**1. Introduction**

The population living in urban areas has been continuously increasing in the last few decades [1,2] and it is estimated by the United Nations that nowadays the majority of people lives in urban areas, not only in Europe but also in less developed countries [3]. Urban environments have then become extremely important in what human health and wellbeing is concerned. Urban soils are a fundamental component of urban ecosystems contributing directly or indirectly to the general quality of life of cities' inhabitants. They play an important role in many processes such as in the cycling of elements, filtration of water, supporting plants and some built infrastructures. However, soils are very often contaminatedbytheanthropogenicactivitieslikevehicletrafficorindustrialactivities[4,5].

 Different anthropogenic sources, just like industry, energy production and fossil fuels combustion in vehicle traffic, release to the atmosphere pollutants containing heavy metals

**Citation:** Silva, H.F.; Silva, N.F.; Oliveira, C.M.; Matos, M.J. Heavy Metals Contamination of Urban Soils—A Decade Study in the City of Lisbon, Portugal. *Soil Syst.* **2021**, *5*, 27. https://doi.org/10.3390/soilsystems 5020027

Academic Editors: Matteo Spagnuolo, Paola Adamo and Giovanni Garau

Received: 10 March 2021 Accepted: 9 April 2021 Published: 13 April 2021

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that can contribute to the contamination of soils by dry or wet deposition [6,7]. Heavy metals, such as Pb, Zn, Cr, Ni or Cd, are the most frequently studied inorganic contaminants in urban soils and can be linked to adverse health hazard effects [8–10] since they cannot be decomposed by micro-organisms having a long-term toxicity for plants, animals and humans. High concentrations of heavy metals have been identified in many studies of urban soils, especially Pb [11] due to the use of leaded petrol [12,13]. With regulations like the one banning leaded petrol [14], emissions from single vehicles have been reduced, although this effect could be compensated by the worldwide increasing road traffic [15,16]. Emissions resulting from vehicle traffic are mostly caused by wear of vehicular components like break lining, tire wear off and exhaust, as well as from incomplete fuel combustion, fuel additives or oil leaking from vehicles [17].

Due to the importance of the quantitative identification of potential sources of heavy metals in soils, source apportionments for these metals concentrations have been performed by principal component analysis (PCA) and positive matrix factorization (PMF) models. These models assume significant correlations of compounds derived from the same sources [18,19]. Moreover, some indexes are fairly used to evaluate the degree of heavy metal pollution in the soil. The most common one is the enrichment factor (*EF*) [18,20,21] but others, namely the geo-accumulation index (*I*geo) [20], the contamination factor ( *C*f) [22–24], the pollution load index (*PLI*) [21,22] or the ecological risk factor [23,24] are also widely used.

In cities, soil pollution is important due to the health risks of its inhabitants. Many particulate pollutants in the air are originated in soil and its composition and size determine its impact on health. In this work, soils from a set of carefully chosen different locations in the city were sampled from 2003 to 2011 and heavy metals contents (Pb, Cr, NI and Cd) were analyzed by GFAAS (graphite furnace atomic absorption spectrometry). The metals under study were considered due to their negative effects on human health and are therefore metals that are subject to surveillance in the European regulation of atmospheric pollution [25] and pollution of soils and groundwater [26]. Results are discussed in terms of the level of contamination of these soils as well as pollution and ecological risks associated with that contamination.

#### **2. Materials and Methods**
