distribution of heavy metals

Distribution of Heavy Metals in the Street Dusts and Soils of an Industrial Cityin Northern Spain

A. Ordonez,1 J. Loredo,1 E. De Miguel,2 S. Charlesworth3

1 Departamento de Explotacioń y Prospeccioń de Minas, Escuela Tećnica Superior de Ingenieros de Minas de Oviedo, Universidad de Oviedo,c/Independencia, 13, 33004, Oviedo, Asturias, Spain2 Departamento de Ingenierıá Quımica y Combustibles, ETSIMM, Universidad Politećnica de Madrid, Madrid, Spain3 Department of Geography, Coventry University, United Kingdom

Received: 22 January 2002/Accepted: 24 July 2002

Abstract. This study characterizes the elemental compositionof street dust and soils in Avile´s (N. Spain), a medium-size cityof approximately eighty thousand inhabitants, where industrialactivities and traffic strongly affect heavy metal distribution. Atotal of 112 samples of street dust were collected within a7-km2 area, encompassing residential and industrial sites (fer-rous and non-ferrous plants). Elevated geometric mean con-centrations of zinc (4,892�g � g�1), cadmium (22.3�g � g�1),and mercury (2.56�g � g�1) in street dust were found insamples located near industrial areas. Two types of anthropicinfluence were distinguishable: the first and most important oneis that related to metallurgical activity and transportation of rawmaterials for local industries. Secondly, exhaust emissionsfrom traffic are an important source of lead concentration inareas with high vehicular density (geometric mean: 514�g � g�1). The zinc content in the dust samples decreased withthe distance from a zinc smelter located in the northern part ofthe city. The same trend was found for other elements inassociation with zinc in the raw materials used by the smelter,such as cadmium and mercury. A simultaneous research cam-paign of urban soils, that involved the collection of 40 samplesfrom a 10-km2 area, revealed geometric mean concentrations of376�g � g�1 Zn, 2.16�g � g�1 Cd, 0.57�g � g�1 Hg, and 149�g � g�1 Pb, and distribution patterns almost identical to thosefound for street dust.

Human activities, in general, exert a substantial impact on theenvironment. Population and industrial centers pollute the air,water, and soil, causing a decline in the quality of the environ-ment. People living in industrial cities are particularly exposedto this decline in environmental quality leading to humanhealth concerns. Urban environments affect and are affected bynatural cycles, where air, water, and soil are altered by products

ultimately returned to the environment in the form of emissionsand wastes. In Europe more than two-thirds of the total popu-lation live in urban areas (EEA 1992), and the United NationsPopulation Division (2001) estimates that nearly all the popu-lation growth in the next 30 years will be concentrated in urbanareas.

Interest in the different factors influencing the presence oftrace elements in the urban environment has been rapidlyincreasing as a consequence of the high levels of contaminationmeasured in a number of cities, and the potential health risksassociated with them. Inhalation and ingestion of suspendedparticles and dust particles (especially in the case of children)are among the possible routes through which people can beexposed to these elevated levels of trace elements (Bartnickietal. 1995; Crosby 1998; Doadrio 1984; Dockery and Pope 1994;Kabata-Pendias and Pendias 1992; Sadiq and Mian 1994).

Pollutants enter the urban atmosphere in the form of gases,particles, or as aerosols, by evaporation of liquids or by co-evaporation of dissolved solvents from water and by winderosion of soil (Magnus 1994). Major air pollutants in cities aresulphur dioxide (SO2), particulate matter, and nitrogen oxides(NOx). As well as other inorganic and organic contaminantsarising from various activities, the urban environment is ex-posed to varying concentrations of heavy metals, emitted to theatmosphere from a vast array of anthropogenic sources as wellas from natural geochemical processes (Shakour and El-Taieb1995). These include metallurgical and other industrial sources,burning of fossil fuels, automobile traffic and resuspension ofsoil particles, etc. Concentrations of heavy metals in street dustparticles vary considerably across cities (Akhter and Madany1993; Chonet al.1995; Davieset al.1987; Dayet al.1975; DeMiguel et al. 1997; Droppoet al. 1998; Fergusson and Ryan1984; Harrisonet al. 1981; Hopkeet al. 1980; Jirieset al.2001; Kimet al.1998; Shakour and El-Taieb 1995; Tong 1998;Wang et al. 1998) depending, among other factors, on thedensity of industrial activities in the area and technologiesemployed, as well as on local weather conditions and windpatterns (Table 1).

