Potentially Toxic Trace Elements in the Urban Soils of Santiago de Compostela (Northwestern Spain)

applied sciences Article Potentially Toxic Trace Elements in the Urban Soils of Santiago de Compostela (Northwestern Spain) Cecilia Herbón, María Teresa Barral and Remigio Paradelo *   Citation: Herbón, C.; Barral, M.T.; Paradelo, R. Potentially Toxic Trace Elements in the Urban Soils of Santiago de Compostela (Northwestern Spain). Appl. Sci. 2021, 11, 4211. https://doi.org/10.3390 /app11094211 Academic Editor: Piero Manna Received: 20 April 2021 Accepted: 1 May 2021 Published: 6 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). CRETUS Centre (Cross-Disciplinary Research in Environmental Technologies), Departamento de Edafoloxía e Química Agrícola, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain; [email protected] (C.H.); [email protected] (M.T.B.) *Correspondence: r[email protected]; Tel.: +34-881-15-042 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, gneiss, 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. Keywords: urban soil; pollution; geoaccumulation; metals 1. Introduction The intensification of urbanization is a key feature of our current society. The spectacular increase in agricultural productivity during the last century has increased global population and reduced the needs of workforce in agriculture, producing a generalized exodus from rural areas towards cities. The 2018 report of the U.N., the World Urbanization Prospects, indicates that the percentage of the world population living in cities increased from 50% in 2008 to 55% in 2018 and will be 68% in 2050 [ 1 ]. For example in Europe, the surface area covered by cities increased by 78% since the 1950s, a trend that is expected to continue globally. This spectacular growth in urban population represents a major challenge for the environment: urban areas are confronted by specific environmental issues (e.g., food dependency, local climate), as they concentrate population, activities and infrastructures. These issues have been recognized by the U.N. in their 2030 Agenda for Sustainable Development, in particular in the Sustainable Development Goal no. 11: “Sustainable cities and communities”, whose objective is “to make cities inclusive, safe, resilient and sustainable”. Among the problems that cities must face in their way towards sustainability, one of the most challenging is the presence of a wide variety of contaminants in the urban environment, due to the presence of several sources of pollution near or within the cities (traffic, coal combustion, industrial facilities, urban waste treatment and disposal, etc.). Many of these substances end up by reaching the soils and accumulating in them. In Appl. Sci. 2021,11, 4211. https://doi.org/10.3390/app11094211 https://www.mdpi.com/journal/applsci Appl. Sci. 2021,11, 4211 2 of 16 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 ]. In comparison, reports of trace element concentrations in urban soils in Spanish cities are still scarce, although they have been increasing in recent years [ 18 – 24 ]. With the objective of contributing information about urban soil contamination in Spain, in this work, we have studied the presence of heavy metals and arsenic in the soils of the city of Santiago de Compostela (Spain), for which these analyses have not been performed to date. We hypothesize that soils in this city will have higher trace element concentrations than surrounding natural and agricultural soils, yet lower values than soils in more industrialized and densely populated cities. The information gathered is important for planning of land uses in the city, in particular in what refers to urban agriculture. 