Scientifica Acta 3, No. 2, 23 – 26 (2009)

Earth Sciences

Environmental indicators for heavy metals pollution: soils andhigher plants

Cecilia DanesinoDipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy.cecilia.danesino@dst.unipv.it

The aim of the study is the evaluation of different heavy metal pollution indicators to efficiently trackatmospheric contamination. The investigation considered both soils and vegetation samples (grass, leavesand pine needles, wood branches, bark). Sampling sites were selected in Northern Italy (Piedmont region),in areas with a known contamination history. Heavy metals were determined on acid digested sampledby ICP-OES. In addition, wood cores were also taken from selected conifer species in order to test theapplicability of LA-ICP-MS to the analysis of heavy metals in tree rings.

Soils can be considered good indicators of environmental quality. Grass samples reflect heavy metalabundances in the top soil while pine needles seem to well reflect the local atmospheric contamination levels.Finally, despite analytical problems, LA-ICP-MS can be considered a suitable technique for quantitativeanalysis of wood samples, with potential application to dendroanalytical investigations.

1 Introduction

Heavy metals are toxic agents and their interaction with the organisms and the environment is the subject ofthe discipline known as ecotoxicology. There are different ways through which heavy metals can come intocontact with the human organisms but the main route of entering the food chain is plant uptake [1].

Soil analysis can be used to apportion the anthropic and litogenic input. Biological material (grass, leaves,bark, pine needles) is analyzed to evaluate the possible uptake of contaminants and the relationship with thepollution sources. Tree rings yearly formed in wood are a wide and complex source of data, potentiallyoffering hints on the impact of atmospheric pollution. They can provide material for dating events at a hightemporal resolution, i.e. yearly or seasonally. In addition, given their broad spatial distribution, trees offerpossibilities of correlations over vast territories [2].

2 Study area

Sampling areas are all located in Piedmont region (Figure 1). They are chosen to investigate different typesof atmospheric pollution (industrial contamination or vehicular traffic), compared to a supposed unpollutedarea. These are:

1. the lower part of the Susa Valley (TO, Italy), a narrow valley axis strongly impacted by anthropogenicemissions, due to the coexistence, in the valley floor, of two major provincial roads, one highway andone railway both connecting Italy to France; moreover, one smelter and one iron-works (S.Didero andFerriere) are located in the same area. In the area, an investigation on the heavy metal contents in soils,grass and vegetation, conducted by IPLA from 1990 to 1995, evidenced and quantified the input of Pband Zn derived from road traffic [3].

2. the site of Villadossola (VCO, Italy), where a serious atmospheric pollution event, due to uncontrolledemissions of black smokes from a smelter, was documented in 1989-1990. Air quality data producedin the 1990’s indicate that the main emitted pollutants were Cd, Zn, Pb, Mn, Ni e Cu [4]. In addition,in 1995, an investigation on the heavy metals content in soils surrounding Villadossola allowed toestimate the deposition, evaluate the bioavailability and calculate the migration rate within the soil [5].

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3. the site of Villarboit (VC, Italy), where, during excavation works, several fossil oaks in standingposition where resumed. The oaks, probably of Roman age, where buried during an exceptional floodevent. The site is selected as a "blank" for atmospheric pollution.

Fig. 1: Sampling sites in Piedmont region.

3 Material and methods

Soils are sampled along depth profiles or with increasing distance from the suspected pollution source.Routine analysis such as pH, Cation Exchange Capacity, Organic Matter content and grain size distributionwere performed. Soil and oven dried vegetation samples were acid digested and analysed by ICP-OESfor heavy metal contents (Co, Cr, Cu, Mn, Ni, Pb, V and Zn). Finally, wood cores extracted from livingconifer trees with an incremental borer have been examined to test the applicability of dendroanalysis forretrospective environmental monitoring.

The determination of trace element composition of single tree rings with a reasonably high spatialresolution and low detection limits is a major challenge in dendroanalysis. The use of LA-ICP-MS, althoughsatisfying both constrains, suffers from the lack of a proper external standard (highly homogeneous and withrelatively high concentration levels) and of a suitable element to be used as internal standard. By performingICP-MS bulk analysis on several individual wood rings, we validated a procedure for determining the heavymetal composition of tree rings with LA-ICP-MS.