Industrialized cities generally show higher concentrations oftrace elements in the atmospheric aerosol, street dust and soil

Correspondence to:Almudena Ordońez Alonso;email: [email protected].

Arch. Environ. Contam. Toxicol. 44, 160–170 (2003)DOI: 10.1007/s00244-002-2005-6

A R C H I V E S O F

EnvironmentalContaminationa n d Toxicology© 2003 Springer-Verlag New York Inc.

than non-industrial urban areas. The spatial distribution ofpollutants largely reflects current or past industrial activities.Large quantities of various raw materials are used both byferrous and non-ferrous industries, and their processing leads toa wide selection of heavy metals with their compounds beingliberated into the surrounding area (Zierock 1994).

Human exposure to urban particles of deposited ambient dustof different sizes has been an object of concern in the lastdecades (Vallack and Shillito 1998). Health effects depend onwhere the particles are preferentially deposited in the respira-tory system, which is a function of their size (Khlystov et al.2001). Dominant particles in terms of concentration are thosein the ultrafine size range (diameters � 100 nm), and are themost problematic, since human defence mechanisms are lessefficient at combating particles of this size (Oberdorster et al.1995). Particulate matter with an aerodynamic diameter lessthan 10 �m (PM10), especially the fine particle fraction (diam-eter � 2.5 �m, PM2.5) have been found to be associated withurban health problems, such as asthma, or even death (Dockeryand Pope 1994), and ambient quality problems such as visibil-ity reduction (Larson et al. 1989; Lin and Tai 2001).

The assessment of trace element pollution in urban environ-ments has often made use of geochemical methods, followedby mapping of the distribution of trace element to establishtheir patterns of dispersion, the spatial boundaries of the areas

affected by individual sources, and finally to zone the areaunder study according to the intensity of pollution (Burenkov etal. 1991). The choice of mapping unit size and the number andmass of the sampling increments taken in each one determinesthe accuracy and precision of the final graphic display and thevalidity of the conclusions drawn from the information con-veyed therein (De Miguel et al. 1997).

This paper reports a study of the heavy metal distribution instreet dusts and soils in a city in northern Spain and attempts todifferentiate between metals produced naturally, those pro-duced as a result of industry and those distributed in relation totraffic movements.

Study Area

Aviles (Asturias) is a port city which, since the 1960s, hasgradually acquired a marked industrial character. It has under-gone accelerated demographic growth—300% in 20 years—asresult of the location of industries such as the iron and steelindustry and non-ferrous metal production processes in thearea. The district as a whole extends to more than 30 km2, thecurrent population exceeds 85,000 inhabitants, with areas ofhigh vehicular traffic density: more than 20,000 vehicles/day of

Table 1. A comparison of the concentrations of heavy metals in street dusts found in studies globally (�g g�1), digestion techniques, andparticle sizes (PS given in �m unless otherwise stated) used

City Ref.* Cd Cu Pb Zn PS Digestion

Amman 1 2.5–3.4 69–117 219–373 n/a �0.045 50% HNO3

Hamilton 2 4.1 129 214 645 n/a �sequentialLondon 3 6250 61–323 413–2241 n/a �1mm concHNO3

London 4 n/a 111–512 544–1636 988–3358 �500 �sequentialHong Kong 4 n/a 92–392 208–755 574–2397 �500 �sequentialLondon 5 6.5 197 3030 1174 �963 4M HNO3

New York 5 8 355 2583 1811 �963 4M HNO3

Halifax 5 1 87 1297 468 �963 4M HNO3

Christchurch 5 1 137 1090 548 �963 4M HNO3

Kingston 5 0.8 65.5 863 765 �963 4M HNO3

Birmingham 1976 6 Residential/industrial 1300/950 n/a �1mm HNO3�Birmingham 1987 6 Residential/industrial 791/527 n/a �1mm HClO4

Urbana, IL 7 1.6 n/a 1000 320 �35mesh 8N HNO3

London 8 2.7 108 2100 539 �600 n/aLancaster 8 3.7 75 1090 260 �600 n/aSeoul 9 3 101 245 296 �2mm HNO3�HClOslo 10 1.4 123 180 412 �100 HNO3�Madrid 10 n/a 188 193 476 �100 HClO4�HFBahrain 11 72 n/a 697 152 �30mesh HNO3�HClGtr. Manchester 12 n/a n/a 970 n/a n/a 2M HNO3