2. Materials and Methods 2.1. Study Area The city of Santiago de Compostela, located in the northwestern corner of the Iberian Peninsula, is the capital of the autonomous region of Galicia and an important political, administrative, religious, and tourist center. The municipality has an area of 222 km 2 and counts 97,000 inhabitants. In addition to the permanent residents, the city has over 20,000 University students and receives over one million visitors every year, many of them in connection to the Way of Saint James, Europe’s first ever pilgrimage route. The city has interesting factors potentially affecting accumulation and dynamics of trace elements: lithology diversity, land use change, short-scale change. Santiago de Compostela has over 5,000,000 m 2 of managed green areas. In the city center, there are 25 green areas with surfaces that range from 1 to 40 hectares, including lawn areas, forest areas, and urban allotment gardens; three different land uses coexist at some places in very short distances. The climate is warm and wet and, according to the Köppen − Geiger Climate Classification, the city is located in the temperate oceanic climate (Cfb) zone [ 25 ]. The mean annual air temperature of 13.0 ◦ C, with August as the warmest month (mean air temperature 19 ◦ C), and January the coldest (mean air temperature 8 ◦ C). The average annual precipita- Appl. Sci. 2021,11, 4211 3 of 16 tion is 1787 mm. The relatively low values for potential evapotranspiration (<300 mm in summer and 50–100 mm in winter) results in a positive water balance (600–800 mm) [26]. The city is in the contact zone between a granitic area and the Santiago Unit of the metamorphic massif known as the Ordes Complex, and therefore presents an important geological diversity. There are four main lithological units in the city, arranged approximately in parallel bands with a N–S direction: (1) granitic rocks, mostly mediumto coarse-grained two-mica granites; (2) Santiago schists, rich in micas and poor in quartz; (3) orthogneisses, with a similar composition to granites; and (4) amphibolites, composed mainly of amphibole and plagioclase [27–29]. 2.2. Soil Sampling and Analysis We sampled soils at 55 points in the city, over several materials, and under different vegetations and land uses (Table 1). Composite samples (soil depth 0–20 cm) were obtained by mixing 4–5 subsamples taken with an auger at each point. Soils were taken to the laboratory, air-dried and sieved (<2 mm) before analysis. Edaphic properties of the soils, including pH, texture, OC, CEC, etc., were previously published [30]. Table 1. List of sampling points. Name Latitude Longitude Altitude Land Use Lithology 1 Carlomagno 1 42◦53000” N 8◦31023” W 290 Urban grassland Amphibolite 2 Carlomagno 2 42◦52052” N 8◦31027” W 285 Urban forest Amphibolite 3 Vieiro 1 42◦52046” N 8◦31038” W 235 Urban grassland Amphibolite 4 Fontiñas 1 42◦53002” N 8◦31047” W 260 Urban grassland Schist 5 Fontiñas 2 42◦53005” N 8◦31057” W 265 Urban garden Schist 6Monte dos Postes 42◦53017” N 8◦31053” W 300 Urban grassland Schist 7 San Caetano 42◦53024” N 8◦32005” W 300 Urban grassland Schist 8 Bouza Brey 42◦53020” N 8◦32021” W 275 Urban grassland Schist 9 Burgo Xixón 42◦53018” N 8◦32034” W 250 Urban grassland Schist 10 Auditorio 42◦53018” N 8◦32041” W 235 Urban grassland Schist 11 Vistalegre 1 42◦53014” N 8◦32049” W 240 Urban forest Schist 12 Vistalegre 2 42◦53008” N 8◦32046” W 230 Urban grassland Schist 13 Santa Isabel 42◦53013” N 8◦32059” W 225 Urban garden Schist 14 Espiño 42◦52059” N 8◦33006” W 245 Urban forest Schist 15 Galeras 1 42◦52056” N 8◦33000” W 225 Urban grassland Schist 16 Galeras 2 42◦53000” N 8◦32058” W 225 Urban grassland Schist 17 Campo das Hortas 42◦52047” N 8◦32052” W 235 Urban garden Schist 18 Fonseca 42◦52045” N 8◦32045” W 245 Urban grassland Schist 19 Alameda 1 42◦52040” N 8◦32058” W 265 Urban grassland Schist 20 Alameda 2 42◦52042” N 8◦32057” W 255 Urban forest Schist 21 Alameda 3 42◦52039” N 8◦32047” W 250 Urban grassland Schist 22 