Scientifica Acta 3, No. 2, 23 – 26 (2009)
Earth Sciences
Environmental indicators for heavy metals pollution: soils and
higher plants
Cecilia Danesino
Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy.
[email protected]
The aim of the study is the evaluation of different heavy metal pollution indicators to efficiently track
atmospheric contamination. The investigation considered both soils and vegetation samples (grass, leaves
and 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 sampled
by ICP-OES. In addition, wood cores were also taken from selected conifer species in order to test the
applicability 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 metal
abundances 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 quantitative
analysis 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 of
the discipline known as ecotoxicology. There are different ways through which heavy metals can come into
contact 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 the
pollution sources. Tree rings yearly formed in wood are a wide and complex source of data, potentially
offering hints on the impact of atmospheric pollution. They can provide material for dating events at a high
temporal resolution, i.e. yearly or seasonally. In addition, given their broad spatial distribution, trees offer
possibilities 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 types
of atmospheric pollution (industrial contamination or vehicular traffic), compared to a supposed unpolluted
area. These are:
1. the lower part of the Susa Valley (TO, Italy), a narrow valley axis strongly impacted by anthropogenic
emissions, due to the coexistence, in the valley floor, of two major provincial roads, one highway and
one railway both connecting Italy to France; moreover, one smelter and one iron-works (S.Didero and
Ferriere) 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 Pb
and Zn derived from road traffic [3].
2. the site of Villadossola (VCO, Italy), where a serious atmospheric pollution event, due to uncontrolled
emissions of black smokes from a smelter, was documented in 1989-1990. Air quality data produced
in 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 to
estimate the deposition, evaluate the bioavailability and calculate the migration rate within the soil [5].
c 2009 Università degli Studi di Pavia
Scientifica Acta 3, No. 2 (2009)
24
3. the site of Villarboit (VC, Italy), where, during excavation works, several fossil oaks in standing
position where resumed. The oaks, probably of Roman age, where buried during an exceptional flood
event. 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 distribution
were performed. Soil and oven dried vegetation samples were acid digested and analysed by ICP-OES
for heavy metal contents (Co, Cr, Cu, Mn, Ni, Pb, V and Zn). Finally, wood cores extracted from living
conifer trees with an incremental borer have been examined to test the applicability of dendroanalysis for
retrospective environmental monitoring.
The determination of trace element composition of single tree rings with a reasonably high spatial
resolution and low detection limits is a major challenge in dendroanalysis. The use of LA-ICP-MS, although
satisfying both constrains, suffers from the lack of a proper external standard (highly homogeneous and with
relatively high concentration levels) and of a suitable element to be used as internal standard. By performing
ICP-MS bulk analysis on several individual wood rings, we validated a procedure for determining the heavy
metal composition of tree rings with LA-ICP-MS.
c 2009 Università degli Studi di Pavia
Scientifica Acta 3, No. 2 (2009)
4
25
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 is
generally 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 the
local atmospheric contamination levels; indeed, soil profiles indicate that the contamination is confined in
the 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 background
content from the bedrock.
On the other hand, the interpretation of heavy metals concentrations in tree rings is not trivial, due to
analytical and standardization difficulties. The lack of a proper external standard results in a relatively
low level of accuracy with respect to conventional LA-ICP-MS analysis of minerals. Nevertheless, with
an independent estimate of the Mg or Ca content of the wood, LA-ICP-MS can be considered a suitable
analytical method for the quantitative analysis of wood samples. Despite the very promising results initially
obtained on Pinus Nigra and Pyrus communis L.[7,8,9], wood cores evidence a strong variability possibly
related to wood physical characteristics. These include density and structures which significantly change
depending on tree species and age.
All data were statistically treated to evidence correlations between environmental indicators. In wood
cores, most metals are correlated together, indicating a rather unselective root uptake for these elements. On
c 2009 Università degli Studi di Pavia
Scientifica Acta 3, No. 2 (2009)
26
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 analysis
of wood samples, with potential application to dendroanalytical investigation. A more widespread application of this microanalytical technique could enhance the knowledge on elemental uptake in higher plants
and translocation or immobilization processes in wood.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
H. B. Bradl (ed.), Interface Science and Technology, Vol. 6 (Elsevier Ltd, London, 2005).
M. M. Savard, C. Bégin, M. Parent, J. Marion, A. Smirnoff Geochemistry: Exploration, Environment, Analysis 6,
237 (2006).
IPLA, internal report (1997).
ARPA Piemonte, internal report (1990).
A. Facchinelli, F. Ajmone Marsan, E. Barberis, Collana Ambiente 26 (Regione Piemonte, 2003).
A. Facchinelli, E. Sacchi, L. Mallen, Environmental Pollution 114, 313 (2001).
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).
A. Facchinelli, C. Folin, C. Danesino, E. Sacchi, M. Tiepolo, R. Vannucci, Proceedings Epitome Geoitalia 2007 Rimini 12-14 Settembre 2007, 2, 448 (2007).
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