Cincinnati 1990 13 n/a 1219 662 n/a �106 2M HNO3

Cincinnati 1998 13 n/a 253 650 n/a �106 2M HNO3

Taejon, Korea(industr./heavytraffic) 14 n/a 47/57 60/52 172/214 �180 HCl�HNO3

Ontario 15 0.85 87 90 227 �2000 HF�HClO4�

Hawaii 16 n/a 167 106 434 �125 HF�HClO4�

Bursa, Turkey 17 3.1 n/a 210 57 �200 HCl�HNO3

n/a � not available* References: 1, Jiries et al. 2001; 2, Droppo et al. 1998; 3, Leharne et al. 1992; 4, Wang et al. 1998; 5, Fergusson and Ryan 1984; 6, Davieset al. 1987; 7, Hopke et al. 1980; 8, Harrison et al. 1981; 9, Chon et al. 1995; 10, de Miguel et al. 1997; 11, Akhter and Madany, 1993; 12, Dayet al. 1975; 13, Tong, 1998; 14, Kim et al. 1998; 15, Stone and Marsalek, 1996; 16, Sutherland and Tolosa, 2000; 17, Arslan, 2001.

Heavy Metals in Street Dust and Soils 161

which 20% is heavy traffic. The dominant wind direction isfrom the northwest, although in general, wind velocities arelow, even approaching zero at times. This enhances the depo-sition of particulates within the city. Concern over the amountand geochemistry of these dusts lead the Aviles city council tomonitor levels, the results of which are outlined below.

Dust Studies Carried Out by the Avile´s City Council

Long-term exposure to contaminated environments pose ahigher health risk than individual pollution events, althoughpeople living in normally dusty areas will tolerate higher levelsof accumulated dust than those living elsewhere, since toler-ance is related to the rates of dustfall normally experienced(Vallack and Shillito 1998). However, a significant correlationwas found between the volume of atmospheric particulatematter in Aviles and a depression in the respiratory function ofchildren (Excelentısimo Ayuntamiento de Aviles 1983).

Rainfall and associated dust have been traditionally moni-tored by the Aviles city council using passive deposit gaugesdistributed around the municipality. These are operated on amonthly basis with results being expressed as the mean dailydeposition rate (mg � m�2 � d�1) of particulate (�0.45 �m) anddissolved (�0.45 �m) solids, as well as total deposited dust(particulate � dissolved), pH of the filtered solution and theconcentration of various elements in the ashed insoluble resi-due. Although there are currently no international standards fordeposited dust measurements and the results from a variety ofdustfall gauges are not necessarily comparable globally (Val-lack and Shillito 1998), several trends were revealed from thesemeasurements:

i) More dust is deposited in the eastern part of the city, dueto the influence of the ferrous industry and the dominantwinds (western direction). Deposition is controlled by rain-fall intensity.

ii) According to Schofield and Shillito (1983), fallout ratesexceeding 200 mg � m�2 � d�1 on a monthly average willgenerally cause nuisance to residential properties at peakperiods within that month. This value has been repeatedlyexceeded in Aviles in the last few decades.

iii) Dust levels above the general tolerance levels of humancommunities, or the first degree emergencylevel, currentlystand at 900 mg � m�2 � d�1 of particulate matter. How-ever, levels of 300 mg � m�2 � d�1 are considered hygien-ically acceptable(Boletın Oficial del Estado 1975). Boththese values have been surpassed in Aviles over the last 15years (Excelentısimo Ayuntamiento de Aviles 1984). Al-though levels are still high currently, the emergency levelis rarely reached nowadays, with a reduction of dust emis-sion evident, particularly in areas of traditionally higherconcentration (Gobierno Pdo. Asturias 1999).

iv) The pH of the filtered samples ranges between 4.7 and 8.7units, reaching its maximum values near the steel industry,probably due to the addition of lime to the load of thefurnaces. Al and Zn content in the ashed insoluble residueof the samples are higher in gauges placed close to theindustries related to both elements. Near the steel industry,Ca, Mg, and Fe content increases.

Given the results of these previous studies, there seems to be acredible need for assessing the levels of heavy metals in sus-pended and deposited particulate matter in Aviles, as well asfor investigating the role of local industries as sources of theseelements. Table 1 compares the concentrations of heavy metalsin street dusts from different cities around the world.

