**4. Conclusions**

In this work, levels of Cd, Cr, Ni and Pb in the soils of six sites with different vehicle traffic characteristics were monitored in Lisbon for a period of about ten years. To our knowledge, this is the longest soil monitoring program carried out in the city of Lisbon. Time wise, in many of the other studies referenced in this work the monitoring periods for soil contamination by toxic metals are typically only one to two years, making this study one of the longest. For each location, 12 samples were collected each year, bringing up 72 samples per year and a total of 648 soil samples. When compared with values obtained for other cities (Table 3), the data in the monitoring period revealed a reasonable low concentration of metals studied. However, these low values of metals in soils are consistent

in the nine years of this study. It should be noted that many of the values listed in Table 3 are single values, obtained generally in a given year and in the weather conditions observed for that year.

The evolution of the levels of metals over the years and in the various places in the city has typically grown, with the exception of the last year. In most locations, the year 2011 showed a decrease in the content of metals in soils. When looking at the average levels for each year, involving all locations, a downward trend in this last year of the campaign is spotted. In 2011 the economic crisis had important impacts on economic activity and this impact was reflected in the circulation of fewer vehicles and, consequently, fewer emissions. That results in a decrease of observed metals in the soil, showing the unequivocal main origin of these pollutants: automotive traffic.

Looking at the levels of metals in each location in the city, the residential area and the city park have the lowest levels of toxic metals in soils, as expected. Exception is noted for Cr in which for those two places the levels are identical to the rest of the city. Still for the same locations, the levels of Cd, Ni and Pb have been maintained or decreased over the years. Cr grew each year, except in 2011. The determination of the pollution indexes (*I*geo, *C*f*, C*d, *PLI*) underlined this global conclusion with values indicating that the most serious levels of contamination are connected to locations with grea<sup>t</sup> traffic density, namely the city entrances, the city center and the urban highway.

Ecological indexes, *E*r and *RI*, also revealed a low or moderate ecological risk except Cd in CE2. When considering the *RI*, in which the joint effect of the metals is considered, moderate ecological risk values are also achieved in CE2. The remaining sites presented a low or moderate level of ecological risk. This index also confirms that the lowest values of potential ecological risk are found in the residential and city park areas.

The low values of metal contents found are also positive for the many urban agriculture projects that are being developed in Lisbon. The municipality has encouraged this practice with the construction of several urban vegetable garden parks throughout the city. This practice should be accompanied by periodic analyzes of soils and plants, as metal levels, even when low, may be transferred from the soil to the plants. It will also be important to follow the pH of the soils and their content in organic matter, which are essential factors in the bioavailability of metals.

Considering that these metals may affect human health by the ingestion of contaminated food (among other forms), the results obtained in this work lead to the conclusion that as the city of Lisbon presents low values of these metals in its soils, it also has a very low potential to affect the health of its inhabitants, workers and visitors.

**Author Contributions:** Conceptualization M.J.M.; data curation H.F.S., M.J.M.; formal analysis H.F.S.; funding acquisition M.J.M., C.M.O.; investigation H.F.S., N.F.S.; methodology H.F.S., M.J.M.; project administration M.J.M., C.M.O.; resources M.J.M., C.M.O.; software M.J.M.; supervision; validation H.F.S.; visualization M.J.M.; roles/writing—original draft H.F.S., N.F.S.; writing—review and editing C.M.O., M.J.M., N.F.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Fundação para a Ciência e a Tecnologia (FCT) through projects UID/QUI/00100/2013 and UIDB/QUI/00100/2020.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data collected, the results of the chemical and physical analysis and the main results obtained, may be requested to the authors. They will be provided, free of charge or not, depending on its future use.

**Conflicts of Interest:** The authors declare no conflict of interest.