Parlamento 42◦52024” N 8◦32032” W 230 Urban grassland Schist 23 Granell 1 42◦51054” N 8◦32048” W 215 Urban grassland Schist Appl. Sci. 2021,11, 4211 4 of 16 Table 1. Cont. Name Latitude Longitude Altitude Land Use Lithology 24 Granell 2 42◦51051” N 8◦32041” W 225 Urban forest Schist 25 Granell 3 42◦51059” N 8◦32057” W 205 Urban forest Granite/Schist 26 Almaciga 3 42◦53011” N 8◦32011” W 325 Urban grassland Schist 27 Almáciga 1 42◦53006” N 8◦32014” W 315 Urban grassland Gneiss 28 Almáciga 2 42◦53008” N 8◦32013” W 320 Urban garden Gneiss 29 Bonaval 1 42◦52058” N 8◦32016” W 290 Urban grassland Gneiss 30 Bonaval 2 42◦53001” N 8◦32017” W 300 Urban forest Gneiss 31 Caramoniña 42◦53003” N 8◦32021” W 285 Urban garden Gneiss 32 Belvís 1 42◦52047” N 8◦32018” W 245 Urban grassland Gneiss 33 Belvís 2 42◦52046” N 8◦32017” W 250 Urban garden Gneiss 34 Belvís 3 42◦52033” N 8◦32020” W 225 Urban forest Gneiss 35 Belvís 4 42◦52048” N 8◦32020” W 245 Urban garden Gneiss 36 Belvís 5 42◦52040” N 8◦32021” W 235 Urban grassland Gneiss 37 Paxonal 1 42◦52007” N 8◦32036” W 205 Urban grassland Gneiss 38 Paxonal 2 42◦52005” N 8◦32037” W 205 Urban garden Gneiss 39 Campus 1 42◦52034” N 8◦33031” W 245 Urban grassland Granite 40 Campus 2 42◦52036” N 8◦33015” W 240 Urban grassland Granite/Schist 41 Campus 3 42◦52041” N 8◦33022” W 235 Urban forest Granite/Schist 42 Campus 4 42◦52026” N 8◦33032” W 215 Urban forest Granite/Schist 43 Campus 5 42◦52032” N 8◦33031” W 240 Urban forest Granite 44 Campus 6 42◦52037” N 8◦33034” W 240 Urban grassland Granite/Schist 45 Campus 7 42◦52026” N 8◦33015” W 230 Urban grassland Schist 46 Campus 8 42◦52023” N 8◦33045” W 200 Urban forest Granite 47 San Lourenzo 42◦52042” N 8◦33023” W 230 Urban forest Granite/Schist 48 Barcelona 42◦52018” N 8◦33037” W 210 Urban grassland Granite 49 Santa Marta 1 42◦52008” N 8◦33021” W 240 Urban grassland Granite 50 Santa Marta 2 42◦51057” N 8◦33030” W 210 Urban garden Granite 51 Santa Marta 3 42◦51053” N 8◦33047” W 190 Urban garden Granite 52 Multiusos 42◦52028” N 8◦31051” W 225 Peri-urban cropland Amphibolite 53 Lermo 42◦53033” N 8◦33000” W 230 Peri-urban cropland Schist 54 Brañas 42◦52017” N 8◦32020” W 210 Peri-urban cropland Gneiss 55 Botánico 42◦52039” N 8◦33042” W 205 Peri-urban cropland Granite For the analysis of total trace elements (Cu, Pb, Zn, Ni, Cr, As), air-dry sieved soils were ground in an agate mortar down to <250 µ m. Ground samples were analyzed by X-ray fluorescence using an energy-dispersive Miniprobe Multielement Analyzer (EMMA-XRF) with two pieces of equipment. Copper, Ni, Zn, Pb, and As were analyzed using a Mo anode, whereas Cr was analyzed with an Ag tube and a secondary anode of pyrographite. The EMMA–XRF and its application for elementary analysis has been described elsewhere [ 31 ]. The instruments are hosted at the RIAIDT (Infrastructure Network for the Support of Research and Technological Development) facility of the University of Santiago de Compostela (Spain). The instruments were calibrated using several reference materials (NIST Appl. Sci. 2021,11, 4211 5 of 16 1515, 1547, and 1575, BCR 60 and 62, and V-1). Standard reference materials CRM143R (sewage sludge-amended soil) and 320R (channel sediment), from the European Commission Community Bureau of Reference (BCR), were used for quality control. Detection limits for the technique in soil are as follows: 3 mg kg −1 for Ni and Pb, 4 mg kg −1 for Cu, 10 mg kg−1for Cr, 5 mg kg−1for Zn, and As. All analyses were performed in triplicates. 