Materials and Methods

The area of study is located in northwestern Spain and lies within thelimits of Aviles (Figure 1). Since 1996, geochemical studies have beencarried out by the University of Oviedo in this area. Studies haveexamined the urban environment by sampling street dust, urban soils,and urban runoff, as well as surrounding areas influenced by the cityand industry by investigating non-urban soils, those on the coast, andbeach sand. The results obtained from the urban sampling strategy arepresented in this paper.

Street Dust

To draw inferences from a limited number of samples, it is essential toselect the appropriate sampling design. This is especially important inthe case of street dust (particulate matter deposited onto urban roadsand pavements), due to its high spatial variability (Duggan 1984;Leharne et al. 1992). For this purpose the urban area was divided intoseven mapping squares of 1 km2 each (Figure 2) and the samplingmethodology followed was that outlined by De Miguel (1995). Sixteensampling points were randomly distributed in each mapping unit for atotal of 112 samples of approximately 150 g of street dust werecollected, including both residential and industrial sectors. Plastic toolswere used to avoid metal contamination of samples (Ordonez 1997).

The 16 sub-samples per mapping unit were composited to representthe whole mapping unit. Some sub-samples were analyzed individu-ally due to their proximity to particular points of interest. In additionto this, in one mapping unit the 16 sub-samples were analyzed indi-vidually to evaluate internal variability. Visman methodology (Visman1947) has been applied to evaluate the precision of the estimate of themean value of the sampling unit. This methodology is based onvariance additivity (total variance is the sum of distribution, compo-sition, preparation, and analysis individual variances), which in thiscase was calculated by duplications at every stage. A detailed discus-sion of the fundamentals and application of this methodology can befound elsewhere (De Miguel et al. 1997).

After drying for 72 h at 45°C, each sub- or composite sample, wassieved through a 2-mm non-metallic mesh to remove large particles,before halving. One half was stored; the second one was thenground—with agate mortar and pestle—carefully homogenized andsieved to obtain the �147 �m fraction (particles that can be easilyresuspended), according to Nicholson (1988) and Sehmel (1980).After reduction by repeated quartering, the sample was digested andanalyzed as outlined below. All procedures of sampling and handlingwere carried out without contact with metals, to avoid potential con-tamination of the samples.

Samples of 0.5 g were digested with 3 ml of 3-1-2 HCl-HNO3-H2Oat 95°C for 1 h and then diluted to 10 ml with distilled water. Thecontent of 27 elements (Ag, Al, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, K,La, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Sr, Th, Ti, U, V, and Zn) wasdetermined by inductively coupled plasma–atomic emission spectrom-etry (ICP-AES), and Hg by flameless atomic absorption spectrometry(FAAS). It should be noted that an aqua regia digestion like the oneused in this study, although recommended by the European SoilBureau for the establishment of background and reference values insoils (European Soil Bureau 2000), will not accomplish a total liber-

162 A. Ordonez et al.

Fig. 1. Geographical setting of the studied area


ation for most elements. All analyses were performed in ACMEAnalytical Laboratories (Canada), accredited under ISO 9002, usingthe same extraction and measuring methods. Quality control methodsinvolving the collection of field duplicates, using control standards(STD DS3, internally certified using CANMET and/or USGS certifiedreference materials) and analytical duplicates have been observed.Approximately 5% of the samples were analyzed as internal andexternal control samples.

Soils

When studying urban soils it is not possible to apply a systematicsampling design as their locations depend on there being open areas,such as parks, gardens, etc., which are not evenly distributed across thecity. The sampling methodology applied in this case was that of DeMiguel et al. (1998), which was used to sample soils in Madrid.

Samples were collected in Aviles from public parks, where there aremany visitors, especially children, who are more sensitive to potentialsoil pollution (Biggins and Harrison 1980 and references therein;Rundle et al. 1985; Watt et al. 1993). Other green areas, such asprivate gardens and undeveloped land were also sampled to cover thedifferent zones of the city and assess possible influences from traffic orindustry as a function of distance from their source. Sampling pointswere chosen from relatively undisturbed areas which were consideredto have a better chance of retaining the heavy metal signature. Sam-

pling points were located at about 15 m from the nearest major road orstreet with high traffic density. In large parks a second sample wascollected to estimate the spatial variation. Some duplicates (8% of thetotal) were also taken to estimate any errors attributable to the sam-pling phase.

