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Abstract We studied the contamination of Cu, Zn, Pb, and Cd in soils under oak ecosystems of urban (U), sub- urban (SU), and rural (R) regions in Sofia, ...
Environ Chem Lett (2006) 4: 101–105 DOI 10.1007/s10311-006-0041-6

ORIGINAL PAPER

Vania Doichinova · Maria Sokolovska · Emilia Velizarova

Heavy metals contamination of soils under oak ecosystems in the Sofia region

Received: 15 October 2005 / Accepted: 20 February 2006 / Published online: 5 April 2006 C Springer-Verlag 2006 

Abstract We studied the contamination of Cu, Zn, Pb, and Cd in soils under oak ecosystems of urban (U), suburban (SU), and rural (R) regions in Sofia, Bulgaria. The urban sampling sites are located in the large forest parks of Sofia under oak ecosystems, while the others are located between the centre of the town and the surrounding mountains and are also from oak ecosystems. The concentrations of Cu, Zn, Pb, and Cd in soils were measured for identifying the sources and degree of contamination, and calculating the concentration coefficients (Hc) and enrichment factors (EF). The result of applying principal component analysis (PCA), showed that Cd and Pb accounted for the anthropogenic pollution and could be inferred as its tracers, whereas Cu is located mainly in the parent material. The Zn concentration levels were controlled both by its original content in the parent material as well as by anthropogenic pollution. The results obtained for the city forest parks allow for their successful use for recreation purposes. Keywords Heavy metals . Oak ecosystems . Urbanization . Concentration coefficient . Enrichment factor . Principal component analysis Introduction Urbanization, a process in which an increasing proportion of a population lives in cities and the suburbs of cities, leads to environmental impacts. Such impacts include: the conversion of land from natural habitats to urban built areas, depletion of natural resources, and the production and disposal of wastes in the atmosphere, hydrosphere and on the surface of the lithosphere. In addition to the disruption of natural soil cycles, urbanization may also contaminate the V. Doichinova · M. Sokolovska · E. Velizarova () Forest Research Institute, Bulgarian Academy of Sciences, 132 “St Kl. Ohridski” Blvd, 1756 Sofia, Bulgaria e-mail: [email protected] Tel.: +359-2-962-04-42 Fax: +359-2-962-04-47

soil with heavy metals. Deposition of polluted atmospheric particles over the years is an important source of heavy metal contamination of soil in urban areas (Sutherland et al. 2000). A similarity in the pattern distribution of heavy metal concentration, population density, and road transport was also established (Pin and Kuo 2000). Different species of trees accumulate the heavy metals depending on their genetic peculiarities (Kabata-Pendias and Pendias 1992). Oak stands are in the group that shows resistance to heavy metal pollution, with high dust- and gas-storage capabilities (Show 1989). In spite of the screening role of trees, a heavy metal contamination of soils in urban areas has been observed. Higher concentrations of Pb and Cd in soils have been found on the outskirts of urban parks, where the road traffic is larger, in comparison with their central regions (Chronopoulos et al. 1997). Several researchers have indicated that the heavy metal content in soils of urban parks increases with time, i.e., the age of the park (Madrid et al. 2002). A risk of heavy metal contamination in soils from suburban or rural regions also exists. The wide array of aerosols emitted from local industries, as well as leaded gasoline, and motor vehicles, are the main reasons for soil pollution (Velizarova 1998). Thus, studies aiming at the investigation of heavy metal pollution include not only the urban region, but also regions located at different distances from the urban centre, in which the changes observed could be related to their degree of urbanisation. A relationship between heavy metal accumulation in forest soils and the distance from the urban centre and industrial activity, has been established (Pouyat and McDonnell 1991; Thorton et al. 1991; Peterson et al. 1996). Previous studies have been directed mainly to the ecological state of soils from urban parks because of their important role in public recreation, microclimate and subsurface hydrology improvement, biodiversity and aesthetic view of the city area. Hence, the differences in soil contamination in urban areas depend on particularities of the population density, industrial activities, and road traffic, which are of special interest in each specific study.