2.3. Pollution Assessment The pollution status of the soils was first assessed taking into account the regulations in the region: trace element concentrations were compared to regional background levels and generic reference levels (GRLs) for the protection of human and ecosystem health, as established in the Spanish and Galician regulations on contaminated soils (Table 2[ 32 ]). Four GRLs are established: industrial use, urban use, other uses (including agricultural) and ecosystem protection (from high to low). For a soil to be declared as polluted, the concentration of a given element should surpass the corresponding GRL by a factor of 100. Table 2. Generic reference levels (GRL, in mg kg −1 ) established for trace elements contents in Galician regulations for contaminated soils [ 32 ]. Lithological exceptions for basic rocks are indicated in brackets, when they are different from the general levels. Pb Cu Zn Ni Cr As GRL—lithological background 55 45 (90) 100 65 (100) 80 (240) 45 GRL—ecosystem protection 80 50 (90) 200 75 (100) 80 (240) 50 GRL—other uses 100 50 (90) 300 75 (100) 80 (240) 50 GRL—urban use 100 100 500 100 100 (240) 50 GRL—industrial use 500 200 1000 200 300 50 Besides, the geoaccumulation index, Igeo [ 33 ] was calculated for the six trace elements at each point as follows: Igeo = log2[Cn/(1.5 ×Bn)], (1) where Cn is the concentration of the element in the soil and Bn is the geochemical background value. Here we used background values for the soils of the region taking into account the lithology of the parent material at each point (Table 2). According to the Igeo values, soils are classed into six degrees: uncontaminated (Igeo ≤ 0); uncontaminated to moderately contaminated (0 < Igeo ≤ 1); moderately contaminated (1 < Igeo ≤ 2); moderately to heavily contaminated (2 < Igeo ≤ 3); heavily contaminated (3 < Igeo ≤ 4); heavily to extremely contaminated (4 < Igeo ≤5); and extremely contaminated (Igeo ≥5). 2.4. Statistics ANOVA mixed model analysis was used to determine the influence of parent material and land use on trace element concentrations. Before analysis, the normality of data was checked using the Shapiro-Wilk test. Data that did not pass the normality test were logtransformed for ANOVA. The homogeneity of variance was tested using the Levene test. When a significant effect of land use or lithology at a level of significance of p< 0.05 was found, the Tukey’s multiple range test was used to separate groups. Pearson’s correlation analyses between the metal concentrations and main edaphic properties of the soils were also conducted. All statistical analyses were performed using the R statistical software for MacOSX version R 3.1.3 [34] and the package R Commander version 2.6-1 [35]. 3. Results As presented in a previous publication, the soils of the city of Santiago de Compostela are acid (pH in water from 4.7 to 6.9), coarse-textured (dominant texture is sandy loam) and rich in organic matter (13–137 g OC kg −1 ), with differences in soil composition and chemical properties that are influenced by the diversity of lithology and land use. Soils developed over amphibolites show heavier textures and higher Fe contents than soils Appl. Sci. 2021,11, 4211 6 of 16 developed over other parent materials, whereas soils of urban allotment gardens present higher nutrient contents, pH and salinity than soils under forest or lawn vegetation [30]. 3.1. Trace Element Concentrations and Influence of Land Use and Lithology The total concentrations of the six trace elements considered in all the soils are shown in Table 3. Figures 1and 2show these concentrations grouped by land use and lithology, along with ANOVA results for the influence of land use and lithology, whereas their spatial distribution is shown in Figure 3. Table 4presents correlations between trace elements contents and soils physicochemical properties. Table 3. Trace element concentrations in urban soils (in mg kg−1, mean ±standard deviation, n = 3). Soil Pb Cu Zn Ni Cr As 1 72 ±3 188 ±5 197 ±9 91 ±20 172 ±22 20 ±6 2 40 ±6 99 ±4 136 ±9 65 ±4 175 ±4 13 ±1 3 45 ±3 90 ±9 123 ±4 70 ±10 221 ±10 32 ±6 4 31 ±1 56 ±8 75 ±8 68 ±12 163 ±24 24 ±7 5 36 ±4 21 ±1 65 ±6 16 ±2 37 ±10 18 ±1 6 20 ±3 47 ±3 76 ±4 41 ±5 113 ±7 17 ±3 7 90 ±5 41 ±7 95 ±8 25 ±3 82 ±13 21 ±2 8 53 ±6 41 ±8 96 ±11 28 ±2 277 ±7 138 ±32 9 103 ±3 49 ±2 99 ±7 31 ±5 75 ±15 45 ±6 10 65 ±1 63 ±3 125 ±0.3 36 ±3 80 ±13 33 ±1 11 101 ±6 52 ±4 87 ±10 34 ±5 83 ±13 27 ±3 12 111 ±4 46 ±3 115 ±5 29 ±6 66 ±17 44 ±1 13 102 ±6 75 ±5 197 ±13 37 ±4 92 ±7 58 ±4 14 68 ±3 29 ±3 95 ±27 37 ±5 94 ±8 20 ±2 15 85 ±5 47 ±6 93 ±8 38 ±6 60 ±19 205 ±6 16 56 ±6 38 ±7 99 ±14 29 ±7 72 ±3 35 ±2 17 289 ±37 151 ±6 294 ±23 30 ±5 38 ±2 33 ±6 18 137 ±15 81 ±3 116 ±6 34 ±3 100 ±13 19 ±2 19 135 ±5 84 ±6 130 ±4 26 ±3 41 ±5 34 ±4 20 105 ±2 40 ±2 104 ±5 25 ±2 57 ±12 29 ±3 21 286 ±22 55 ±8 84 ±6 11 ±2 40 ±8 27 ±5 22 37 ±1 66 ±1 123 ±6 45 ±5 124 ±12 24 ±3 23 92 ±37 29 ±1 65 ±5 28 ±0.4 71 ±21 35 ±3 24 40 ±6 34 ±4 69 ±2 34 ±2 68 ±10 27 ±2 25 57 ±10 28 ±6 59 ±7 24 ±2 88 ±10 35 ±2 26 83 ±3 79 ±22 131 ±8 18 ±3 57 ±1 38 ±0.4 27 32 ±6 20 ±6 68 ±4 17 ±5 22 ±12 58 ±3 28 58 ±1 23 ±2 83 ±5 11 ±2 17 ±87 60 ±2 29 78 ±4 69 ±9 80 ±7 13 ±1 23 ±8 43 ±1 30 78 ±5 25 ±5 55 ±7 13 ±3 29 ±15 55 ±1 31 156 ±38 157 ±58 144 ±12 19 ±2 28 ±5 44 ±8 32 268 ±28 109 ±7 167 ±4 23 ±3 48 ±20 40 ±8 33 155 ±8 64 ±2 161 ±9 16 ±4 40 ±7 33 ±4 34 82 ±14 43 ±3 93 ±7 11 ±1 22 ±11 35 ±4 35 293 ±10 124 ±25 248 ±2 23 ±1 50 ±11 46 ±3 36 155 ±12 76 ±5 151 ±7 20 ±3 31 ±9 37 ±3 37 93 ±1 65 ±6 107 ±7 18 ±5 49 ±4 31 ±4 38 72 ±2 46 ±2 123 ±9 28 ±5 62 ±18 40 ±1 39 35 ±3 17 ±1 70 ±9 22 ±5 43 ±2 18 ±3 40 62 ±5 29 ±4 86 ±9 19 ±4 44 ±10 33 ±2 41 61 ±3 30 ±1 90 ±13 26 ±5 63 ±4 17 ±2 42 52 ±2 29 ±4 91 ±3 20 ±7 49 ±3 33 ±2 43 41 ±0.4 28 ±3 88 ±5 24 ±5 47 ±9 19 ±2 44 48 ±1 27 ±1 77 ±4 25 ±1 54 ±17 23 ±1 45 39 ±4 26 ±2 72 ±10 23 ±3 51 ±8 19 ±1 46 62 ±1 30 ±3 78 ±14 17 ±6 33 ±9 20 ±2 47 85 ±1 48 ±3 110 ±5 27 ±1 61 ±8 42 ±1 Appl. Sci. 2021,11, 4211 7 of 16 Table 3. Cont. Soil Pb Cu Zn Ni Cr As 48 55 ±5 34 ±3 105 ±2 26 ±6 58 ±2 30 ±3 49 63 ±5 29 ±1 91 ±12 20 ±3 41 ±8 25 ±5 50 58 ±2 28 ±2 85 ±6 18 ±1 43 ±18 32 ±3 51 80 ±14 41 ±2 115 ±6 24 ±2 53 ±7 30 ±3 52 60 ±3 91 ±10 109 ±5 70 ±4 274 ±33 14 ±1 53 71 ±1 72 ±9 122 ±18 30 ±3 71 ±11 38 ±6 54 566 ±38 126 ±5 484 ±7 36 ±5 65 ±7 44 ±1 55 52 ±3 38 ±8 89 ±4 24 ±7 93 ±10 22 ±3 Mean 97 58 116 29 74 37 Median 71 46 96 25 58 33 Standard deviation 91 37 67 16 56 29 Minimum 20 17 55 11 17 13 Maximum 566 188 484 91 277 205 Average content in natural soils a29 ±15 21 ±14 (50 ±32) 60 ±29 56 ±25 (66 ±32) 80 ±72 (129 ±79) 21 ±20 aAveraged values in non-agricultural soils in Galicia [36]. Values for soils developed over basic rocks are indicated in brackets. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 17 Soil Pb Cu Zn Ni Cr As Median 71 46 96 25 58 33 Standard deviation 91 37 67 16 56 29 Minimum 20 17 55 11 17 13 Maximum 566 188 484 91 277 205 Average content in natural soils a 29  15 21  14 (50  32) 60  29 56  25 (66  32) 80  72 (129  79) 21  20 a Averaged values in non-agricultural soils in Galicia [36]. Values for soils developed over basic rocks are indicated in brackets. 3.1. Trace Element Concentrations and Influence of Land Use and Lithology The total concentrations of the six trace elements considered in all the soils are shown in Table 3. Figures 1 and 2 show these concentrations grouped by land use and lithology, along with ANOVA results for the influence of land use and lithology, whereas their spatial distribution is shown in Figure 3. Table 4 presents correlations between trace elements contents and soils physicochemical properties. Figure 1. Soil trace element concentrations split by land use. Different letters mean statistically significant differences between land uses in the Tukey test at p < 0.05. Figure 1. Soil trace element concentrations split by land use. Different letters mean statistically significant differences between land uses in the Tukey test at p< 0.05. In most soils, trace element concentrations followed the sequence Zn > Pb > Cr > Cu > As > Ni. Zinc was the most abundant element in 