Soils collected were not necessarily restricted to the urban area,unlike the street dust campaign, and therefore covered a larger area (10km2), including soils surrounding the city. Some samples taken farfrom the city center were used to assess the gradient of heavy metalconcentrations across the city.

Soil samples were composites of five sub-samples (of about 0.5 kg),collected from the upper 15 cm of the soil profile, and spatially distributedaccording to a cross pattern, with 5 m distance between sub-samples, toreduce any variability associated with the sampling point. Soil samplingwas carried out using plastic trowels at the 40 sampling points shown inFigure 2. The composite samples (average mass 2.5 kg) were transportedin plastic bags to the laboratory, where they were subjected to homoge-nisation, drying, sieving, quartering, and analysis phases, analogous tothose applied to the street dust samples.

Results and Discussion

Before the results of the street dust analysis are presented, theaccuracy and precision of the methodology are explored toassess the representativity of the results obtained. A similardiscussion is given before presentation of the soils data.

Precision of Street Dust Results

To evaluate variability associated with field and laboratoryphases, as well as the representativity of each individual result,duplicate samples were tested at each step.1 During sampling,which can be a major contributor to variability, 4% of thesamples were duplicated, where it was found that for Ag, As,Zn, and Cd, the results varied by ��10%, whereas for the restof elements, variation was less than 10%. Of the sub-samplestaken to the laboratory, 8% were duplicated before their prep-aration for analysis by grinding, sieving, etc. At this step of theprocedure, variability was found to be �10% in all cases.Before geochemical analysis in the laboratory, 11% of the indi-vidual samples were duplicated, where the contribution to totalvariability was found to be �10% for all elements analyzed.

A map is considered “stable” or “ robust” when the internalvariability within mapping units, sw

2 , is small compared to thevariability between different mapping units, sb

2 (Miesch 1976).The analysis of the 16 individual subsamples taken from one ofthe mapping units allowed to calculate sw

2 (the assumption ismade that the internal variability does not change significantlyfrom one mapping unit to another throughout the area underinvestigation). Similarly, the variability between mapping unitscan be estimated from the variance of the set of mean elementalconcentrations of each mapping unit and sw

2 .

sb2 � s2 (X� i) �

sw2

n

where s2(X� i): variance of the set of mean elemental concentra-

1As outlined in the previous section (Materials and Methods).

Fig. 2. Squared units used for street dust sampling and soil samplingpoints


tions of the different mapping units. n: number of samplingincrements taken from each mapping unit.

The ratio sb2/sw

2 gives an indication of the stability of the mapsprepared for each chemical element. This ratio is shown inTable 2 for some studied metals. The average stability for thedifferent elements analyzed was 0.5%, i.e. for the averagevariability between mapping units, any two of them will bedisplayed as significantly different with a significance level of0.5% (De Miguel 1995).

Results and Discussion of the Street Dust Study

While the results of statistical analyses carried out on the wholedatabase are shown in Table 3, the distributions of someelements such as Zn, Cd, Al, and Pb will be discussed in furtherdetail as representative of the distribution found in general.Isoline maps in Figure 3 show the spatial distributions of Zn,Cd, and A1 found in street dust. These maps were generatedusing SURFER software and the interpolation algorithm usedwas kriging. They show the origin of the heavy metals wherearrows indicate increasing concentration of the element to-wards its source.

Zinc concentration is high in the whole study area and tendsto steadily increase in a northerly direction as shown in Figure3a. It was found that the trend of decreasing concentration withdistance from the industrial zinc smelter was also common toCd and Hg. The reason for this is obvious: while concentrationsof Zn are uniformly high across the whole area (geometricmean concentration of 4,892 �g � g�1), particular enrichment isfound in the north where the zinc smelter is located (23,400�g � g�1). Since Cd, Hg, and other trace elements, like Pb orAg, occur as impurities of sphalerite, the Zn ore, enrichment ofthese elements in street dust is found in the same areas. Thisrelationship is further confirmed by high coefficients of corre-lation between Zn and Cd (r2 � 0.998) and Zn and Hg (r2 �0.979). The similar spatial distributions of Zn and Cd can becompared in Figure 3a and b, respectively, which show thehighest concentrations of both elements occur in the north, e.g,the maximum for Cd is 104 �g � g�1.

Mercury concentrations in dust particles ranged from 1.20 to10.8 �g � g�1 with a geometric mean of 2.56 �g � g�1 showinga similar spatial distribution to that found for Zn.