102 Table 1

Main soil characteristics

SSa

pH Clay content (%)

OMb (%) K (mg/kg)

Concentration (mg/kg) Na (mg/kg) Ca (mg/kg)

Mg (mg/kg)

Fe (mg/kg)

Mn (mg/kg)

U1 U2 U3 U4 SU1 SU2 SU3 SU4 R1 R2 R3 R4

6.5 4.8 5.7 6.6 6.5 5.1 5.1 6.4 6.0 5.1 5.4 6.0

2.1 2.1 2.5 4.1 2.2 3.1 1.9 3.8 2.0 3.0 1.8 2.7

2040.3 690.7 1037.7 2673.7 2665.3 1056.3 1285.3 1288.3 1815.3 1119.0 1024.3 1696.7

951.3 890.4 1338.9 2963.8 3111.6 1982.0 3032.6 766.2 2595.3 1092.3 3830.0 1482.3

14328.6 9910.8 24838.5 25649.8 26877.9 19962.4 30770.0 9072.2 23502.3 21549.3 31219.7 23504.2

318.1 465.6 577.6 463.1 656.5 275.0 758.3 1586.3 1027.5 687.0 791.4 758.3

a b

29.28 27.37 51.19 38.51 49.43 39.95 45.78 39.76 47.67 48.51 46.34 55.96

1429.0 1118.0 1838.8 10831.0 1751.3 896.2 477.5 2048.3 1012 1341.7 569.3 2573.0

3727.0 1768.0 7607.7 8466.7 7247.3 4707.3 3305.3 6774.0 6930.7 5391.3 6163.0 5352.0

SS—sampling site OM—organic matter

Sofia is the capital of Bulgaria, with the largest urban region and population in the country. The aim of the current investigation is to characterize the content of some macroelements, and assess the heavy metal contamination of soils under oak ecosystems from urban, suburban, and rural areas of the Sofia region including mountains flanked around the Sofia valley. These ecosystems differ in their degree of urbanization and anthropogenic impact.

Sampling and analysis Within each sampling site, five replicate surface soil samples (0–20 cm in depth) were taken by means of a stainless trowel. Air-dried soil samples were weighed, sieved through a 2 mm mesh, and homogenized. The concentrations of elements were determined in extracts of aqua regia (ISO method 11466) by an AAC—Perkin Elmer 360 A analyzer. The main soil properties are presented in Table 1.

Experimental Data analysis Study area In this study, an urban–rural gradient approach with decreasing urban load was implemented (Pouyat and McDonnell 1991). Twelve sampling sites with different degrees of urbanization were investigated in the Sofia region. These sites were established for the purpose of the international project GLOBENET applied to the Sofia region for monitoring the urban impact on components of the forest ecosystems (Ecology of the City of Sofia 2004). The urban sampling sites (U1—Boris garden, U2—Loven park, U3—Western park, U4—Northern park) were located in the city’s largest forest parks, and the others were located at an increasing distance from the urban centre through suburban areas (SU1—village German, SU2—village Vladaia, SU3—village Vilite, SU4—village Gradetz) to the mountains surrounding the Sofia valley, denoted as rural areas (R1—Losen mountain, R2—Vitosha mountain, R3—Lulin mountain, R4—Balkan mountain) (Scheme 1). The studied soils according to the FAO (1974) soil classification were as follows: U1 and U3—Haplic Vertisols, U2—Haplic Luvisols, U4—Anthrosols urbogenic terri-cumulic (Gencheva 1995). The soils from SU and R sampling sites were Chromic Luvisols and Dystric/Eutric Cambisols, respectively, in dependence of the altitude (700–800 m). In all sampling sites studied, the forest vegetation was oak stands (Quercus sp.)