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4 Results and discussion

Results indicate that the Susa Valley is characterised by a high natural background level of Cr, Co and Ni,due to the presence of numerous outcrops of ultramafic rocks in the drainage basin [6]. On the other hand,Pb and Zn are systematically enriched in the topsoil due to an anthropogenic input (Figure 2).

The most heavily contaminated area is located close to industrial plants, while vehicular pollution isgenerally not detectable beyond 10m distance from the major roads.

Fig. 2: Distribution of heavy metal contents in top soil (Susa Valley). Mean values in ppm.

Grass samples reflect heavy metal abundances in the top soil while pine needles seem to well reflect thelocal atmospheric contamination levels; indeed, soil profiles indicate that the contamination is confined inthe upper 20 cm, and therefore a transfer to pine needles by root uptake is unlikely.

The low metal levels in the Villarboit soils confirm a minor anthropic impact and a low natural backgroundcontent from the bedrock.

On the other hand, the interpretation of heavy metals concentrations in tree rings is not trivial, due toanalytical and standardization difficulties. The lack of a proper external standard results in a relativelylow level of accuracy with respect to conventional LA-ICP-MS analysis of minerals. Nevertheless, withan independent estimate of the Mg or Ca content of the wood, LA-ICP-MS can be considered a suitableanalytical method for the quantitative analysis of wood samples. Despite the very promising results initiallyobtained on Pinus Nigra and Pyrus communis L.[7,8,9], wood cores evidence a strong variability possiblyrelated to wood physical characteristics. These include density and structures which significantly changedepending on tree species and age.

All data were statistically treated to evidence correlations between environmental indicators. In woodcores, most metals are correlated together, indicating a rather unselective root uptake for these elements. On

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Fig. 3: An example of a metal trend in a wood core.

the other hand, no clear relationship could be evidenced between the metal contents in soil and vegetation,except for V, nor between soil physical-chemical parameters and metal contents in wood, except for Cd.

5 Conclusion

Soils can be considered a good indicator of environmental quality and an accumulator of heavy metals.Concerning vegetation matrixes, the elemental content is always very low, often below the detection limits;the most interesting results are obtained from conifers needles and grass samples.

Despite analytical problems, LA-ICP-MS can be considered a suitable technique for quantitative analysisof wood samples, with potential application to dendroanalytical investigation. A more widespread applica-tion of this microanalytical technique could enhance the knowledge on elemental uptake in higher plantsand translocation or immobilization processes in wood.

References[1] H. B. Bradl (ed.), Interface Science and Technology, Vol. 6 (Elsevier Ltd, London, 2005).[2] M. M. Savard, C. Bégin, M. Parent, J. Marion, A. Smirnoff Geochemistry: Exploration, Environment, Analysis 6,

237 (2006).[3] IPLA, internal report (1997).[4] ARPA Piemonte, internal report (1990).[5] A. Facchinelli, F. Ajmone Marsan, E. Barberis, Collana Ambiente 26 (Regione Piemonte, 2003).[6] A. Facchinelli, E. Sacchi, L. Mallen, Environmental Pollution 114, 313 (2001).[7] A. Facchinelli, C. Folin, C. Danesino, E. Sacchi, M. Tiepolo, R. Vannucci, Consoil 2008 - Proceedings of 10th

International UFZ-Deltares/TNO Conference on Soil-Water Systems (Milano, 3-6 June 2008), 164 (2008).[8] A. Facchinelli, C. Folin, C. Danesino, E. Sacchi, M. Tiepolo, R. Vannucci, Proceedings Epitome Geoitalia 2007 -

Rimini 12-14 Settembre 2007, 2, 448 (2007).[9] V. Re, M. Tiepolo, E. Sacchi, R. Vannucci, G. Dolza (2003) - Geoitalia 2003: IV Forum Italiano di Scienze della

Terra, Bellaria, 17-19/09/2003, 430.

c© 2009 Università degli Studi di Pavia