Aluminium in urban street dust exhibited values rangingfrom 0.72% in the southern squares with the highest (1.12%)located in the northern square, with a geometric mean of0.85%. However, Figure 3c indicates that the main source ofA1 is in fact found to the northeast, where A1 metallurgy islocated.

In the case of Pb, which has been described as the mostimportant toxic hazard in the development of civilization(Magnus 1994; Salomons et al. 1995), values are uniformlyhigh (Figure 4) across the whole area (geometric mean of 514�g � g�1). High concentrations of Pb (964 �g � g�1) have beenfound in the north of the area, similar to Zn, Cd, or Hg, aroundthe Zn smelter itself. However, not all of the Pb found in Avilesstreet dust is associated with the Zn metallurgical industrybecause one of the major sources of this element at the timethat the sampling was done was traffic, as highlighted by the

particular enrichment of Pb in street dust along busy roads(Figure 4). The spatial distribution of Pb in the dust of Avilesis a result of the combination of both sources: metallurgicalindustry and traffic. In spite of the high Pb concentration instreet dust, a public study (Gobierno Pdo. Asturias 1999)determined that the annual lead content in air was below thelimit of 2 �g m�3.

Iron concentrations in the street dust of the studied area havegeometric mean value of 4.22%, and ranges from 3.24 to5.74%; the maximum values being found in the proximity ofthe steel industry located in the east.

Precision of Soils Results

Variability in soils was for most elements lower than that foundin street dust, and was mainly associated with the samplingstage, rather than the preparation and analytical phases.

Results and Discussion of Soils Study

Geometric mean, minimum, and maximum values for the 28elements analyzed are shown in Table 3. Of particular concernare the high maximum values for As, Cu, Pb, and Zn. Geo-metric mean concentrations of Zn, Cd, Hg, and Pb in soils areappreciably lower than those found in the surrounding streetdust (approximately, 13, 10, 5, and 3.5 times lower, respec-tively). However, Fe and Al values in soils are similar to thosefound in the dust. Elements such as K, Mg, La, etc., arisingfrom natural sources, mainly by means of soil resuspension, areconsequently more abundant in soils than in dust and homo-geneously distributed in the area. Compared to backgroundlevels found after sampling a neighboring area not affected byurbanization or industrialization, the Aviles soils are enrichedin Zn, Cd, and Pb with factors of 6, 6, and 4, respectively.

Comparison between the street dust and soil studies withrunoff studies carried out by Ordonez (1997) show a geochemi-cal relationship between those three environments which hasalso been found by other authors elsewhere in Spain (DeMiguel et al. 2001). Anthropic sources from industry andtraffic are thought to be the main cause of metal enrichment inthe street dust of Aviles, this is then combined with naturalelements from the original geological matrix. The deposition ofparticles transported by air increases heavy metal concentrationin urban soils, which are subsequently a source of natural andanthropogenic elements in dust by means of resuspension.Finally, runoff washes particles off urban surfaces which havebeen enriched anthropically.

Statistical Analysis

To better describe and interpret the analytical information,multivariate analysis techniques, i.e. factor analysis (principalcomponents method, rotation varimax) and cluster analysis(ward method), were applied using SPSS software, to thedatabase of street dust and soil results.


Street Dust

Cluster analysis of the variables (Figure 5) revealed twogroups of elements: the first contained Zn, Cd, Hg, and Pb,elements of predominantly anthropic origin and found inexcess quantities in association with high traffic density andindustry. The second group contained elements such as Mg,La, Th, etc., which are homogeneously distributed in thearea and mainly natural in origin, but mixed with elementslike Ni, Cu, and Al whose enrichment in the dust is probablydue to anthropic activities, but whose spatial distribution isless related to industry and traffic than in the first group.Additionally, cluster analysis distinguished samples col-

lected in the city center (enriched in anthropogenic ele-ments) from those taken in the periphery (with higher con-centrations of natural elements).