In order to assess the heavy metal pollution in the regions studied, the concentration coefficients (Hc) and enrichment factors (EF) for each metal were calculated for all sampling sites. The concentration coefficient (Hc) was calculated as the ratio of the element concentration in the soil sample to the maximum permissible concentration (MPC), or Hc = element concentration/MPC. MPC, according to the Bulgarian National Standard Reference Material, refers to the limit in concentration of a certain heavy metal, above which a toxic effect is revealed. These values have been published in a Government Newspaper (36/1979; 54/1997). The enrichment factor (EF) for a heavy metal in a soil was calculated as the ratio of the measured target element concentration to its average natural concentration for the Sofia region, which is assumed to be the geochemical background concentration (BC). Thus, EF = element concentration/background concentration. The Cu, Zn, Pb, and Cd background concentrations in natural soils in the Sofia region are defined by Atanasov (2002) as 61.4, 94, 42, and 0.4 mg/kg, respectively. Besides the afore-mentioned assessment of heavy metal contamination in soils, the degree of soil pollution was also estimated in accordance with the three-stage scale of the Bulgarian State Standard (BSS, 2, 17.4.1.04-88), in which

103 Scheme 1 Location of the sampling sites

soils are classified as weakly, moderately and highly polluted. In weakly polluted soils, the content of a given heavy metal does not exceed the maximum permissible concentration, but is above the background concentration for the studied region. In moderately polluted soils, the content of a given heavy metal exceeds up to three times the maximum permissible concentration, but obvious changes in soil properties are not observed. In highly polluted soils, the content of a heavy metal exceeds from 3 to 5 times the maximum permissible concentration and unfavorable soil changes have occurred. Precautionary values (PV) of heavy metals were used for recognition of unfavorable functional changes of soils (“Government Newspaper” 21/2002). Precautionary values are defined in accordance with the soil textural characteristics. Since the studied soils were sandy clay (clay content 20–60%), the relevant PV for Cu, Zn, Pb, and Cd are 60, 160, 45, and 0.8 mg/kg, respectively. Therefore, the soils are defined as: weakly polluted—with Hc ≤ 1 and EF>1; moderately polluted—with 3 ≥ Hc>1 and heavy metal concentration 3 and

the concentration of heavy metals >PV, and dangerously polluted—with Hc>5. The data sets of element concentrations were treated statistically by the “Statistica” software Version 5.0 for Windows. A multivariate statistical analysis (Principal component analysis—PCA) was applied using the algorithm described by Manta et al. (2002). Results and discussion The arithmetic means of the heavy metal concentrations (in mg/kg), the respective standard deviations ( ± SD), as well as concentration coefficients (Hc) and EF are presented in Table 2. The data shows that the concentration of Cu in urban soils U3 and U4 exceed the background concentration, and according to the criteria discussed above, these soils may be classified as weakly polluted in terms of Cu. The measured pH values of the soils taken from sampling sites U3 and U4 were 5.7 and 6.6, respectively (moderately and slightly acidic), thus the Cu pollution did not provoke

104 Table 2 Average concentration (arithmetic mean) of heavy metals, standard deviation ( ± SD) (mg/kg), concentration coefficients (Hc) and enrichment factors (EF) SS

Cu Mean ± SD

Hc

EF

Zn Mean ± SD

Hc

EF

Pb Mean ± SD

Hc

EF

Cd Mean ± SD

Hc

EF

U1 U2 U3 U4 SU1 SU2 SU3 SU4 R1 R2 R3 R4

24.5 ± 6.2 24.3 ± 15.0 61.9 ± 16.9 70.4 ± 21.4 24.7 ± 5.8 25.2 ± 9.7 47.4 ± 4.5 14.1 ± 3.7 33.6 ± 9.3 25.7 ± 5.8 53.0 ± 5.6 22.5 ± 4.6

0.1 0.6 0.8 0.3 0.1 0.4 0.8 0.1 0.3 0.4 0.9 0.2

0.4 0.4 1.0 1.1 0.4 0.4 0.8 0.2 0.5 0.4 0.8 0.4

35.5 ± 9.3 30.6 ± 5.2 71.7 ± 18.8 98.8 ± 17.6 36.7 ± 4.6 29.15 ± 3.7 47.5 ± 8.8 31.6 ± 6.5 38.6 ± 6.7 36.0 ± 7.0 39.3 ± 5.3 130.5 ± 24.1