33 out of the 55 points, ranging from 55 to 484 mg kg −1 with a mean of 116 mg kg −1 and a median of 96 mg kg −1 . A significant influence of lithology was not observed, but higher Zn concentrations were observed in agricultural soils (Figure 1,p= 0.004**) and if we look at the spatial distribution (Figure 3), Zn concentrations were clearly higher in the city center. Zn concentrations were positively correlated to Pb and Cu (Table 4). Zn was positively correlated to oxalate-extractable Fe but not total Fe or free iron (Table 4); thus, suggesting a preferential association to poorly-crystalline iron forms. Weak positive correlations were found with pH and clay content that could reflect Zn accumulation in the soil due to a reduction of mobility under conditions close to neutrality and interaction with clay minerals. Appl. Sci. 2021,11, 4211 8 of 16 Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 17 Figure 2. Soil trace element concentrations split by lithology of the parent material. Different letters mean statistically significant differences between lithologies in the Tukey test at p < 0.05. In most soils, trace element concentrations followed the sequence Zn > Pb > Cr > Cu > As > Ni. Zinc was the most abundant element in 33 out of the 55 points, ranging from 55 to 484 mg kg−1 with a mean of 116 mg kg−1 and a median of 96 mg kg−1. A significant influence of lithology was not observed, but higher Zn concentrations were observed in agricultural soils (Figure 1, p = 0.004**) and if we look at the spatial distribution (Figure 3), Zn concentrations were clearly higher in the city center. Zn concentrations were positively correlated to Pb and Cu (Table 4). Zn was positively correlated to oxalate-extractable Fe but not total Fe or free iron (Table 4); thus, suggesting a preferential association to poorlycrystalline iron forms. Weak positive correlations were found with pH and clay content that could reflect Zn accumulation in the soil due to a reduction of mobility under conditions close to neutrality and interaction with clay minerals. Lead was the element with the highest concentrations in 12 out of the 55 soils; its concentrations ranged from 20 to 566 mg kg−1, with an average of 97 mg kg−1 and a median of 71 mg kg−1. The point with the highest concentration could be related to past shooting activities, given that weathered bullets have been found in this soil. Overall, Pb concentrations were not significantly affected by land use or lithology, but a very clear pattern of spatial distribution was observed, with the highest concentrations found in the city center. Significant correlations with edaphic properties have not been found, but Pb was positively correlated to Cu and Zn concentrations. Copper concentrations ranged between 17 and 188 mg kg-1, with an average of 58 mg kg-1 and a median of 46 mg kg−1, and it was the most abundant element in two soils. Lithology of the parent material has an influence on this element, with higher Cu contents in soils developed over amphibolites (p < 0.001***), whereas differences related to land use were not observed. Spatial distribution shows higher Cu concentrations in the city center, similarly to Zn and Pb, as well as in the eastern area of the city where the soil on amphibolites are located. Positive correlations observed with clay content and total and free Fe could be an indirect effect of the highest Cu contents in amphibolite soils, which are also the soils richer in iron compounds and clay, but also an effect of preferential association of Cu to iron compounds and clay minerals. In addition, Cu was positively correlated with Pb and Zn. Figure 2. Soil trace element concentrations split by lithology of the parent material. Different letters mean statistically significant differences between lithologies in the Tukey test at p< 0.05. Table 4. Correlations