Data were also subjected to Factor Analysis, with the aimof reducing the number of original variables to a smallernumber of associations. Four factors were extracted (eigen-values over 1.0), which account for 78.1% of the variation inthe dataset. Factor 1 exhibits a strong association betweenZn, Cd, Hg, Pb, and As, and reflects the influence of the Znindustry. Ni, Mo, Cu, Fe, and Cr, grouped in factor 2, arevery likely associated with emissions from the dominantlycoal-powered domestic heating systems. Factors 3 (Al, Mg,La, Ti, and Th) and 4 (Ca, Sr, and Ba) probably reflect the

Table 2. Variability between different mapping units, internal variability within mapping units, and the stability of the maps prepared foreach chemical element

ElementVariability betweendifferent mapping units, sb

2Internal variabilitywithin mapping units, sw

2 Ratio sb2/sw

2Stability ofthe maps

Al 2124762 4243958 0.501 0.5366%Cd 1105 550 2.008 0.0003%Fe 81958214 136532000 0.600 0.2799%Hg 11.28 8.89 1.269 0.0069%Pb 66419 52517 1.265 0.0070%Zn 56622467 31815282 1.780 0.0007%

Table 3. Descriptive statistics of the analytical data found in street dust and soils of Aviles

Element

Min. Max. Geometric mean Geom. SD Coef. variation

Str. dust Soil Str. dust Soil Str. dust Soil Str. dust Soil Str. dust Soil

Ag (�g � g�1) 0.40 0.27 4.55 11.0 1.31 0.69 1.43 1.33 0.81 1.69Al (%) 0.72 0.60 1.12 3.03 0.85 1.52 1.07 1.18 0.17 0.35As (�g � g�1) 11.0 4.50 26.0 117 17.5 20.9 1.15 1.41 0.30 0.93B (�g � g�1) 7.00 2.90 11.5 38.0 9.57 8.98 1.09 1.32 0.18 0.65Ba (�g � g�1) 244 76.0 489 790 361 189 1.10 1.29 0.21 0.74Ca (%) 9.08 0.24 12.0 10.9 10.1 1.09 1.05 1.55 0.11 1.18Cd (�g � g�1) 9.60 0.80 104 7.80 22.3 2.16 1.42 1.29 1.07 0.67Co (�g � g�1) 5.00 2.00 11.5 182 7.03 8.91 1.15 1.42 0.33 1.95Cr (�g � g�1) 32.0 9.00 54.5 110 41.6 23.9 1.09 1.23 0.20 0.61Cu (�g � g�1) 104 19.0 374 1,040 183 62.5 1.24 1.46 0.51 1.65Fe (%) 3.24 1.67 5.74 9.83 4.22 3.56 1.10 1.23 0.21 0.53Hg (�g � g�1) 1.20 0.17 10.8 2.41 2.56 0.57 1.38 1.33 0.99 0.77K (%) 0.09 0.07 0.23 1.03 0.14 0.20 1.14 1.28 0.36 0.71La (�g � g�1) 8.00 7.00 11.5 36.0 10.3 15.5 1.06 1.17 0.13 0.37Mg (%) 0.94 0.09 1.28 3.53 1.10 0.51 1.05 1.41 0.12 0.94Mn (�g � g�1) 1,194 185 3,148 4,261 1,661 690 1.18 1.39 0.41 0.97Mo (�g � g�1) 3.00 0.90 5.00 10.0 4.16 1.28 1.08 1.27 0.17 1.00Na (%) 0.03 0.01 0.05 0.05 0.04 0.01 1.10 1.22 0.25 1.00Ni (�g � g�1) 18.0 5.00 50.0 48.0 27.5 16.7 1.16 1.28 0.36 0.54P (%) 0.04 0.03 0.23 0.61 0.09 0.07 1.36 1.28 0.64 1.13Pb (�g � g�1) 330 54.0 964 1,160 514 149 1.21 1.32 0.46 0.95Sb (�g � g�1) 7.00 1.80 9.00 15.0 7.98 4.60 1.04 1.29 0.08 0.67Sr (�g � g�1) 222 10.0 310 381 264 33.0 1.06 1.46 0.13 1.32Th (�g � g�1) 4.00 1.80 8.00 9.00 6.36 3.91 1.11 1.22 0.21 0.44Ti (%) 0.02 0.01 0.03 0.02 0.02 0.01 1.09 1.11 0.00 0.00U (�g � g�1) 4.50 4.50 21.0 12.0 4.96 5.78 1.52 1.15 0.83 0.37V(�g � g�1) 25.0 22.0 34.5 67.0 28.1 34.1 1.05 1.11 0.12 0.27Zn (�g � g�1) 2,422 110 23,400 1,959 4,892 376 1.42 1.35 1.09 0.76


different nature (alumino-silicate, carbonate) of the geolog-ical substrate in street dust.