0.1 0.5 0.7 0.3 0.1 0.3 0.5 0.1 0.2 0.4 0.5 0.6

0.4 0.3 0.8 1.0 0.4 0.3 0.5 0.4 0.4 0.4 0.4 1.4

27.5 ± 6.60 33.0 ± 3.08 24.0 ± 3.76 31.8 ± 5.53 17.8 ± 2.44 25.2 ± 2.86 29.9 ± 4.41 35.4 ± 3.24 21.5 ± 2.94 27.5 ± 5.10 27.9 ± 5.37 58.0 ± 5.08

0.3 0.8 0.4 0.4 0.2 0.5 0.6 0.4 0.3 0.5 0.6 0.8

0.6 0.8 0.6 0.8 0.4 0.6 0.7 0.8 0.5 0.7 0.7 1.4

0.5 ± 0.1 0.2 ± 0.1 0.3 ± 0.1 0.5 ± 0.1 0.3 ± 0.1 0.4 ± 0.1 0.3 ± 0.0 0.7 ± 0.2 0.3 ± 0.1 0.2 ± 0.1 0.3 ± 0.1 0.4 ± 0.1

0.2 0.4 0.3 0.3 0.2 0.6 0.3 0.4 0.3 0.3 0.4 0.3

1.2 0.5 0.8 1.2 0.8 0.9 0.7 1.6 0.8 0.4 0.7 0.9

SS—sampling site

toxicity. The concentration coefficients for U3 and U4 were Hc = 0.8 and 0.3 for the afore-mentioned sampling sites. The urban soils, taken from U1 and U4, were weakly polluted in terms of Cd (see Table 2). The EFs for the soils from both sampling sites were equal to 1.2. Previous studies also documented contamination with this element under deciduous trees in Sofia city parks (Gencheva 1995). A relatively high EF value of 1.6 was calculated for the soil from SU4, which could be a result of airborne particles from a mineral pitch factory in close proximity. Considerable contamination of Zn and Pb was found in the soil of the rural sampling site—R4. The lithological studies in this region have shown prevailing distribution of materials, containing Zn and Pb (Atanasov 2002). The small differences in the heavy metal concentrations in the soil sampling sites with the different degrees of urbanization, suggests that slight changes have occurred as a result of anthropogenic load. The previous studies showed the anthropogenic origin of the Pb, Cd in urban soils, which is connected mainly with vehicle transport (Gencheva 1995). Research performed in Seville (Spain), indicated that the concentrations of Pb, Zn, and Cu in the soils from major parks and green areas closer to the historic centre often exceeded the acceptable limits for residential, recreational and institutional sites (Madrid et al. 2002). In order to determine the predominant metal in the anthropogenic load of the studied territory, principal component analysis (PCA) was applied to the data obtained. The grouping was performed on the basis of linear combinations of the quantities of the studied elements, their correlations and rotation. In our case, two groups of factors, related to the source of elements in a studied region, were obtained (Table 3). Factor 1 with 26.0% of variance, is dominated by Fe, K, Ca, and Mg (shown in bold) with relatively high loadings. These elements are mainly crustal hence the first factor can be identified as a “lithological”. Most probably, the presence of these elements could be responsible for the soil aluminosilicate texture. Cu and Zn have a considerable

