAHs), pesticides and antibiotics, or microplastics [2; 3]. The presence of contaminants in urban soils represents risks for human health and well-being, as well as problems for the functioning of the urban ecosystems, in which soils play a key role. Urban soils prevent flooding by allowing infiltration of water, thus reducing surface water run-off; improve air quality, and are important for regulating the microclimate and the \"heat island\" effect through vegetation and evaporation. They store a considerable amount of soil organic carbon; can be a source of food through urban agriculture; and they serve as fertile islands for plants and animals in increasingly densified cities [4; 5]. Several of these functions are negatively influenced by the presence of contaminants, in particular those more directly related to plant production or habitat for biodiversity, as contaminants can be mobilized towards other compartments of the environment, in especial waters or the food chain. Cultivating urban soils can increase human exposition to contaminants through consumption of vegetables grown on contaminated soil, incidental soil ingestion, soil resuspension, and subsequent exposure [6]. Urban soil contaminants can influence biodiversity by their impact on both aboveground flora and soil biota populations [7]. In summary, urban soil contamination compromises ecosystem functions, biodiversity, human health and food security and consequently the presence and location of contaminants in urban soils must be clearly established to ensure safe land uses, an essential condition for the development and planning of the sustainable cities of the future. Research about trace elements in urban soils in Europe include works in many cities, such as Karlsruhe (Germany) [8], Lisbon (Portugal) [9], Glasgow (UK) [10], Ljubljana (Slovenia) [10], Vienna (Austria) [11], Berlin (Germany) [12], Paris (France) [13], Naples (Italy) [14], Novi Sad (Serbia) [15], Zurich (Switzerland) [5], Uppsala (Sweden) [16], or Torun (Poland) [17].