Soils

Cluster analysis on the soil elements grouped the elements intotwo main conglomerates. The first one represents elementsassociated with anthropic activity such as Pb, Zn, Cd, Ag, etc.,their distribution across the city reflecting urban and industrialinfluences. The second group includes elements which areregularly distributed across the city and of natural origin suchas Mg, Mn, Th, La, K, etc., as well as other elements of mixedsources, like Fe, V, As, Sb, or Hg. Factor analysis corroboratedthe previous results.

Most soil samples with high As, Cd, Cu, Hg, Pb, Ni, andZn concentrations were taken in parks (35%), gardens(30%), and undeveloped land (25%), whereas only a few(10%) were collected from privately-owned land used togrow vegetables for domestic consumption and for cattlegrazing. In particular, sample number 18 (Figure 2), locatedin the city center, was taken in a public park, more specif-ically from the sand covering a children’ s playground. Theanalysis revealed that this particular sample had concentra-tions of Zn, Hg, Pb, Ag, Sr, Ca, and As as high as samplestaken to the north of the area and much higher than sample19 (Figure 2) which was a soil sample taken from the samepark. This sand was found to come from a beach located tothe north of the city, close to the Zn industry.

In later studies undertaken by the University of Oviedo(Gallego 2000), soil sampling campaigns have been carried outin areas surrounding the city, on both sides of the Avilesestuary. Using similar laboratory procedures to this study, highconcentrations of Zn and Al have been found outside the cityboundaries, ranging from 169 to 1150 �g � g�1 and from 0.66

Fig. 4. Spatial distribution of Pb concentration in street dust and majorroads of Aviles

Fig. 3. Isolines of concentrations of (a) Zn, (b) Cd, and (c) Al in the street dust of Aviles


to 2.4%, respectively. These concentrations, however, are stilllower than those found in Aviles urban soils. Several studiesare currently being undertaken by the same university to mon-itor the environmental geochemistry of the city and its sur-rounding areas.

Conclusions

The sources of the different elements in street dust and soils aretypically common to most urban environments (traffic, heatingsystems, industry, natural substrate, etc.), but their intensitiesand patterns of distribution vary accordingly to the peculiaritiesof each city. In the case of Aviles, the urban dust and soilsamples within the residential and industrial areas, there areclearly some locations with significant heavy metal accumula-tions. The following conclusions can be drawn:

Geochemical analysis of the street dust in Aviles revealedvariable concentrations of contaminants, being severely en-riched in some points. Mapping the metal content of street dustshows that the isolines are orientated perpendicularly to theelement dispersion direction. This reflects the location of theZn, steel and Al industries and, in the case of Pb, areas withhigher traffic density. There is a clear positive correlationbetween metal content in street dust and either proximity tothose industries, or location of service roads.

The concentration of some heavy metals in urban deposits isvery high in comparison to their average concentration found inthe urban soils, as street dusts deposition is believed to be themain source of these elements in the soils. Zinc, Cd, Hg, and

Pb, in particular, all have elevated concentrations. These ele-ments are clearly of anthropic origin being associated with theZn industry and its raw materials.

Natural elements, such as Mg, La, K, etc., are homogeneousin distribution, their natural source being reflected in the factthat their concentrations are greater in soils than in street dust.

Based on the multivariate statistical analysis of the geo-chemical data, two types of geochemical anthropic influencescan be distinguished in the city: the first is related to industry,and the second to traffic, which is thought to be implicated inthe transport of particles from other sources.

Street dust and soil compositions vary within the urbanenvironment of Aviles from central to suburban zones. This iscommon in cities where industrial development and trafficintensity are unique, and exert characteristic influences overany general geochemical “fi ngerprint.”

This study has shown the significant contribution of localindustry to the heavy metal contamination of the street dustand soils of Aviles. By means of sample duplication it wasfound that the variability introduced by different phases ofthe study is acceptable. It has also been shown that the streetdust sampling strategy has produced an accurate reflectionof the geochemical fingerprint in each mapping unit. More-over, isoline maps appear to have satisfactory levels ofstability and resolution and can therefore be used in spatialdistribution studies. These maps of heavy metal concentra-tion in street dust illustrate well the source of contaminantsand allow identification of any areas which are particularlyenriched.

Fig. 5. Clustering of variables analyzed in streetdust (Linkage method: Ward)


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