Table 3

Values of the factor loadings for studied elements

Variable

Factor 1

Factor 2

Cu Zn Pb Cd K Na Fe Mn Mg Ca Percent variance

0.73 0.62 0.08 0.17 0.73 0.49 0.62 0.01 0.60 0.73 26.0

− 0.37 0.46 0.73 0.64 0.28 0.12 − 0.58 0.42 − 0.51 0.08 22.0

Values of factor loadings of the dominant elements are presented in bold

contribution to the group of elements in the first factor, as indicated by low values of their EF. The absence of specific industrial sources of Cu pollution in the territory of Sofia does not support the potential risk of soil contamination. Factor 2 accounts for 22% of total variance, which is dominated mainly by Pb and Cd. The distribution of these two elements is affected chiefly by anthropogenic activity, which is confirmed by EF data. Lead deposition is greatly influenced by vehicle emissions. The Pb contamination of SU- and R-regions is a result of the activity of the biggest source of pollution in this region—the metallurgical plant “Kremikovci”. Cadmium is specified as the main pollutant from emissions of industrial processes and power stations. Zinc exhibits a higher loading value (0.62) in Factor 1 and a smaller one (0.46) in Factor 2. Obviously, the distribution of Zn in soils from these sampling sites indicate that both lithogenic and anthropogenic factors are important. There is a potential risk for future soil contamination of this element, as it is emitted by vehicle transport. Based on the results obtained, the Pb and Cd may be suggested to be indicators of anthropogenic pollution for the

105

studied region. In spite of the weak contamination of urban soils by Cd and Cu, the city parks could be successfully used as residential and recreational territory. Conclusion The investigation performed with soils under oak ecosystems in urban, suburban, and rural regions in Sofia, revealed a contamination of Cu and Cd in urban soils, and Zn and Cd in rural soils. In these cases, the concentrations of heavy metals in the soil samples exceeded their respective background concentrations less than twice. The calculated EF values were in the range of 1.0–1.6. Through the application of the multivariate statistic approach (PCA), it was found that Cd and Pb accounted for the anthropogenic pollution and could be inferred as its tracers, whereas Cu was inherited mainly from the parent material. The concentration levels of Zn were controlled both by its original content in the parent material as well as by anthropogenic pollution. The results obtained for the city forest parks (U sampling sites) allow for their successful use for recreation purposes. Acknowledgement We are grateful to the Bulgarian National Board of Forestry at the Ministry of Agriculture and Forestry for financial support to this research project.

References Atanasov I (2002) Studying and developing of precautionary values for heavy metals and metalloids. In: Report. MOEW, Sofia, 188 p

Chronopoulos J, Haidouti I, Chropoulou-Sereli A, Massas I (1997) Variations in plant and lead and cadmium content in urban parks in Athens, Greece. Sci Total Environ 196:91–98 Ecology of the City of Sofia (2004) In: Penev L, Niemel¨a J, Kotze D, Chipev N (eds) Pensoft, Sofia, p 456 Gencheva S (1995) Classification and some peculiarities of anthropogenic soils, FU, Sofia, p 284 Kabata-Pendias A, Pendias H (1992) Trace elements in soils and plants, 2nd edn, CRC Press, FL, p 365 Madrid L, Diaz-Barrientos E, Madrid F (2002) Distribution of heavy metal content of urban soils in parks of Sevillie. Chemospere 49:1301–1308 Manta D, Angelone M, Bellanca A, Neri R, Sprovieri M (2002) Heavy metals in urban soils: a case study from city of Palermo (Sicily) Italy. Sci Total Environ 300(1–3):229–243 Peterson E, Sanka M, Clark L (1996) Urban soils as pollution sinks— a case from Abardeen, Scotland. Appl Geochem 11:129–131 Pin L, Kuo C (2000) Simulated annealing and kriging method identifying the special patterns and variability of soil heavy metal. J Environ Sci Health, Part A 35:1089–1115 Pouyat R, McDonnell M (1991) Heavy metal accumulation in forest soils along an urban – rural gradient in Southeastern New York, USA. Water Air Soil 57–58:797–807 Show AJ (1989) Heavy metal tolerance in plants: evolutionary aspects. CRC Press, FL, p 268 Sutherland R, Tolosa C, Tack F, Verloo M (2000) Characterization of selected element concentrations and enrichment ratios in background and anthropogenically impacted road side areas. Arch Environ Contam Toxicol 38:428–438 Thorton I (1991) Metal contamination of soils in urban areas. In: Bullock P, Gregory PJ (eds) Soils in the urban environments. Blackwell Scientific Publications, Oxford, pp 47– 75 Velizarova E (1998) Heavy metal pollution of forest soils. Part 1. Distribution with distance and depth. J Balkan Ecol 3:56– 59