between trace element concentrations and edaphic properties. Significance of correlation is indicated as follows: * significant at a p-value of 0.05; ** significant at a p-value of 0.01; *** significant at a p-value of 0.001. pHwpHKCl Clay C CEC Fetotal FeDCB Feox Pb Cu Zn Ni Cr Pb 0.10 0.11 0.02 −0.08 −0.01 −0.21 −0.15 0.15 1 Cu 0.36 ** 0.36 ** 0.50 *** −0.08 0.10 0.40 0.42 *** 0.41 0.55 *** 1 Zn 0.27 * 0.25 0.30 * −0.05 0.11 0.11 0.13 0.36 ** 0.84 *** 0.70 *** 1 Ni 0.05 0.02 0.75 *** 0.10 0.13 0.94 *** 0.90 *** 0.50 *** −0.11 0.48 0.20 1 Cr 0.01 0.02 0.51 *** 0.15 0.11 0.78 *** 0.76 *** 0.30 * −0.18 0.24 ** 0.02 0.75 *** 1 As −0.001 −0.06 −0.20 −0.16 −0.06 −0.13 −0.17 −0.11 0.06 −0.03 0.03 −0.09 0.08 Lead was the element with the highest concentrations in 12 out of the 55 soils; its concentrations ranged from 20 to 566 mg kg −1 , with an average of 97 mg kg −1 and a median of 71 mg kg −1 . The point with the highest concentration could be related to past shooting activities, given that weathered bullets have been found in this soil. Overall, Pb concentrations were not significantly affected by land use or lithology, but a very clear pattern of spatial distribution was observed, with the highest concentrations found in the city center. Significant correlations with edaphic properties have not been found, but Pb was positively correlated to Cu and Zn concentrations. Copper concentrations ranged between 17 and 188 mg kg −1 , with an average of 58 mg kg −1 and a median of 46 mg kg −1 , and it was the most abundant element in two soils. Lithology of the parent material has an influence on this element, with higher Cu contents in soils developed over amphibolites (p< 0.001 ***), whereas differences related to land use were not observed. Spatial distribution shows higher Cu concentrations in the city center, similarly to Zn and Pb, as well as in the eastern area of the city where the soil on amphibolites are located. Positive correlations observed with clay content and total and free Fe could be an indirect effect of the highest Cu contents in amphibolite soils, which are also the soils richer in iron compounds and clay, but also an effect of preferential association of Cu to iron compounds and clay minerals. In addition, Cu was positively correlated with Pb and Zn. Appl. Sci. 2021,11, 4211 9 of 16 Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 17 Figure 3. Spatial distribution of trace element concentrations. Chromium was the most abundant trace element considered in eight soils, with concentrations that ranged from 17 to 277 mg kg−1 (mean value of 74 mg kg-1 and median value of 58 mg kg−1). Influence of land use was not significant, whereas a very clear influence of lithology was observed (p < 0.001***), also reflected in the spatial distribution, with the soils on amphibolites as the richest ones. Significant positive correlations have been found with clay and free iron that, similarly to Cu, could reflect an actual association of this element to iron oxides and clay minerals or an indirect effect of the higher Fe and clay Figure 3. Spatial distribution of trace element concentrations. Chromium was the most abundant trace element considered in eight soils, with concentrations that ranged from 17 to 277 mg kg −1 (mean value of 74 mg kg −1 and median value of 58 mg kg −1 ). Influence of land use was not significant, whereas a very clear influence of lithology was observed (p< 0.001 ***), also reflected in the spatial distribution, with the soils on amphibolites as the richest ones. Significant positive correlations have been found with clay and free iron that, similarly to Cu, could reflect an actual association of this element to iron oxides and clay minerals or an indirect effect of the higher Fe and Appl. Sci. 2021,11, 4211 16 of 16 33. Muller, G. 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